Read more

Greetings, and welcome to the RLRH newsletter!

Endurance training stats usually get compressed into two numbers: how much did you run, and how hard was it?

That is why we spend so much time debating mileage, the 80/20 rule, and whether an athlete’s training is polarized, pyramidal, or something in between. These summaries are useful. But they also flatten a workout into a single dimension.

A new study of elite Ethiopian distance runners shows what can disappear in that flattening.

On a standard easy/moderate/hard chart, this group’s training looked relatively familiar to most of us—lots of easy running, very little moderate running, and a modest amount of hard work (characteristic of the polarized training model). But when the researchers added where the athletes ran, what was underfoot, how the terrain changed, how long the session lasted, and who was running together, a much richer structure emerged.

So let’s take a look…

Reconstructing 97 days of elite training

The new paper followed 14 elite Ethiopian distance runners—six men and eight women—for 97 consecutive days between September and December 2025. Most specialized in the marathon or half marathon, with one 10K road runner.

Their watches produced 22,605 GPS segments containing pace, heart rate, altitude, location, and elapsed time. Field information identified eight training venues between 2,065 and 3,100 meters of altitude and across six different surfaces. That’s a LOT of data…

The average athlete ran about 75 miles/119 kilometers per week across roughly eight and a half training sessions, but the runners ranged from 40 to 100 miles (64 to 161 kilometers) per week.

The week followed a recognizable hard-easy rhythm.

• Tuesday, Thursday, and Saturday carried the most running—typically 13–14 miles (21–22 kilometers).

• Monday, Wednesday, and Friday were lighter.

• Sunday was a rest day in about 80% of athlete-weeks.

• About 40% of training days included a second run (a “double”), usually a short, low-heart-rate recovery session in the afternoon after the main morning run.

The intensity distribution looked ordinary

The researchers classified training intensity several ways because neither pace nor heart rate offers a perfect answer in this setting.

Across the altitude-corrected pace methods, roughly:

• 75% to 93% of training time was easy

• 3% to 5% was moderate, and

• 4% to 23% was hard.

There was also a revealing pattern when the researchers removed the zones entirely and simply plotted training time by altitude-adjusted pace. The data formed two preferred pace regions: a clearly easy mode and a faster mode closer to marathon pace, with relatively little running in between. Eleven of the 14 athletes showed this same dip individually!

Place was part of the prescription

The researchers identified eight venues in which training was performed. Each one offered a particular combination of altitude, surface, and terrain, and the athletes used them for different purposes.

• Long aerobic runs concentrated on gently rolling, mid-altitude gravel roads. Those roads accounted for about two-thirds of all long-run time.

• High-altitude trails hosted slower, more variable running over routes that often changed from one session to the next.

• The synthetic track hosted short, structured repetitions.

• Easy recovery running was spread across several other venues, influenced partly by where individual athletes lived (we all can relate… my “preferred” route is right out my front door).

In other words, a track session offered precision and repeatability. A road offered a stable setting for sustained work. A trail offered variation—different climbs, descents, turns, and footing—even when the athlete’s effort remained controlled.

More than one third of all training time occurred as easy running on high-altitude soft trails, at roughly 71% of maximum heart rate and a pace near 9:00 per mile or 5:40 per kilometer. Taken out of context, that pace might look almost comically slow for a world-class distance runner. Put it back at high altitude, on soft and rolling terrain, and it makes sense.

The group did some of the individualizing

The social structure of the training mattered too.

About three-quarters of all sessions were completed in groups, usually with around nine athletes. Long runs and harder sessions were especially likely to be shared: 93% of long steady runs, 97% of long mixed sessions, and 80% of interval sessions were group runs.

But “group training” did not mean every runner received the same dose. In runs of 5 athletes or more, the group separated into two pace bands about 30 seconds per kilometer apart. Athletes with performances closer to the world record were (obviously) more likely to occupy the faster band.

The researchers describe this as a form of group-based individualization. Instead of every athlete receiving a different written pace target, the group appeared to sort itself by fitness while still completing the session together.

What this means for runners

Read more

Read more

Greetings, and welcome to the RLRH newsletter!

Does a 1% Treadmill Incline Really Match Outdoor Running?

Many runners have long operated by a simple rule when it comes to indoor training: Set the treadmill to 1%, and the energetic cost will roughly match running outside (supposedly because it accounts for windspeed, which is absent indoors).

That rule was never meant to be universal (and personally, I’ve never bought into it), and a new study suggests it may not hold even in highly trained runners!

Twelve national-level male endurance runners completed the same test twice. They ran from 5 to 9 miles per hour (8 to 15 kilometers per hour) in three-minute stages, first on a treadmill set to a 1% grade and then, 48 to 72 hours later, on a 400-meter outdoor track. The researchers measured oxygen consumption, oxygen cost per kilometer, total energy cost, and fuel use across different running intensities.

Overground running was consistently more “expensive.”

• Oxygen cost of running averaged 204.2 versus 197.1 milliliters per kilogram per kilometer outdoors and on the treadmill, respectively—a difference of about 3.6%.

• At 7 miles per hour/12 kilometers per hour, the energy cost was 0.93 versus 0.87 kilocalories per kilogram per kilometer. Near the runner’s second ventilatory threshold (denoting a heavy intensity), the gap widened.

• Carbohydrate use was also higher outdoors between the first and second ventilatory thresholds: the runners used 3.20 versus 2.75 grams per minute. Near the second threshold, it reached 3.70 versus 3.03 grams per minute.

What this means for runners

A treadmill remains an excellent tool for controlled training. But treadmill pace and outdoor pace are not interchangeable, even when the belt is raised to 1%. If an outdoor threshold session feels a little harder than the same speed indoors, that does not necessarily mean your fitness changed.

The practical move here is to calibrate by effort, heart rate, or lactate rather than forcing one universal incline correction. Keep treadmill conditions consistent when tracking progress, and complete some race-specific work on the surface where you plan to compete. I wouldn’t turn this small study into a command to raise the belt another half-percent.

Can Your Body Tell You Whether Super Shoes Will Work?

The appeal of advanced footwear technology is easy to understand. Put on the shoe, spend less energy to run at the same pace, and… at least in theory… run faster.

The frustrating part is that the response varies. Some runners receive a large metabolic benefit, while others get little or none. A new study asked whether we can identify those runners before they buy the shoe.

Researchers recruited 64 runners—32 women and 32 men—ranging in age from 14 to 72 years. Each runner completed two six-minute treadmill trials in a carbon-plated racing shoe and two in a non-plated control shoe, all at 70% of the speed associated with their VO₂ peak. The researchers also measured lower-leg strength, Achilles tendon stiffness, hamstring stiffness, foot posture, arch flexibility, and several other anatomical and neuromuscular characteristics.

• On average, the advanced shoe improved running economy by 4.13%. The individual responses ranged from a 2.62% worsening to an 11.01% improvement, and 53 of the 64 runners improved by at least 2.5%.

That is an impressive average response. But the prediction part was much less impressive.

Runners who cleared the 2.5% threshold had greater hamstring stiffness. And calf-muscle stiffness and navicular drop—a measure of how much the arch of the foot lowers under load—were significant predictors of the shoe response.

But together, nearly four-fifths of the runner-to-runner response could not be explained by their individual characteristics or the researchers’ model.

What this means for runners

The reassuring finding is that most runners in this study did receive a meaningful running-economy benefit. The less satisfying finding is that a calf-stiffness test or arch measurement cannot yet tell you which shoe will work best for you.

So at this point, we should treat anatomy as an interesting research clue, not a shopping rule. If the decision matters, compare shoes under repeatable conditions at a pace relevant to your racing. A lab test is the cleanest option, but even repeated controlled workouts—paired with comfort, stability, and perceived effort—will tell you more than a single static foot measurement alone. The shoe that works for the average runner is not automatically the shoe that works for you.

Do Probiotics Improve Endurance Performance?

Probiotics are often sold to athletes through a chain of plausible promises, such as improved gut health, reduced inflammation, faster recovery, and, ultimately, better performance. Each link sounds reasonable. But do they actually work?

A new study enrolled 30 male distance runners who received either a five-strain probiotic providing 2 billion colony-forming units per day or a placebo. Training continued, and the researchers provided standardized meal boxes to reduce dietary noise.

• The probiotic did not improve time to exhaustion, Wingate performance (a test of maximal power output on a stationary bike), ventilation, respiratory exchange ratio (a measure of fuel utilization during exercise), lactate, creatine kinase (a marker of muscle breakdown), interleukin-6 (a marker of inflammation), or tumor necrosis factor-alpha (another marker of inflammation) compared with placebo.

• However, the probiotic did produce a small but significant 3.5 ml/kg/min difference in the runner’s peak oxygen uptake.

What this means for runners

Do not treat “probiotic” as a single performance-enhancing ingredient. Effects, when they exist, are likely to depend on the strain, dose, outcome, and athlete. This five-strain product did not make trained runners faster or blunt their exercise-induced inflammatory, lactate, or muscle-damage responses over four weeks.

A probiotic may still be useful for a specific gastrointestinal or clinical reason, but that is a different claim from improved endurance. If performance is the goal, the more reliable priorities remain adequate carbohydrates, sufficient total energy, sleep, and a solid training plan (boring… I know).

Can Believing You Took Caffeine Make You Run Faster?

Read more

Greetings, and welcome to the RLRH newsletter!

There is something slightly strange about the way we study the marathon.

In the laboratory, researchers can control almost everything: treadmill speed, temperature, nutrition, airflow, and when a blood sample is taken. That control is the great strength of laboratory science. But it also creates a problem. Marathon performance unfolds over several hours, under changing environmental conditions, after months or years of training, and in the presence of thousands of other runners. Reproducing all of that in a lab is… well… impossible.

Then there is the sample-size problem. You cannot ask 100,000 runners to complete multiple all-out marathons while randomly changing their training, pacing, altitude, heat exposure, or air pollution. It would be impractical, not to mention costly.

Luckily, runners perform this experiment for us every weekend!

A new review by Daniel Muniz-Pumares and colleagues argues that the marathon can and should be treated as a giant natural experiment. The researchers do not assign the exposure. Instead, they study what happens when large groups of runners encounter different environments, adopt different behaviors, or arrive with different training histories.

The data are messy and the “experiment” is uncontrolled, but this type of analysis can reveal patterns that a small, tightly controlled study would never be able to detect.

The conclusion is not that big data should replace the laboratory. It is that the two answer different parts of the same question: What actually determines marathon performance in the real world? That’s what today’s newsletter is about.

The physiological boundary that matters

To understand the marathon, it helps to begin with critical speed.

A marathon therefore needs to be run close to, but below, critical speed. The question is how close.

Traditionally, determining critical speed requires several maximal efforts lasting roughly two to 15 minutes. Researchers graph speed against duration and estimate a runner’s maximum sustainable speed (or critical speed) from that relationship. It is informative, but it is also demanding and time-consuming.

Wearable data offer another route. In more than 25,000 recreational runners, researchers estimated critical speed from each runner’s best training performances over distances from 400 to 5,000 meters. The resulting critical-speed estimate predicted marathon performance with an average error of about 7.7%.

Across the group, runners completed the marathon at roughly 85% of their predicted critical speed. But that average conceals the most interesting result:

• The fastest recreational runners sustained more than 90% of critical speed.

• The fraction declined progressively as finishing time increased.

• Elite runners in separate data completed the marathon at approximately 96% of critical speed.

Faster marathoners, in other words, do not merely have a higher physiological ceiling. They also spend the race closer to it.

That distinction points toward another quality that I’ve talked about a lot in this newsletter: durability.

The marathon exposes durability

Most physiological testing describes a runner while fresh. A marathon asks whether those same characteristics still exist after two, three, or four hours of running.

One way to observe this deterioration in the field is to compare internal workload with external workload. Heart rate is the internal cost; speed is the external output. If heart rate rises while speed stays the same—or speed falls despite a similar heart rate—the two measures have “decoupled.”

In an analysis of 82,303 recreational marathoners, runners with high decoupling had a heart-rate-to-speed ratio at the end of the race that was at least 30% higher than during the 5-to-10-kilometer segment. The gap between heart rate and speed widened as the race went on! They finished in an average of 3:58. Runners with low decoupling, defined as an increase of less than 10%, averaged 3:37.

Critical speed alone predicted finishing time with an error of about 6.5%. Adding decoupling improved the prediction, reducing the error to about 5.2%.

So here, among a large group of marathoners, a fresh measure of capacity did not tell the whole story. Knowing how the relationship between effort and output changed during the race improved the ability to predict a runner’s finishing time.

The course becomes part of the physiology

Read more

Greetings, and welcome to the RLRH newsletter!

Before we get to the research, Jonathan Gault’s conversation with Charles Hicks is worth your time. Hicks talks about building from roughly 110–115 miles per week before New York to a planned 130–135 before Chicago, with 150 as a long-term target. The thread running through the interview is that mileage, data, and fueling are tools, and I think the way Charles approaches training is truly unique and incredibly evidence-based.

The biomechanics of “hitting the wall”

The marathon wall is usually explained metabolically: glycogen runs low and pace falls apart. That story is real, but it may be incomplete. A new Journal of Applied Biomechanics study suggests the wall also has a recognizable mechanical signature.

Researchers analyzed 297 finishers across 58 marathons in Japan. Each runner wore a waist-mounted sensor that estimated pace, step length, step frequency, ground-contact time, vertical motion, and vertical stiffness for every kilometer. The runners separated into three pacing patterns:

• 119 maintained a relatively even pace.

• 103 “hit the wall” with a sharp slowdown after about 30 kilometers (18 miles).

• 75 began fading gradually from roughly 15 kilometers (9 miles) onward.

The even-paced group finished fastest. The wall group was next, and the early pace-down group was slowest. Both slowdown groups shortened their stride, spent longer on the ground, and lost vertical stiffness as the race progressed. But the timing differed. The pace-down group deteriorated gradually, while the wall group changed abruptly late in the race. Step frequency (cadence) also declined late in both slowdown groups while remaining stable among the even pacers.

What this means for runners

The wall is not simply a moment when motivation (or glycogen) disappears. It seems that by the time your pace collapses, the spring-like behavior of the legs and the ability to preserve step length may be deteriorating, too.

Do not interpret this as a cue to consciously “force” a longer stride late in a marathon. A better goal is to arrive at the ~20-mile mark with enough metabolic and muscular reserve that your normal stride remains available. Conservative early pacing, practiced carbohydrate intake, progressive long runs, and strength training are still the practical levers, even if this study cannot tell us which one prevents the wall.

What predicts running durability?

The ability to resist deterioration in running economy and other performance variables is increasingly called durability or physiological resilience. A new Scandinavian Journal of Medicine & Science in Sports study asked what predicts it among recreational and competitive runners.

Thirty-three recreational-to-competitive runners completed a long treadmill run near their first ventilatory threshold (the point where breathing rate starts to spike), surrounded by tests of running economy, VO₂ max, peak speed, ventilatory thresholds, strength, jumping, fat oxidation, and maximal lactate production. The investigators then calculated how much each performance variable deteriorated.

Resilience depended on the outcome.

• Runners with higher VO₂ max and faster threshold or peak speeds tended to preserve their first ventilatory threshold and peak speed better.

• Greater leg-press strength was associated with a smaller decline in running economy.

• Higher maximal lactate production was associated with a larger loss of peak speed.

Competitive runners preserved their first ventilatory threshold better than recreational runners, yet recreational runners appeared to preserve running economy better. That last result needs care, though, because it seems that runners with worse economy at baseline had less deterioration, which may partly reflect less room to worsen.

What this means for runners

There may be no single “durability score.” Aerobic fitness, threshold speed, muscle strength, and carbohydrate-burning characteristics may protect different parts of your physiology.

For training, the safest inference is to build your aerobic ceiling, raise the speeds you can sustain below it (your threshold pace), and maintain strength. Those were the key variables here associated with better durability. Then occasionally evaluate those qualities after a long but controlled run, not only when fresh. This was a small correlational study, so it identifies candidates for durability training; it does not prove that improving any one variable will make you more durable.

Why sprint speed may be important for female distance runners

VO₂ max gets most of the attention, but runners with similar aerobic ceilings can perform very differently. A small study in trained female runners is a good reminder of what may separate athletes inside a relatively similar-looking group.

Researchers evaluated 13 female runners who averaged 10:07.8 for their season-best 3,000-meter performance.

• Faster 3,000-meter times were associated with faster 100-meter sprint times, better running economy, a faster speed at the onset of blood-lactate accumulation, and a higher estimated maximal rate of lactate accumulation, or vLamax.

• VO₂ peak was not meaningfully associated with performance.

In simple terms, the faster women were more economical, reached the lactate marker at a higher speed, sprinted faster, and—somewhat surprisingly—showed a higher accumulation of lactate. And these were all better predictors of how the women would perform than their maximal aerobic capacity!

What this means for runners

This is a friendly reminder that VO₂ max is one part of performance—not a complete ranking system.

Once aerobic fitness is reasonably high, performance may depend more on how economically and quickly you can use it. Strides, short hill sprints, and carefully dosed speed work can improve coordination and preserve your “top end,” but that doesn’t mean you need to turn your endurance program into sprint training.

Runners’ hearts don’t have to work as hard

Read more

Runners are used to thinking about fuel in terms of totals.

How many calories did I eat? How many did I burn? Did I replace what the long run took out of me?

And of course, getting enough energy (calories) is important.

But calorie balance may not tell the whole story.

When runners eat less than their training demands, they often consume less carbohydrate at the same time. Sometimes that is intentional. More often it happens quietly as mileage rises, appetite or meal planning fails to keep pace, and a diet that looked adequate during an easy week becomes inadequate during a high-volume one.

That creates two shortages at once. There is an energy shortage, and there may also be a carbohydrate shortage.

A new study asks whether the body can tell the difference.

Energy availability is the energy left for the rest of the body after the energetic cost of exercise has been subtracted:

Energy availability = (energy intake − exercise energy expenditure) ÷ fat-free mass

Imagine a runner with 60 kilograms of fat-free mass who eats 2,500 calories and expends 1,000 calories during training. Their energy availability is 25 calories per kilogram of fat-free mass per day—a level low enough to produce measurable physiological changes within days.

The important point is that low energy availability does not necessarily mean someone is eating very little. A runner can consume what looks like a substantial amount of food and still end the day with too little energy available because the training bill is so large.

Carbohydrate availability is a separate calculation:

Carbohydrate availability = carbohydrate intake − carbohydrate used (oxidized) during exercise

Two runners can therefore finish a day with the same energy availability but very different carbohydrate availability. One may replace most of the carbohydrate used during a long workout. The other may consume the same number of calories but obtain more of them from fat. Their energy budgets match, but their carbohydrate budgets do not!

Carbohydrates support moderate- and high-intensity exercise, but their role is not limited to just fuel. Carbohydrate availability also interacts with endocrine signals involved in growth, tissue repair, appetite regulation, bone health, and metabolism.

So when several hormones fall during low energy availability, how much of that response comes from the calorie deficit itself—and how much comes from low carbohydrate layered on top of it?

Separating calories from carbohydrates

The new randomized crossover study recruited 16 young, recreationally active adults: nine women and seven men.

Each participant completed two four-day periods of low energy availability. In both conditions, energy availability was held at about 25 calories per kilogram of fat-free mass per day. Participants ate roughly 2,600 calories but also completed enough stationary cycling at 60% of peak oxygen uptake to expend around 1,150 calories—or approximately two hours of exercise—each day.

The calorie shortage was essentially the same in both trials. The carbohydrate remaining after exercise was not.

• In the lower-carbohydrate-availability condition, participants consumed 4.2 grams of carbohydrate per kilogram of body mass per day. About 1.4 grams per kilogram per day remained after exercise.

• In the higher-carbohydrate-availability condition, participants consumed 6.7 grams per kilogram per day. About 3.7 grams per kilogram per day remained after exercise.

Notice that 4.2 grams per kilogram per day does not sound like an especially low-carbohydrate diet. It became low relative to the workload. Nearly two hours of cycling every day changed the meaning of the number.

Energy and protein intake were closely matched. To raise carbohydrate without raising total calories, the researchers reduced fat: carbohydrate supplied 46% of energy in the lower-carbohydrate condition and 73% in the higher-carbohydrate condition, while fat supplied 37% and 11%, respectively.

Before and after each trial, the researchers measured body composition, resting metabolism, fasting hormones, fuel use, blood lactate, peak oxygen uptake, and maximal cycling power.

• Both conditions produced nearly identical body-mass losses of about 1 kilogram, or 2.2 pounds, across four days. Resting metabolic rate did not change.

• The most convincing difference appeared in insulin-like growth factor 1, or IGF-1, a hormone involved in growth and tissue repair. IGF-1 fell by 20.2% when carbohydrate availability was lower, compared with 9.4% when it was higher.

In other words, preserving more carbohydrate cut the decline roughly in half. It did not prevent it. IGF-1 still fell despite the higher-carbohydrate diet because the participants were still under-fueled.

Several other hormones appeared to tell a similar story at first glance.

• Insulin and testosterone declined significantly during the lower-carbohydrate condition but not during the higher-carbohydrate condition

• Leptin (a hormone involved in appetite regulation) appeared to fall less in women when more carbohydrate was available.

A “better” lactate curve that probably wasn’t better

Read more

Could Dehydration Be Undermining Your Fuel Strategy?

Endurance fueling advice usually treats carbohydrate and fluid as separate targets: hit a certain number of grams per hour, then drink enough to manage thirst and heat. A small new experiment suggests the two may be more connected than that.

Nine trained male cyclists and triathletes completed two 160-minute rides at 50% of their peak power in temperate conditions, about 73°F (23°C). In both trials, they consumed 60 grams of glucose per hour. In the high-fluid trial, they also received enough water to match sweat losses closely; in the low-fluid trial, they received only 15 milliliters of water after each carbohydrate feeding.

The low-fluid condition did exactly what it was intended to do. By the end of the ride, participants had lost 2.8% of their body mass, compared with just 0.2% in the high-fluid condition. Plasma volume fell, and heart rate and thirst climbed. Core temperature and gastrointestinal discomfort, however, were not different between trials.

Dehydration reduced how much of the ingested glucose the athletes used (oxidized).

• From 40 to 160 minutes, average exogenous glucose oxidation was 0.51 grams per minute in the low-fluid trial versus 0.61 grams per minute with more fluid, a difference of roughly 16%.

• Peak glucose oxidation was also about 12% lower, 0.70 versus 0.79 grams per minute.

• The largest gap appeared at 60 minutes, when oxidation was 0.33 versus 0.49 grams per minute, even though body-mass loss at that point was only about 1%.

What this means for runners

If you are taking gels or a concentrated drink during a long run or race, the fluid consumed alongside those carbohydrates may affect how well they move through the gut and become available to working muscle. The practical target is not to replace every drop of sweat. It is to test a combined carbohydrate-and-fluid plan that limits excessive dehydration without creating sloshing or gastrointestinal distress.

This study does not establish a universal fluid dose, and it does not prove that a 16% reduction in exogenous glucose oxidation will slow your race. Treat it as a reason to rehearse fueling and hydration together—especially for long events, if you have a high sweat rate, or in a race where carrying or accessing fluid is difficult.

How Heat Risk Categories Affect the Body

Wet-bulb globe temperature, or WBGT, compresses heat, humidity, wind, and radiant heat into a single number. Race organizers and athletes use it to sort conditions into risk bands. The question is whether runners’ physiological responses separate as neatly as these risk bands assume.

A new systematic review and meta-analysis analyzed 43 studies, involving trained adults performing self-paced runs ranging from 5K to the marathon. A total of 58% of the studies were conducted indoors.

The researchers grouped conditions using American College of Sports Medicine (ACSM) thresholds: low risk at 72.0°F WBGT or below, moderate from 72.1°F to 78.1°F, high from 78.3°F to 82.0°F, and very high at 82.2°F or above (in Celsius, that’s 22.2°C, 22.3°C to 25.6°C, 25.7°C to 27.8°C, and 27.9°C or above for low, moderate, high, and very high risk, respectively).

• Runners’ core temperatures at the end of exercise rose from 102.61°F (39.23°C) in low-risk conditions to 102.79°F (39.33°C) in moderate risk and 103.10°F (39.50°C) in high risk, then dipped to 102.85°F (39.36°C) in very-high-risk conditions.

• Heart rate showed a similar separation: 181 beats per minute in both low and moderate risk and 185 beats per minute in both high and very high risk.

• Body-mass loss averaged 1.4% to 1.8% across categories, with no significant differences between the temperature conditions.

• Running speed declined as heat risk increased, even though there wasn’t much of a statistical difference when comparing conditions.

What this means for runners

A higher WBGT should probably push you toward a more conservative pace, more attention to access to fluids and cooling, and a lower threshold for adjusting the day’s goal. But the same number can create very different strain depending on acclimatization, fitness, health, clothing, sun exposure, air movement, race duration, and how hard you choose to run.

The overlap (or lack of a “statistical difference) between risk bands is not permission to ignore a high-risk forecast. If anything, it is a reminder that a population-level cutoff cannot tell you exactly what your core temperature or performance will be on the day.

Is Even Pacing the Best Way to Race a Marathon?

The first miles of a marathon often feel suspiciously easy. That is precisely what makes them dangerous: the pace that feels relaxed at mile three can become very… “unrelaxed” at mile 23.

A large observational study analyzed 78,912 finishers from the 2015 through 2017 Boston Marathons. The runners were 18 to 84 years old, and 45.5% were women. Using split times from nine checkpoints, the researchers grouped pacing profiles into four patterns: even pacing, mild positive splitting, strong positive splitting, and variable pacing.

Nearly half of the runners, 47.9%, landed in the even-pacing cluster. Another 31.9% showed a mild positive split, 12.2% a strong positive split, and 8.0% a highly variable pattern. “Even” did not mean metronomic: runners in that cluster generally stayed within about 5% of their average pace and still had a modestly faster first half.

• Even pacing was associated with the fastest average finish time, 216.9 minutes, compared with roughly 276 minutes in the strong-positive-split group.

• Variable pacers finished about 39.7 minutes slower than even pacers on average.

• The pattern also appeared within performance bands: even pacing was associated with a 4.6-minute advantage over strong positive splitting among sub-three-hour runners and a roughly 13-minute advantage among recreational and slower runners.

The most dramatic result involved “hitting the wall,” defined here as running the final segment more than 20% slower than the average pace through 22 miles (35 kilometers). Overall, 15.2% of runners met that threshold. It occurred in only 0.2% of the even-pacing group but 85.4% of the variable-pacing group! Runners whose opening 10K was more than 5% faster than their eventual average pace hit the wall 26.6% of the time, compared with 1.5% among those who opened near their average.

What this means for runners

Base your opening pace on training, tune-up races, course demands, and conditions—not on how comfortable the first 10K feels. On a hilly course, aim for even effort rather than mechanically identical splits.

My advice is to practice that restraint during long runs. A pacing plan becomes much easier to follow when you have repeatedly felt the difference between sustainable marathon effort and the slightly-too-fast pace that only reveals its cost late in the race.

Does Mental Health Influence Late-Race Slowing?

Read more

Believe it or not, one of the most frequently asked questions I get about running isn’t even about running. It’s about how I use cycling to run better.

Indeed, cycling has become a huge part of my overall training in recent years, partly out of necessity and partly out of utility. I wanted to find a way to train more without putting myself at risk of inj…

Read more

Just tasting carbs makes hard efforts feel easier

Carbohydrates are usually framed as fuel. You eat or drink them, they get absorbed, they help maintain blood glucose and carbohydrate availability, and you perform better.

But carbohydrate can also act before it becomes fuel.

A recent randomized trial tested carbohydrate mouth rinsing against music listening and a placebo mouth rinse during a hard 4-kilometer cycling time trial.

Eleven trained cyclists completed all three conditions. In the carbohydrate condition, they rinsed with a 6.4% maltodextrin solution—five 25 mL rinses, each swished for 10 seconds and spat out. The researchers measured prefrontal cortex oxygenation (essentially how much oxygen is in this particular brain region), executive function, perceived exertion every 500 meters, time-trial performance, heart rate, and blood lactate.

• The carbohydrate rinse increased oxygenation in both sides of the prefrontal cortex after the intervention and after the time trial compared with music and placebo.

• Executive function was also better after the time trial in the carbohydrate condition.

• Perceived exertion was lower with carbohydrate mouth rinsing than placebo at every distance interval, and lower than music through the middle of the trial.

• Performance improved modestly: completion time averaged 336.64 seconds with carbohydrate mouth rinsing versus 339.36 seconds with placebo and 339.43 seconds with music.

• Heart rate and lactate did not differ between conditions.

What this means for runners

For short, hard efforts—5Ks, final surges, hill reps, hard track sessions, or moments when your stomach does not want more fuel—carbohydrate mouth rinsing is a low-risk tool worth testing. It should not replace actual carbohydrate intake in longer races where fueling matters metabolically. But as a perceptual tool, especially late in a hard effort, swishing a carb drink and spitting or swallowing depending on the context may help reduce effort just enough to hold pace. Treat it like you would any race strategy: practice it before race day.

Can cycling replace some of your running?

Every runner eventually has to negotiate with impact.

That is where cycling becomes tempting. You can work hard, accumulate a lot of aerobic volume, and wake up the next day without the same “musculoskeletal hangover.” The question is whether that fitness actually transfers back to running.

A new systematic review and meta-analysis looked at exactly this question: does training on the bike improve or maintain running-related fitness and performance compared with running itself?

The researchers included seven randomized controlled trials lasting at least four weeks, comparing running-only, cycling-only, or combined run-bike training in recreationally active, trained, or competitive participants. Outcomes included VO2 max measured on a treadmill, VO2 max measured on a bike, and running performance over 1 mile, 3,000 meters, or 5,000 meters.

• There were no statistically significant differences between running-only and cycling-only training for treadmill VO2 max, cycling VO2 max, or running performance.

• Run-only training favored treadmill VO2 max.

• Cycling-only training favored cycling VO2 max.

• For running performance, the effect between training modalities was essentially zero.

That sounds like a win for cross-training. But the nuance matters.

The review does not prove cycling is interchangeable with running. The authors are careful to note that the evidence base was small, many protocols were old, and several studies had some concerns regarding their methods. The more accurate interpretation is that, within the available evidence, researchers did not detect clear differences over short-to-moderate training periods between running and cycling.

What this means for runners

I’d treat cycling as one of the best tools runners have for increasing aerobic work while controlling orthopedic load, especially when the alternative is forcing more easy mileage into already tired legs. But I would not use it as a full substitute for the key running-specific pieces that make you race well, including long runs, race-pace work, strides, hills, and enough weekly running to keep your tissues prepared for impact. The practical move is to swap cycling for some easy or recovery volume, not for every run that teaches your body how to run fast, long, and efficiently.

Can jumping protect your bones when you’re underfueling?

Runners love to think about trade-offs. If we are not eating enough, maybe strength training protects us. If we are in a deficit, maybe impact loading keeps bones stimulated.

A new study tested a version of that logic, and the result is a useful warning for all runners.

Researchers studied 12 recreational female runners in a randomized crossover design. Each runner completed two five-day periods of controlled low energy availability, set at 15 calories/kg fat-free mass per day. That is low. The deficit came from both dietary restriction and daily treadmill running at 65–70% VO2 max. In one condition, participants only ran. In the other, they also completed 50 supervised depth jumps per day, separated from the run by at least four hours.

The idea was reasonable. Bone responds to mechanical loading, and previous work suggests that brief high-impact jumping might blunt some of the negative bone-marker changes seen during short-term underfueling. But here, when the jumping was layered on top of daily running and controlled low energy availability, it did not rescue bone turnover; in fact, it pushed bone metabolism in a direction consistent with bone breakdown.

• The bone resorption marker beta-CTX increased over time in both conditions.

• Bone formation markers, including P1NP and sclerostin, did not significantly change over time or differ between conditions.

The study was small and short, and it measured bone markers rather than long-term changes in bone mineral density or stress fracture risk. It also studied young female recreational runners without current low energy availability risk, hormonal contraceptive use, recent bone injury, or low bone density. So we should not overgeneralize. But the direction of the finding is hard to ignore.

What this means for runners

If you are underfueling, your bones may start showing unfavorable metabolic signals within days, not months. And no amount of jumping or plyometrics will help. The more practical approach is to treat energy availability as the foundation: eat enough total calories, enough carbohydrates around training, adequate calcium and vitamin D, and include recovery days that are actually recovery days. Jump training can be useful for bone and tendon health, but it works best when the body has the resources to adapt to it.

Can vitamin D status inform us about athlete cardiovascular health?

Read more

In the final day or two before a marathon, we’re all given the same advice. Eat more carbohydrates, reduce training, top off glycogen, and arrive at the start line with the tank full.

It is one of the most familiar rituals in endurance sport, and the basic physiology behind this idea is solid. Muscle glycogen (think of it as stored carbohydrates) is a critical fuel source during prolonged moderate- to high-intensity exercise. When glycogen availability falls, the ability to sustain pace or power can suffer.

But how much of a performance boost does “carb-loading” really provide?

A new commentary in the International Journal of Sport Nutrition and Exercise Metabolism asked whether traditional carbohydrate loading provides an additional performance benefit once athletes are already fueling before and during the race in ways that reflect current practice.

The paper does not argue that glycogen is unimportant, but it does challenge the certainty around one specific idea—that pushing muscle glycogen above normal through aggressive 24–48 hour carbohydrate loading reliably improves performance beyond what is achieved by adequate pre-race fueling and optimized carbohydrate intake during the event.

The authors reviewed the carbohydrate-loading literature with two methodological questions in mind.

First, were the studies double-blind and placebo-controlled? In other words, did participants and researchers actually not know whether the athlete was receiving the carbohydrate-loading intervention or the comparison condition? (This is important for reasons we’ll discuss later).

Second, did the studies provide carbohydrates during exercise in a way that resembles real-world endurance competition?

Both questions are important because performance is physiology filtered through perception, expectation, motivation, pacing, and belief. If an athlete knows they are carbohydrate-loaded, that knowledge may influence how hard they are willing to push, how they interpret fatigue, or how confident they feel late in a test. Likewise, if an athlete is asked to perform prolonged exercise without carbohydrate intake, the test may exaggerate the benefit of pre-exercise glycogen stores compared with a real race where carbohydrate is consumed throughout.

To evaluate this, the authors identified carbohydrate-loading studies that compared a moderate or “normal” carbohydrate intake (roughly 4–6 grams per kilogram per day) with a higher carbohydrate intake (above 7 grams per kilogram per day for at least 24 hours). The studies also had to include an endurance performance or work-capacity test lasting at least 90 minutes.

From an initial search of more than 10,000 studies, they narrowed the field to (just) 14 eligible studies.

That alone tells us something—carbohydrate loading is one of the most deeply embedded practices in endurance sport, yet the performance evidence base that directly tests it under relevant conditions is not especially large.

• Of the 14 studies, only two used double-blind, placebo-controlled designs.

• Only four provided carbohydrates during the performance test.

• The two studies that did both—double-blinding plus in-exercise carbohydrate provision—did not show a clear performance benefit from carbohydrate loading.

Those two studies were small, with only 7 and 9 well-trained athletes, and only produced relatively modest increases in muscle glycogen: 18% in one study and 25% in the other. Classic carbohydrate-loading protocols can sometimes increase glycogen by 30–90%, so it is possible that these trials did not even create a large enough separation between “normal” and “loaded” conditions to fully test the question.

The largest benefits in the literature tended to come from studies that were not double-blind and did not provide carbohydrates during exercise. These studies often used time-to-exhaustion tests, where athletes continue at a fixed pace (or work output) until they can no longer maintain it. Time-to-exhaustion tests can be useful for investigating fatigue, but they do not fully mimic racing, where athletes pace themselves, respond to discomfort, and make constant decisions about effort.

This is where the interpretation gets a bit tricky. If carbohydrate loading improves your time to exhaustion in a fasted or underfueled test, that may demonstrate that glycogen availability matters; but it does not necessarily prove that aggressive carbohydrate loading adds a meaningful benefit for you arriving at the start line fed and consuming carbs during the race!

Even among the studies that did provide carbohydrate during exercise, the doses were typically around 60–70 grams per hour. That was consistent with older guidelines and still represents solid fueling, but it is below the 90–120 grams per hour now being discussed and used by some highly trained endurance athletes. So these studies still don’t fully reflect the highest end of modern fueling practice.

The placebo problem

Read more

As we get older, fitness drops.

Some of that is biology. But some of it might be the result of a lifetime of underestimating what the body can maintain when it keeps moving.

A new mini-review in the Journal of Applied Physiology is a scientific time capsule. Dr. Douglas Seals and colleagues look back at a research program that started at the University of Colorado Boulder in the early 1990s, when women were still wildly underrepresented in exercise physiology and cardiovascular aging research (author’s note: they still are).

Their approach was simple: study female masters endurance athletes, mostly runners, and compare them with healthy but non-exercising women of similar age, as well as younger women. If we want to know what aging does to the cardiovascular system, we should not only study people who stopped training decades ago, but also study women who kept running.

That’s what the team did, and the results are a pretty strong argument that the aging body is far more adaptable than old assumptions suggested. And I think these findings have implications for all runners, no matter your age or gender.

The historical context here is important.

For a long time, male physiology was treated as the default in biomedical research. Women were often excluded because menstrual cycles, pregnancy, hormonal contraceptives, and menopause were viewed as “complicating factors.” This is scientifically lazy and leaves major gaps in our understanding of women’s health.

Exercise science had (has?) the same problem. Women were underrepresented, and older women were even less studied. Meanwhile, there were persistent cultural and medical myths that strenuous exercise was somehow inappropriate or risky for women.

This review is partly about that history. But it’s also about what the researchers found when they actually started measuring. That’s what we’ll talk about today.

That is incredibly relevant for runners. Most of us are trying to figure out how to keep doing this for decades and what the result of that long-term training might be.

The basic idea of the masters athlete model is to compare older endurance athletes with older nonexercising adults, and sometimes with younger athletes and younger nonexercising adults too. This lets researchers ask two related questions.

• First, do older endurance athletes have better cardiovascular function than older nonexercising adults?

• Second, does endurance training appear to reduce the typical age-related decline in those functions?

The advantage is obvious. Masters athletes represent a real-world model of long-term training. They’ve accumulated years of aerobic work, not just a few months in a lab study.

The limitation is also obvious. These athletes are not randomly assigned to become lifelong athletes. They may differ from nonexercising peers in genetics, diet, body composition, sleep, social support, motivation, injury history, and a dozen other factors. So we should not interpret every difference as purely caused by running.

Still, as an initial probe into what long-term endurance exercise might preserve, this model is incredibly useful. And in women, it was especially useful because so little work had been done.

Aerobic fitness is the biggest and clearest difference

In one early CU Boulder study, researchers compared 13 highly trained middle-aged and older female distance runners with 17 age-matched healthy nonexercising women. The runners were around 55 years old, had been training for about 18 years, ran roughly 31 miles per week on average, and performed at least an hour of high-intensity exercise daily.

Their VO2 max was 83% higher than that of the nonexercising women: about 48.6 versus 26.5 mL/kg/min.

The runners also had about 40% greater total blood volume relative to body weight, including greater plasma and red blood cell volume. That matters because blood volume is a major contributor to endurance performance. More blood volume can support higher stroke volume, oxygen delivery, thermoregulation, and overall cardiovascular capacity.

But VO2 max still declined…. in everyone.

Female masters runners had much higher VO2 max than nonexercising women at any given age. But they still experienced declines with aging.

In fact, when researchers looked at the absolute decline in VO2 max, endurance-trained women sometimes showed a larger drop per decade than sedentary women. That sounds bad until you remember that baseline status matters. If you start higher, you often have more room to fall.

When the decline was expressed as a percentage, trained and untrained women lost VO2 max at a similar relative rate, roughly around 9–11% per decade.

Running does not make you immune to aging. Even highly trained women lose aerobic capacity over time.

The encouraging part is that, because trained women start from a much higher level, they remain far fitter than their inactive peers for decades. A runner can lose fitness with age and still maintain a level of aerobic capacity that is dramatically higher than someone who never built the base in the first place. You can see that clearly from the figure below.

One longitudinal study followed eight female masters runners and sixteen nonexercising women for about seven years.

VO2 max declined in both groups, and the absolute decline was about twice as large in the runners. But among runners who maintained their training volume, the decline was much smaller than in runners who reduced training.

That is the runner’s version of “use it or lose it.” Or maybe more accurately: build it, use it, and keep using it.

Body composition and metabolism declines aren’t inevitable

In nonexercising women, aging and menopause are commonly associated with increases in body mass, total body fat, abdominal fat, and reductions in fat-free mass.

The masters runners showed a very different pattern.

Across several studies, endurance-trained women gained far less body mass and fat mass with age and menopause than nonexercising women. In one four-group comparison, the postmenopausal-to-premenopausal differences in body mass, BMI, body fat percentage, fat mass, waist circumference, and fat-free mass were about 40–80% smaller in runners than in nonexercising women.

That doesn’t mean female runners were completely protected from body composition changes. Some increase in adiposity still occurred with age and menopause. But the magnitude was much smaller.

The researchers also looked at resting metabolic rate.

• In nonexercising women, resting metabolic rate was about 10% lower in postmenopausal women compared with premenopausal women.

• In endurance athletes, that age-related drop was not observed.

The runners appeared to maintain resting energy expenditure better, possibly because they preserved more fat-free mass and had a higher overall “energy flux” due to training and diet.

Blood pressure and arteries

Read more

Read more

Does a marathon temporarily stress the heart?

Marathon running is good for you.

Marathon running is also a massive physiological stressor.

Both things can be true.

A new systematic review and meta-analysis looked at the acute (short-term) effects of marathon running on the heart. The researchers included studies of healthy adults who completed a road marathon and had cardiac measurements before and after the race.

In total, 69 studies were included in the systematic review, covering 3,274 participants, mostly men, generally between 30 and 55 years old. Forty-nine studies were included in the meta-analysis. The researchers looked at cardiac biomarkers, echocardiography, and cardiac MRI.

Marathon running consistently increased circulating markers associated with cardiac stress or injury.

• Troponin T increased by about 42.4 ng/L.

• Troponin I rose by about 74.5 ng/L.

• NT-proBNP increased by about 113.8 ng/L.

Troponins are used clinically when evaluating possible heart injury. But context matters. In endurance athletes, post-race biomarker elevations are common, often transient, and do not automatically mean pathological damage.

The bigger pattern was a temporary shift in how the heart relaxed, filled, and pumped after 26.2 miles. Measures of left ventricular diastolic function—basically how well the heart relaxes and fills between beats—decreased. Early filling velocity dropped by 12.7 cm/s, while late filling velocity increased by 13.1 cm/s, meaning the heart appeared to rely more on the final “push” from the atria to fill the ventricle. The E/A ratio, a common marker of this filling pattern, fell by about 33 percent.

The right side of the heart also showed signs of strain. The right ventricle, which pumps blood to the lungs, had larger end-diastolic and end-systolic volumes after the marathon. In simple terms, it was holding more blood before and after contraction. Right ventricular ejection fraction dropped by about 3.5 percentage points, and TAPSE—a measure of how strongly the right ventricle contracts—declined slightly. Together, these changes suggest that after a marathon, the heart may pump a bit less efficiently for a short period, especially on the right side, while also showing signs of altered filling on the left side.

The authors emphasize that the magnitude of these changes was generally modest and often remained within normal, physiological ranges. Cardiac MRI, which is better for structural assessment, showed a small increase in left ventricular ejection fraction and no clear signs of clinically meaningful structural heart muscle injury in the available data.

The marathon appears to produce acute cardiac stress, but the meaning of that stress depends on the runner. Responses varied by age, sex, training status, and finish time. NT-proBNP increased more in studies with longer marathon finish times, older participants, and a higher proportion of women. Troponin I tended to be higher in less-trained cohorts.

That does not mean slower runners are damaging their hearts. But it does suggest that preparation matters. A marathon is not just 26.2 miles on race day. It is the cardiovascular load of the race layered on top of your training history, age, health status, heat exposure, pacing, hydration, and recovery.

What this means for runners

This study should not scare healthy runners away from marathons. But it should make us respect the distance. A marathon is an acute cardiac stress test, especially for underprepared runners, older athletes, and anyone with symptoms or known cardiovascular risk. Train progressively, do not race marathons off chaotic preparation, and take post-race recovery seriously. And of course, consult a healthcare professional if you’re ever unsure about your personal risk.

Why do some elite runners last while others burn out?

Every runner knows someone who got really fit, really fast—and then disappeared.

Sometimes it’s injury. Sometimes it’s burnout. Sometimes it’s the toll of the psychological grind. In elite running, where the incentives are enormous and the margins are tiny, this pattern is probably more common than we’d like to think.

A new study tried to unpack this problem in one of the most fascinating running environments in the world: Ethiopia. The authors call it the “Ethiopian Paradox.” Ethiopia produces some of the greatest distance runners in history, yet within that same system, many talented athletes burn out, get injured, or leave the sport early.

The study included 60 retired elite Ethiopian endurance runners, split evenly by sex. All had competed at national championship or international level in events from 5000 meters to the marathon. The researchers compared two groups: a “Durable & Thriving” cohort with elite careers lasting 10 or more years, and a “Burnout & Injury” cohort whose careers ended within seven years because of injury, overtraining syndrome, or burnout.

• The durable runners averaged 14.2 years at the elite level and retired around age 36.8.

• The burnout/injury group averaged 5.4 years and retired around age 27.9. That alone is a huge difference. The question was why.

The researchers pulled historical training logs, calculated training load volatility using weekly acute: chronic workload ratios, administered psychological surveys, and conducted semi-structured interviews.

The durable group had significantly lower training load volatility—their training was steadier. The burnout/injury group had more frequent spikes in workload, often pushing acute: chronic workload ratios above 1.5, a zone commonly discussed as higher risk for injury when load jumps too quickly.

Durable athletes scored higher on coping with adversity, 10.2 versus 8.7, and much lower on concern over mistakes, 22.1 versus 28.5. Personal standards were similar between groups, which matters. The durable athletes were not less ambitious; they just seemed less likely to turn every setback into a personal catastrophe.

Three themes separated the durable runners from those who broke down:

• Guided autonomy in coaching (durable runners described coaches who adjusted training based on how the athlete felt. Burned-out runners described more rigid, top-down systems).

• An identity outside of running (durable runners had family, work, community, or other roles that gave them some psychological distance from performance. Burned-out runners often described running as their entire identity).

• A strong community buffer.

I like this study because it pushes the durability conversation beyond physiology. VO2 max, running economy, and lactate threshold matter. But so do training stability, coach-athlete communication, recovery, identity, and whether your training environment gives you permission to adapt. That’s especially true for those of us who aren’t professional athletes.

What this means for runners

Durability often comes from hard training that is absorbed consistently. Watch the spikes: sudden mileage jumps, stacking workouts when life stress is high, racing too often, or trying to “make up” missed sessions. Also pay attention to the psychological side. If every bad workout feels like evidence that you are failing, that is a risk factor. The more practical approach is to build a system where training can flex, recovery is respected, and running is important without becoming your entire identity. A lot of these things come with experience in the sport (speaking personally).

Can a mitochondrial supplement make you faster?

CoQ10 is an enzyme that sits inside the mitochondria, helps shuttle electrons, and has antioxidant properties. Ubiquinol (the reduced form of CoQ10) is more bioavailable than another form called ubiquinone. On paper, that makes it an appealing endurance supplement.

A new randomized study tested whether six weeks of ubiquinol supplementation could improve mitochondrial function and severe-intensity exercise capacity. Fifty-four healthy, recreationally active men were randomized to either 300 mg per day of ubiquinol or placebo for six weeks. The dose was split into three 100 mg servings with meals.

Before and after supplementation, participants completed severe-intensity cycling tests to exhaustion. A smaller group also had muscle biopsies to measure mitochondrial function, mitochondrial proteins, mitochondrial enzyme activity, and markers related to mitochondrial “leak” (think of it as energy inefficiency).

• The supplement clearly did what it was supposed to do in the blood. Total plasma CoQ10 levels increased in the ubiquinol group, while placebo stayed essentially unchanged.

• Ubiquinol improved oxidative phosphorylation efficiency—more efficient linking of oxygen use to ATP (energy) production.

• Despite the improvement in mitochondrial coupling efficiency, ubiquinol did not improve time to exhaustion during severe-intensity cycling. It also did not improve oxygen uptake at the end of exercise, mitochondrial enzyme activity, or mitochondrial proteins.

This is a classic example of why supplement studies need performance outcomes. A supplement can shift a biological marker and still not make you faster, more durable, or better recovered.

What this means for runners

I would not treat ubiquinol or CoQ10 as a reliable endurance performance supplement based on this study. It may improve some markers of mitochondrial efficiency, but in healthy recreationally active men, that did not translate into better exercise capacity. If you are deciding where to spend money for performance, training consistency, carbohydrate availability, sleep, strength training, and race-specific workouts are still much higher on the list.

Thanks to ProBio Nutrition for sponsoring this post.

ProBio Nutrition is the daily all-in-one that’s been developed by Dr. Marc Bubbs, a nutrition expert and consultant to top athletes, and is formulated with a focus on performance, longevity, and recovery. I’ve been taking ProBio ProBlend for months, and my main takeaways are that this covers all your bases, like a multivitamin for people who train regularly, is a better value than an old supplement stack, and it actually truly tastes great.

RLRH has partnered with ProBio Nutrition because we believe in the benefits, and our readers can get a flat 40% off using this link (if you subscribe, you’ll get an extra 10% off plus free shipping).

Ice slurries as a recovery tool?

Read more

Read more

Elite marathoners, pro cyclists, triathletes, and ultrarunners are pushing carbohydrate intake higher than what most sports nutrition guidelines have traditionally recommended. The old 30 to 60 grams per hour advice gave way to 90 grams per hour. And now the cutting edge of endurance fueling has moved even further to 100, 110, 120 grams per hour… sometimes more (though I find some claims by athletes to be somewhat dubious).

This has become one of the most interesting nutrition shifts in endurance sport. I’ve felt it personally, too. A few years ago, I would have considered even 40 grams per hour a solid fueling plan for a long run or marathon workout. Now, for long runs and race-specific sessions, I’m often thinking in the 70 to 90 gram range, and I know many runners who are experimenting above that.

But are athletes actually performing better because they are taking in ultra-high amounts of carbohydrate, or have we started confusing “more advanced” with “more”?

That’s the question asked in a new commentary article, titled “Fuelled or Fooled? Examining the Evidence and Mechanisms Behind Ultra-High Carbohydrate Intake in Endurance Athletes.” It takes a critical look at this new carbohydrate revolution.

And the authors’ argument is not anti-carb. Far from it. The case for carbohydrates during prolonged endurance exercise is strong. The question is whether pushing beyond 90 grams per hour is broadly supported by the evidence.

Their answer: not yet.

The authors define ultra-high carbohydrate intake as anything above 90 grams per hour during endurance exercise. That’s important because 90 grams per hour has long been treated as something like the upper evidence-based range, especially when using multiple transportable carbohydrates like glucose and fructose.

Where things get murkier is the jump from “carbs help performance” to “more carbs always help.”

The authors point out that ultra-high carb fueling has gained momentum largely because of elite athlete practice, anecdote, and impressive race performances. We see athletes breaking records while reportedly consuming huge amounts of carbohydrate, and it is tempting to connect the dots: more carbs must be one of the secrets.

But controlled studies have not consistently shown that taking in more than 90 grams per hour produces additional performance benefits. In fact, much of the research suggests that carbohydrate ingestion improves performance compared with little or no carbohydrate, but the dose-response curve flattens out somewhere below the ultra-high range.

More carbs do not seem to spare more muscle glycogen

One of the classic arguments for fueling during endurance exercise is that taking in carbohydrate “spares” your stored carbohydrate, especially muscle glycogen. Since glycogen depletion is one of the major contributors to fatigue during long races, the idea makes intuitive sense: eat more carbs, burn more external fuel, preserve more internal fuel, run stronger late. But the evidence does not cleanly support that idea at ultra-high intakes.

• The authors highlight studies showing that increasing intake from 90 to 120 grams per hour can increase exogenous carbohydrate oxidation, meaning you burn more of the carbohydrate you consume.

• However, that extra oxidation does not necessarily reduce endogenous carbohydrate oxidation, meaning it does not reliably spare muscle glycogen.

If an athlete takes in 120 grams per hour and oxidizes more external carbohydrate, but their muscles keep burning stored carbohydrate at the same rate, then the main benefit is not glycogen sparing. The extra carbohydrate may simply be replacing some fat oxidation rather than preserving precious muscle glycogen.

This matters because a lot of the practical messaging around high-carb fueling still rests on the idea that higher intake protects your internal carbohydrate stores. According to this commentary, that explanation is probably too simple, and possibly even wrong at the highest intake levels.

The authors also point out that oxidation efficiency drops as intake rises.

• In one cited study, oxidation efficiency fell from about 86% at 90 grams per hour to about 76% at 120 grams per hour. As you push intake higher, a larger fraction of what you consume may not be immediately burned for energy.

That unoxidized carbohydrate has to go somewhere. Some may remain in the gut. Some may be retained in the liver. Some may contribute to better recovery after the race or workout. But it may not be directly powering your legs in the moment.

And for runners, the gut piece is not trivial. It is one thing to tolerate 120 grams per hour on the bike or in a controlled lab session. It is another thing to do it at marathon pace, late in a race, with heat, jostling, dehydration, and rising sympathetic stress. The paper notes that laboratory trials may be too short to capture the point at which gastrointestinal distress often appears in long events. Anyone who has raced a marathon knows that the stomach can be perfectly fine at mile 10 and not so fine by mile 22.

The performance evidence doesn’t match the hype

The authors argue that, while carbohydrate ingestion reliably improves endurance performance compared with placebo or no fuel, the evidence for a clear dose-response benefit above 90 grams per hour is weak.

Read more

Greetings, and welcome to the RLRH newsletter.

Thanks to ProBio Nutrition for sponsoring this post.

ProBio Nutrition is the daily all-in-one that’s been developed by Dr. Marc Bubbs, a nutrition expert and consultant to top athletes, and is formulated with a focus on performance, longevity, and recovery. I’ve been taking ProBio ProBlend for months, and my main takeaways are that this covers all your bases, like a multivitamin for people who train regularly, is a better value than an old supplement stack, and it actually truly tastes great.

RLRH has partnered with ProBio Nutrition because we believe in the benefits, and our readers can get a flat 40% off using this link (if you subscribe, you’ll get an extra 10% off plus free shipping).

Are Endurance Athletes Matching Their Carbs to Training Load?

Most of us know the basic advice when it comes to nutrition: eat more carbohydrates when training is harder, eat a little less when training is easier, and fuel the work that matters most. That sounds simple. But nobody is perfect.

A new observational study tried to answer a practical question: Are endurance athletes actually adjusting carbohydrate intake based on training load in the real world?

Researchers followed 46 endurance athletes for 12 weeks as they self-selected their training and logged both dietary intake and exercise using a smartphone app. That gave the researchers a large day-to-day dataset: 3,718 days of dietary assessments and 3,160 days of training. The main focus was carbohydrate intake and how it related to training volume, exercise duration, and fasted-state training.

Higher weekly training volume was associated with higher average daily carbohydrate intake, and longer exercise sessions tended to come with more carbohydrate. So the broad picture was not that endurance athletes are completely ignoring fueling demands.

But the individual-level data were much more variable. Average carbohydrate intake ranged widely, from just 1.2 to 7.2 grams per kilogram per day, with a group average of 3.9 g/kg/day. That is not very high for endurance athletes, especially during heavier training. Fasted training was also common: 65% of athletes did it regularly, and it was more common among men than women. Athletes who trained fasted more often tended to have lower carbohydrate intake overall.

The most interesting part of the study is that some athletes adjusted carbohydrate intake closely with training load, while others barely did. Individual correlations between daily training load and carbohydrate intake ranged from negative to strongly positive. In other words, some athletes were fueling more on bigger days, some were not changing much at all, and some appeared to eat less carbohydrate when training more.

This is where the study becomes useful. The average athlete may understand carbohydrate periodization in theory, but applying it day after day is a different skill. The study does not tell us whether the lower-carb or fasted-training athletes performed worse, recovered worse, or had more health issues. It simply shows that “fuel for the work required” is not yet how many endurance athletes actually behave.

What this means for runners

Carbohydrate intake should probably move more intentionally with your training. Easy short run? You may not need much beyond normal meals. Long run, workout, race-specific session, or a big-volume day? That is when carbohydrate availability becomes performance nutrition, not just calories. I’d especially pay attention to whether your hardest sessions are being accidentally under-fueled because they happen early, because you’re busy, or because you’re trying to keep daily intake artificially low.

The overall goal is not to micromanage every gram, but to avoid treating a 30-minute jog and a 2-hour long run like they have the same fueling demands. Just be a bit more intentional about fueling.

Can Portable Sweat Sodium Devices Actually Be Trusted?

Sweat testing has moved from the lab into the hands of everyday athletes. That sounds exciting because sweat sodium varies massively between runners, and for long races in hot conditions, knowing whether you are a salty sweater can be genuinely useful. But there is always a catch with consumer-friendly sports tech. Just because a device gives you a number does not mean that number is reliable.

A new study tested two portable sweat sodium analyzers, the MX3 and LAQUAtwin-Na-11, against a reference analyzer in warm, humid exercise conditions. Nineteen healthy, active adults completed two 60-minute running or cycling trials in the heat, around 81 degrees Fahrenheit and 51% relative humidity. Sweat was collected using an absorbent patch on the forearm, then analyzed with the portable devices and the reference method.

The question was twofold: are these devices reliable from trial to trial, and are they valid compared with the reference analyzer?

Both devices performed reasonably well, but not perfectly. The MX3 showed acceptable-to-high between-trial reliability, while the LAQUAtwin showed acceptable-to-moderate reliability. The coefficients of variation were still fairly large, around 23–24%, which means individual readings can bounce around. That matters because athletes often want sweat sodium testing to give them a precise hydration prescription. The reality is probably more like a useful estimate than a laboratory-grade truth.

On validity, both devices looked better. They were strongly associated with the reference method. Overall, the authors concluded that both portable analyzers provide good validity for lab-based sweat sodium assessment, with MX3 showing somewhat better reliability.

The study is useful, but it also reinforces an important limitation: this was a controlled lab-style setup using forearm sweat patches, not a chaotic race-day environment with changing pace, wind, fluid intake, aid stations, and hours of exercise. Sweat sodium itself also varies by body site, sweat rate, heat acclimation, and collection method. So while these tools can be helpful, I would be cautious about building an overly precise sodium plan from a single test.

What this means for runners

Portable sweat sodium testing can be useful, especially if you are training for long races in the heat or have a history of heavy salt loss, cramping, or white salt marks on clothes. But treat the number as a starting range, not a commandment. If a device tells you that you lose a lot of sodium, it may justify practicing with higher-sodium fluids or gels during long runs, but the real test is how your gut, thirst, performance, and post-run symptoms respond. I’d use sweat sodium data alongside sweat rate, race duration, heat exposure, and fueling tolerance rather than turning it into a rigid formula.

Do Super Shoes Work on Trails Too?

We have gotten used to the idea that advanced footwear technology improves running economy on the road. But trails are not roads. The surface is uneven, pace changes constantly, foot placement matters, and stability can become just as important as bounce.

So the question here is simple: do advanced trail shoes actually improve running economy on real trails?

This study tested eight trail runners during an outdoor trail running session. Each runner completed four 1,500-meter trail trials at a self-selected 50K race effort, alternating between an advanced footwear technology trail shoe and a control trail shoe. The order was counterbalanced, and oxygen consumption was measured using a portable metabolic device. Running economy was calculated as the cost of transport, which adjusts oxygen use for speed.

The result: the advanced trail shoe improved running economy by about 1.1% compared with the control shoe, which is smaller than the 2–4% improvements often discussed in road racing shoes, especially when tested at faster treadmill speeds.

That smaller effect makes sense. Trail running likely dilutes some of the benefits of advanced footwear. If you are braking on descents, navigating rocks, turning, stabilizing, and varying stride length, the energy return from the shoe may not translate as cleanly as it does on smooth pavement. Also, the athletes ran slower than elite road-shoe testing speeds. Super shoe benefits can depend on speed, mechanics, and how well the shoe interacts with the runner.

Still, 1.1% is not nothing. Over an ultra or long trail race, even a small reduction in energy cost could matter, particularly if it delays fatigue or reduces muscle damage. But the key word is could. This study measured running economy over relatively short trail segments, not actual race outcomes, downhill durability, ankle stability, technical terrain performance, or late-race fatigue.

What this means for runners

Advanced trail shoes may offer a real but smaller economy benefit than road super shoes, so I’d think of them as a tool rather than a magic upgrade. On smoother, faster, runnable trails, they may be especially useful. On highly technical courses, stability, traction, confidence, and downhill control may matter more than foam rebound. The best trail shoe is not necessarily the one with the most technology; it is the one that lets you run efficiently without fighting the terrain. Practice in them before race day, especially on descents and technical sections, because the shoe that saves oxygen on paper is only helpful if you can actually control it when the trail gets messy.

Can Your “Training Stress Score” Predict Running Injuries?

Read more

Greetings, and welcome to the RLRH newsletter.

Thanks to ProBio Nutrition for sponsoring this post.

ProBio Nutrition is the daily all-in-one that’s been developed by Dr. Marc Bubbs, a nutrition expert and consultant to top athletes, and is formulated with a focus on performance, longevity, and recovery. I’ve been taking ProBio ProBlend for months, and my main takeaways are that this covers all your bases, like a multivitamin for people who train regularly, is a better value than an old supplement stack, and it actually truly tastes great.

RLRH has partnered with ProBio Nutrition because we believe in the benefits, and our readers can get a flat 40% off using this link (if you subscribe, you’ll get an extra 10% off plus free shipping).

VO₂ max, lactate threshold, and running economy are the tidy little triangle that’s been used to explain endurance performance for a long time.

If you want to run fast, long (or ideally, fast for a long while), you need a big aerobic engine, the ability to hold a high fraction of that engine, and the mechanical efficiency to turn oxygen into speed without wasting energy.

But anyone who has trained for a marathon, raced an ultra, or watched their heart rate drift upward during what was supposed to be an “easy” long run knows this model doesn’t capture everything about performance.

Two runners can have similar lab values and can even look nearly identical at marathon pace for the first hour. Then, somewhere after 90 minutes, one runner keeps rolling while the other starts leaking speed, form, and confidence.

That difference is what sports scientists are increasingly trying to describe with a new set of terms: durability, fatigability, repeatability, and resilience.

This week’s paper by Benedikt Meixner, Michael Joyner, and Billy Sperlich is not a traditional experiment. It’s a viewpoint article, which means the authors are not testing a new training intervention or reporting a new dataset. Instead, they are trying to clean up the language around these emerging concepts. And honestly, it matters a lot.

Because right now, endurance science has a terminology problem.

A lot of people are using words like durability, fatigue resistance, resilience, and repeatability almost interchangeably. Coaches use them. Researchers use them. Athletes use them. I’ve probably done it too. But these terms do not all mean the same thing. And if we want to measure them, train them, and actually apply them to performance, we need to be more precise.

That is the central idea of this paper. The classic endurance model is still foundational, but it may be incomplete unless we also account for how athletes respond when the race gets long, messy, hot, hilly, mentally demanding, or unpredictable.

The old model still (mostly) works

The authors begin with the traditional Joyner-style endurance model: VO₂ max, lactate threshold, and movement economy.

These three variables explain a lot. VO₂ max reflects the maximum capacity to take in, transport, and use oxygen. Lactate threshold describes the highest intensity that can be sustained in something close to a metabolic steady state. Economy reflects the energy cost of moving at a given speed or power output.

In a clean lab test, these are incredibly useful. But races are not clean lab tests.

Marathons last long enough for fuel availability, neuromuscular fatigue, cardiac drift, muscle damage, thermoregulation, pacing, and psychology to matter. Trail races add terrain and eccentric loading. Ultras add sleep loss, heat, altitude, gut issues, and emotional volatility.

The authors’ argument is not that VO₂ max, threshold, and economy are outdated. It’s that they describe what an athlete can do under relatively fresh and controlled conditions. They do not fully explain what happens after two hours, after repeated surges, after environmental stress, or after fatigue starts changing the body.

Durability

Durability is probably the most important concept here for marathoners and long-distance runners.

A durable runner does not just have a good lactate threshold in the lab. They still have a strong lactate threshold after 90 or 120 minutes of running. They do not just have good running economy when fresh. They maintain economy deep into the race. Their heart rate does not drift wildly upward at the same pace. Their stride does not collapse. Their fuel use does not become catastrophically inefficient.

Durability captures what every marathoner eventually learns (usually the hard way)—your fitness on paper is not always the same as your fitness at mile 22.

The authors point to several mechanisms that may contribute to durability, including cardiovascular drift, neuromuscular fatigue, metabolic shifts, carbohydrate depletion, and changes in economy or critical power. They also note that durability can be assessed by looking at physiological drift during prolonged exercise or by testing performance markers before and after a long endurance effort.

For runners, I think durability is one of the missing links between “I’m fit” and “I can execute the race I trained for.” It is not enough to hit marathon pace in a workout when fresh. The question is whether marathon pace still feels metabolically, mechanically, and psychologically manageable after you have already done a lot of work.

Fatigability

Fatigability is related to durability, but it is not exactly the same.

This concept is especially obvious in cycling, where athletes may need to respond to surges, climbs, attacks, and stochastic power demands. But it applies to running too, especially in hilly races, trail races, cross-country, tactical racing, and workouts with repeated hard segments.

A runner with high fatigability might look great early but lose speed rapidly as the session or race continues. Their power, pace, mechanics, or perceived effort starts to deteriorate quickly. A runner with low fatigability can absorb the work and keep producing.

The key difference from durability is that fatigability often involves variable or repeated intensity while recovery is limited. Durability is more about preserving physiological function over prolonged steady or semi-steady exercise. Fatigability is more about how quickly the athlete cracks as workload accumulates.

This matters because not all “fatigue resistance” is the same. A marathoner may need durability to preserve pace for 26.2 miles. A trail runner may need low fatigability to handle repeated climbs, descents, accelerations, and technical terrain. A road racer may need to tolerate pace changes without falling apart.

The authors also emphasize that realistic testing matters. A lab test that does not include a race-relevant preload, fueling conditions, or variable intensity might miss the point. That is a big theme of the paper. If we want to understand endurance performance, we need assessments that resemble the actual demands of the sport (which unfortunately, doesn’t always happen in well-controlled laboratory experiments).

Repeatability

Read more

Greetings, and welcome to the RLRH newsletter.

Thanks to ProBio Nutrition for sponsoring this post.

ProBio Nutrition is the daily all-in-one that’s been developed by Dr. Marc Bubbs, a nutrition expert and consultant to top athletes, and is formulated with a focus on performance, longevity, and recovery. I’ve been taking ProBio ProBlend for months, and my main takeaways are that this covers all your bases, like a multivitamin for people who train regularly, is a better value than an old supplement stack, and it actually truly tastes great.

RLRH has partnered with ProBio Nutrition because we believe in the benefits, and our readers can get a flat 40% off using this link (if you subscribe, you’ll get an extra 10% off plus free shipping).

Do Super Shoes Make You Faster at a Cost?

Super shoes have become one of the strangest training decisions in running. On the one hand, they are clearly fast. The combination of highly resilient foam, tall midsoles, and stiff embedded plates can improve running economy and make race pace feel just a little more forgiving. On the other hand, almost every runner now has a story: the friend who started doing workouts in plated shoes and suddenly had calf issues, foot pain, or a bone stress injury that seemed to come out of nowhere.

That does not mean super shoes are dangerous. But it does raise the question of when these shoes do change how we run, where does that stress go?

A new study looked at exactly that question in elite distance runners. The researchers had 23 healthy elite runners, 11 women and 12 men, run in three different shoe conditions: a neutral shoe, a lightweight responsive foam shoe, and an advanced footwear technology shoe with highly cushioned foam and a stiff embedded plate. The runners tested each shoe in randomized order at three self-selected speeds: training effort, tempo effort, and 5K race pace. During each condition, the researchers measured movement and force patterns that have previously been associated with bone stress injuries.

In the advanced footwear condition, runners showed a lower cadence, meaning they took fewer steps per minute. Lower cadence often goes along with longer strides or slight overstriding, which can increase loading demands on the body over time. The researchers also found more inward collapse of the arch/rearfoot compared with the neutral shoe, another mechanical pattern that has been linked with bone stress injury risk. These changes were described as small, but small changes repeated thousands of times per run can matter when an athlete is stacking workouts, long runs, and high weekly mileage.

But there was also a potentially protective signal. In the super shoes, runners pushed off less with their ankles. That suggests the shoe may reduce demand on the ankle plantarflexors, including the calf-Achilles complex, during propulsion.

This is the part that makes the interpretation more complicated. Super shoes may not simply increase or decrease injury risk; they may redistribute load. Less ankle demand could be helpful for some runners, while altered cadence and greater rearfoot motion could shift stress elsewhere, including toward the bones of the foot or lower leg.

What this means for runners

If you race in plated shoes, you probably need some exposure to them in training so race day does not become a brand-new mechanical stimulus. But I would be cautious about making them your everyday trainer, especially during high-volume blocks or when returning from injury. Rotate shoes, introduce plated models gradually, and pay attention to early warning signs in the foot, shin, calf, and Achilles. The key idea is that super shoes may help you run faster by changing how load is distributed, but your bones, tendons, and muscles still need time to adapt to that new loading pattern.

Do Carbs Still Work When It’s Hot?

Racing or training in the heat has a way of making every normal endurance rule feel a little shakier. Paces that usually feel controlled suddenly feel too ambitious. And fueling (which is already part science and part personal experiment) gets even more complicated when your gut is bouncing and your sweat rate is climbing.

Do carbohydrates still help endurance performance in the heat, or does heat stress change the equation?

A new systematic review looked at carbohydrate supplementation during endurance exercise in hot environments. The researchers synthesized nine randomized crossover studies. In practice, these were all lab-based cycling studies performed in environmental chambers between 80–95 degrees Fahrenheit (27 to 35 degrees Celsius), with humidity ranging from 20% to 78%. That is important right away: this review is highly relevant to endurance athletes, but it is not directly a running review.

The carbohydrate strategies varied a lot. Some studies used glucose or maltodextrin, others used multiple transportable carbohydrates like glucose, sucrose, fructose, or maltodextrin-fructose combinations, and one used sago. Intake rates ranged from a very low 14 grams per hour to an extremely high 140 grams per hour. Exercise trials lasted roughly 50 to 152 minutes, with performance measured through either time-to-exhaustion tests or time trials.

The results were mixed, which is probably the most honest answer here. Five studies found performance benefits from carbohydrate supplementation in the heat, while four found no clear effect. In the positive studies, carbohydrates improved time to exhaustion by 13.4% to 19.3% and improved time-trial performance by 3.3% to 12.7%. That sounds impressive, but the picture gets messier when you look closer. Some of the biggest benefits came from time-to-exhaustion tests, which are useful in the lab but less reliable than real-world time trials.

The bigger physiological question is whether carbohydrate intake in the heat actually changes fuel use. Heat stress can increase glycogen breakdown, which creates the theory that athletes might need more carbohydrate when racing hot. But in this review, most studies did not find major differences in respiratory exchange ratio (a marker of relative fat and carbohydrate use during exercise) or carbohydrate oxidation between carbohydrate and placebo (no-carb) conditions. Taking in carbs did not consistently appear to reduce the body’s reliance on stored carbohydrate. That does not mean fueling is useless; it means heat may create a more complicated metabolic environment where hydration, thermoregulation, gut function, and central fatigue all compete with simple “more carbs equals more performance” logic.

The gut may be the hidden variable. Only two of the nine studies actually measured gastrointestinal symptoms, which is a major limitation because heat is notorious for making the gut more fragile. One study found more nausea, fullness, and stomach upset with a 12% glucose drink providing about 100 grams per hour compared with a 6% drink and placebo. Another found greater fullness with a 16% maltodextrin-fructose drink at 90 grams per hour. The review also noted that one study using a very concentrated 14% carbohydrate drink, equal to roughly 140 grams per hour, produced worse performance on average, which may have been related to gut distress, even though GI symptoms were not measured.

Hydration and body temperature did not seem to be strongly affected by carbohydrate intake overall. Most studies found no meaningful differences in hydration markers, core temperature, or perceived exertion between carbohydrate and placebo.

What this means for runners

I would not take this review as a reason to fuel less in the heat, but I would take it as a reason to fuel smarter. Carbohydrates can still help, especially for longer races and harder efforts, but hot conditions make the gut less forgiving, so forcing huge carb targets without practice may backfire. The practical hierarchy should probably be: start well-fueled, prioritize hydration and sodium based on sweat losses, use carbohydrate amounts you have practiced in similar conditions, and be cautious with very concentrated drinks or aggressive fueling rates when it is hot and humid. Since all included studies were cycling-based, runners should be especially careful extrapolating the higher intake ranges, because running tends to create more gut jostling and GI distress.

Female Runners are More Durable than Males

Most runners think about fitness in terms of what they can do when they’re fresh. What’s your VO2 max? What’s your threshold pace? What can you run for a 5K, half-marathon, or uphill time trial when the legs are ready to go?

But racing rarely asks that question. Racing asks a much more interesting one—what can you still do after two or three hours of accumulated fatigue?

That’s the idea behind “durability” (a.k.a “physiological resilience”), one of the more useful concepts in endurance performance right now. It’s not just how strong your engine is at the start. It’s how much of that engine you can keep using after glycogen drops, muscles get beat up, stride mechanics shift, and perceived effort starts climbing. A new study asked whether male and female runners differ in that quality.

Researchers studied 11 highly trained female trail runners and 11 highly trained male trail runners who were matched by performance level using International Trail Running Association rankings. The runners completed three lab visits: a graded exercise test, a fresh 12-minute uphill time trial, and then a 3-hour treadmill run at moderate intensity with another 12-minute uphill time trial inserted every 60 minutes. During the prolonged run, the athletes consumed 90 grams of carbohydrate per hour and drank water as desired, while the researchers measured physiology, biomechanics, perceived exertion, and the strength of muscles in the thigh and surrounding the knee.

The women held up better. After three hours, female runners had only a 1.1% decline in uphill time-trial speed, while male runners slowed by 9.9%. That is a massive difference in durability. This was not because the women were working less hard in the time trials; heart rate, perceived effort, and peak oxygen uptake during the time trials were broadly similar in how they responded across the protocol. Instead, the difference seemed to come from greater metabolic and neuromuscular resilience.

During the 3-hour steady-state run, both groups shifted toward greater fat use over time, but the shift was much larger in men. By three hours, carbohydrate oxidation had dropped by 29% in male runners versus only 9% in female runners. Respiratory exchange ratio also fell more in men, suggesting a bigger move away from carbohydrate metabolism. During the repeated uphill time trials, the same pattern showed up again: men had larger drops in carbohydrate oxidation, larger increases in fat oxidation, and bigger reductions in peak blood lactate.

That lactate finding is important. In a hard uphill effort, peak lactate is partly a sign that the runner can still access high-intensity carbohydrate-driven energy production. By the final time trial, male runners showed a 53% drop in peak lactate, compared with a 27% drop in females. That suggests the men were losing some ability to hit the same high-intensity metabolic gear as fatigue accumulated.

Muscle strength also declined more in men, with males showing an 18% reduction after two hours, while females were essentially unchanged at that time point. Meanwhile, running economy worsened similarly between sexes, and both groups made fatigue-related biomechanical adjustments: longer ground contact time, reduced stride length, and lower leg stiffness. In other words, the durability advantage in women did not seem to come from some radically different stride pattern. It looked more like better preservation of the metabolic and muscular systems that support performance late in a long effort.

There is one big caveat: the study matched runners by duration, not distance. Because the men ran faster, they covered more total distance during the protocol: about 26 miles versus 22 miles for women. They also expended more energy relative to body mass. The researchers tried to account for this statistically, and the findings largely held up, but this still matters. If the study had matched distance instead of time, the gap might have been smaller.

What this means for runners

The practical takeaway is that durability deserves more attention than fresh fitness. A runner with a great threshold test or fast standalone workout may not be the runner who performs best after two or three hours, especially on hilly terrain. For everyone, one of the best ways to assess durability may be to place short, controlled hard efforts late in long runs and see how much performance, form, and perceived effort deteriorate. The race is not decided by what your physiology looks like when you’re fresh; it’s decided by what’s still available when the easy miles are behind you.

Running Hundreds of Marathons Keeps VO₂ Max Younger

Read more