Dehydration Makes Carbs Less Effective
Plus—the pacing pattern linked to faster marathons; mental health history and late-race slowing; what one sleepless night does to running; whether weight lifting improves uphill running, and 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.
