How Much Slower Will the Heat Make You on Race Day?

If you’ve ever had your goal marathon derailed by an unseasonably warm day (recent London marathoners will be keenly aware), you know how much weather can mess with performance. But how much does it actually matter? Can you plan for it? Adjust for it?

John Davis dives deep into those questions in a recent breakdown of marathon performance across a wide range of temperatures and humidity levels, using data from nearly 4,000 marathoners across 754 races. Here are the key insights and how to apply them to your race-day strategy.

Unlike wind or hills, heat doesn’t just increase your energy cost—it adds another layer of fatigue that hits early and builds as the race goes on. Interestingly, runners begin to slow before their core temperature spikes, suggesting it’s the perception of effort—not just overheating—that forces you to back off.

Davis analyzed marathon results in conjunction with weather data and discovered a U-shaped relationship between temperature and performance.

The best performance window was 35–55°F (2–13°C), with ~48°F (9°C) as the optimal temperature for personal best times. Performance drops quickly above 65°F (18°C), and that’s where humidity begins to compound the problem.

Cold slows you down less than heat. A race in 30°F is better than one in 70°F. Things get worse when it’s both hot and humid. A steamy race day (e.g., 75°F with high humidity) can slow marathon pace by 6–8%, or more than 15 minutes at 3:00 marathon pace.

Davis encourages using heat index—which combines air temp and humidity—to adjust your pacing expectations. For example, if the heat index is 80°F, expect to run ~3% slower than your goal pace in ideal weather. He provides a table mapping heat index to adjusted paces (e.g., 6:00/mile in cool weather becomes ~6:11/mile at 80°F). You don’t need a fancy calculator—just knowing where the temps fall relative to that 35–55°F zone helps you set smarter expectations and avoid crashing mid-race.

RELATED ARTICLE: Summer Running Guide: 12 Top Tips To Beat The Heat​

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Can Your Gut Handle 90 Grams of Carbs per Hour?

If you’ve ever pushed the envelope on mid-race fueling—maybe going for that 90g/hour sweet spot—you’ve probably also experienced the dreaded tradeoff of your stomach saying “no thanks.” A new study puts that fueling strategy to the test and asks whether sex, age, or history of gastrointestinal (GI) issues affects how well you tolerate high-carb fueling during a long run.

The study included 53 endurance-trained runners (43 men, 10 women with an average age of 34). All participants ran for 2 hours at 60% VO₂max on a treadmill while ingesting a total of 180g of carbs (90g/hour) from a custom, low-fructose gel. They followed this up with a 1-hour self-paced run (a proxy for race-day performance).

Over 70% of athletes experienced some form of GI distress during the run, but symptom severity didn’t differ significantly by sex, age, or history of GI symptoms. However, women did report more upper abdominal pain and lower abdominal bloating, and athletes with a history of GI symptoms had more belching, heartburn, and abnormal bowel movements after exercise.

Only 9% of athletes exhibited poor carbohydrate absorption, and no group differences were observed. Carbohydrate tolerance was, not surprisingly, lowest in athletes with a history of GI issues—they reported less interest in food and drink and a lower appetite during and after exercise compared to the other runners. Women also showed lower interest in food during exercise and reduced tolerance to food during recovery. They also had higher blood glucose levels during and after the run, likely due to a higher relative carb intake per body mass and lower overall energy expenditure.

Despite all of these differences in gut comfort and metabolism, the distance covered in the 1-hour time trial was similar across all groups.

What this means for runners

Most runners can tolerate a high-carb intake, especially when using a low-fructose gel, with manageable symptom severity, even if they’ve had past GI issues. Because women exhibited more discomfort and lower fat oxidation, and appeared less inclined to eat or drink, their sensory and physiological cues may differ, and fueling plans may need to address this. The most significant predictor of poor feeding tolerance wasn’t gender or age—it was whether athletes had experienced GI issues in the past. Gut-training strategies could be most useful here.

RELATED ARTICLE: Energy Gels For Runners: How Often Should You Take A Gel for Max Race Performance?​

A Pre-Run Coffee Might Boost Post-Run Recovery

The ritual of a strong cup of coffee before a morning run feels almost sacred. It wakes you up, sharpens focus, and maybe even gives your pace a little kick. But beyond performance, caffeine could also impact what happens after the run—how your body recovers.

A new study from researchers in Japan investigated whether caffeine could enhance the natural increase in blood flow that occurs in the muscles after high-intensity running.

The study tested ten highly trained male long-distance runners, with an average VO2 max of ~70 ml/kg/min. Each runner completed two test days—one with a placebo and one with 6 mg/kg of caffeine (roughly equivalent to 400–500 mg for most of them, or about 3–4 cups of coffee), administered 30 minutes before an 8-minute treadmill run at ~92% VO2 max (close to 10k race pace).

After the run, the runners laid down while researchers measured blood flow in the femoral artery (the major vessel supplying the leg) using ultrasound. The focus was on the 30-minute post-exercise window—a critical period for recovery processes, including oxygen replenishment, metabolic waste clearance, and glycogen resynthesis.

Caffeine increased post-exercise leg blood flow by 17% compared to placebo. The increase wasn’t due to a difference in blood pressure, suggesting the change came from vasodilation (caffeine promoted a widening of the arteries). Caffeine also elevated breathing rate and reduced carbon dioxide levels, which may help make blood less acidic. Despite these changes in blood flow and ventilation, oxygen uptake (VO2) and CO2 output (VCO2) only modestly increased post-exercise. Unfortunately, while glycogen resynthesis wasn't directly measured, it may have been supported by enhanced nutrient delivery. Notably, there were no significant changes in lactate levels, cardiac output, or perceived exertion between conditions, suggesting the enhanced blood flow was a true caffeine effect, not a response to harder running in one condition versus the other.

What this means for runners

Previous studies have shown that pairing caffeine with carbs enhances glycogen storage post-exercise. This study suggests one mechanism might be improved muscle perfusion, especially in highly trained runners. Interestingly, caffeine didn’t seem to impair sleep in this study, based on a short questionnaire. But that may not hold true for everyone, especially with afternoon or evening training (and the extremely high dose used in this study). You don’t want to trade faster recovery for worse sleep.

RELATED ARTICLE: Coffee Before Running? Performance Benefits + How Much To Drink​

Here's what else you would have received this week if you were a subscriber to the complete, full-text edition of “Run Long, Run Healthy.” SUBSCRIBE HERE.

• Music alters your stride (but only on the treadmill)

• How to choose the right carbs for hydration

• Does lactate threshold really separate the elites from the rest?
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​Thanks for reading. As always—Run Long, Run Healthy
​​
~Brady~