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The Fine Art (and Science) of Fueling for Hot Races

As temperatures rise, we need to adjust more than our pace and fluid intake—science says there are foods and supplements that can specifically help your body's systems in the heat.


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When the temperature changes, your fueling needs to change with it—and it doesn’t take much mercury to trigger a new approach. When race day temperatures rise into the mid-70s or higher, your entire strategy should change. Racing in the heat presents a whole host of challenges: core temperature regulation, hydration, potential GI distress, higher perception of effort, and higher heart rate. You simply cannot race the same in hot weather as you can in cooler climes.

Let’s break down what exactly happens when you toe the line in the heat, so we can understand how to help your body adapt.

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Systems of Checks and Balances

Heat production during exercise is 15-20 times greater than at rest, and it is sufficient to raise core body temperature by 1.8 degrees F (1 degree C) every 5 minutes if there are no thermoregulatory adjustments. But of course, our bodies do make adjustments. The brain, more specifically the hypothalamus, controls thermoregulation. If the hypothalamus senses internal temperatures growing too hot or too cold, it will automatically send signals to the skin, glands, muscles, and organs. For example, if the body is producing heat from high-level exercise or if the external ambient temperature is elevated enough to cause a rise in the core temperature, signals to the hypothalamus result in signals to the skin to produce sweat. Sweating is one mechanism the body uses to cool itself, as heat is lost through the process of sweat evaporation, but sweat evaporation also has a side effect: dehydration.

One of the first physiological responses to exercise, whether in the cold or heat, is to raise the heart rate to increase cardiac output. This is because there are competing interests in your body for blood flow: First, the working muscles need oxygen but also need to have heat and metabolic waste removed; second, there needs to be enough blood to maintain a minimum perfusion pressure to keep our organs functioning; and third, a major shift in skin blood flow occurs—increasing the amount of blood that passes through the superficial vessels to help offload body heat. The trade-off in this scenario is that blood flow to the gut and liver decreases—at 70% of VO2max it decreases by almost 80%; there are greater decreases as exercise intensity increases. With such a decreased blood flow and lowered oxygen supply, there is a change in nutrient absorption, motility, and the mucosal integrity of the gastrointestinal tract.

All of this is simply part of your body’s system of checks and balances when you’re racing at a high effort. That system goes into overdrive when the ambient temperature increases. What does this mean for fueling and hydrating effectively when racing in the heat—particularly if you want to avoid GI distress and still maintain performance?

RELATED: Hot Stuff: The History and Science of Heat Acclimation

Photo: Hannah DeWitt
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How You Can Help (Yourself)

First and foremost, it is essential to stay on top of your hydration. As you sweat, you lose water from your blood (and as the plasma volume drops, the body pulls water from other spaces to try to keep blood volume up). This means if you are behind on your hydration, you compromise your blood volume, which means compromised blood circulation to the muscles and the skin—thus less water for sweating. Not ideal for racing in the heat.

There are specific things the body needs to create a net water gradient in the small intestines (where 95% of water flux takes place). These key things are: glucose, sucrose, and sodium. On a smaller scale, you also need potassium, magnesium, chloride, and calcium. Without glucose and sucrose, the constant flow of sodium and water into the body becomes rate-limited (i.e., it slows down as the body tries to find glucose to work with the sodium). But the critical thing is that the concentration of carbohydrates in your drink needs to be no more than 4% (~1.2grams of carbohydrate per fluid ounce)—especially in the heat. Remember, your gut’s ability to absorb fluid and nutrients is significantly compromised from the heat and the lack of blood flow, so the more concentrated your fluid, the longer it sits in the intestines—increasing the osmotic pressure. The body’s response to all of  this is to pull water into the intestines to help reduce the pressure. This internal tug-of-war is a surefire way to make dehydration and GI distress worse.

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Create a Strategy

Because your appetite and thirst is muted when exercising in hot conditions (and of course, by the excitement of race day), it is best to set a reminder alarm to tell you to sip throughout the many hours you are racing. This becomes even more important as the race gets longer and longer, because you are only able to slow the rate of dehydration with key hydration strategies, and your thirst sensation really takes a hit the longer your race goes—meaning you won’t remember to drink when you need to drink the most!

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What to Eat in the Heat

  1. Minimize fat and concentrated carbohydrates when your gut is compromised. Fat slows gut transit time, and this is compounded in the heat. As mentioned above, concentrated carbohydrates can cause osmotic diuresis (water into the gut) and GI distress. Your body does need carbohydrate for fuel, and as we have seen in the research, amino acids can help in long-distance races, both as a fuel source but also to help with gut integrity.
  2. Glucose-rich foods reduce the incidence of GI issues. The best thing to do is to trial certain foods during high-intensity training sessions, knowing that as you spend more time out there racing, your gut becomes less able to cope with large amounts of nutrients at one time.
  3. Eat small amounts, often. While high carbohydrate fueling is trendy and can be tolerable (without GI distress), the data in this research is primarily from just a handful of elite, 25-year-old male cyclists in cool conditions—which doesn’t translate to most of us racing in the heat. In the heat, it’s best to spread out your calorie intake over the race.
Photo: Brad Kaminski
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Train Your Gut

If you have enough time before your race, you can do specific gut heat acclimatization by using a sauna post-workout or short, intense sessions in the heat. These brief, but high heat stress exposures will not only improve your thermoregulatory capacities (increased blood volume, sooner time to sweat onset, more dilute sweating, better heat tolerance), but will also force the expression of heat shock proteins. These little proteins are synthesized rapidly following exposure to high stress. They work to restore normal function to proteins that have been broken down by the heat stress—a significant boon to the gut, as heat shock protein expression is a significant factor in adaptation of the intestines to heat stress. Once you have adapted, your body will respond to temperatures above 80 degrees F as if you were racing in 60 degrees.

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What About Supplements?

Two of the most widely-used supplements in the endurance space are caffeine and nitrates—both of which work well in cool or temperate environments. But in hot conditions, they have been shown to have a negative impact on performance. Caffeine specifically is known to increase core temperature without any performance benefits in the heat. Nitrates are a vasodilator, meaning they dilate your blood vessels, but their effectiveness dissipates when the body is under heat stress, as the physiological response to vasodilate is greater than what the nitrates can supply. Basically, your body is already doing its maximum to vasodilate the vessels to offload heat and no supplement can do more.

Beta alanine is still effective in the heat by the action of reducing muscle fatigue, both through its buffering capabilities and its ability to improve calcium release for muscle contraction. Another supplement you may want to load in the three days prior to your race is curcumin. Recent research has demonstrated that 500mg a day for three days before an exercise heat stress test improved gut function and physiological strain responses.

You can race well in the heat, but, like almost everything, it takes some planning and preparation to adjust for the increased stress your body will encounter.

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Heat-Tested, Sims-Approved Sports Nutrition

ProBalance Protein Water

$2.40, 17 oz

Protein water? It’s a thing. This light and refreshing drink packs 15 grams of protein, along with vitamins and electrolytes to help you function at your best. Try the wild cherry lime or tropical coconut flavors for an especially summery sip.

nuun Endurance

$20, 16 servings

Nuun Endurance* was designed to deliver only the minerals and nutrients that your body needs (at highly absorbable ratios) during hot and hard sessions. The easy-to-digest carbohydrates go down smoothly and absorb quickly, helping you to avoid GI distress.

Momentous L-Glutamine

$39, 60 servings

Designed for proper digestion, absorption, and protein synthesis, one small scoop of this easy-to-mix powder every morning primes your body to perform at optimal levels in the heat.

*Editor note: Sims was hired by Nuun to work on Nuun Endurance