As the temperature changes, so does your energy supply—and it doesn’t take a lot of mercury to trigger a new approach. If race-day temperatures soar into the mid-70s or higher, your entire strategy should change. Running in the heat presents a whole host of challenges: core temperature regulation, hydration, potential gastrointestinal upset, higher exertion perception and higher heart rate. You just can’t race in hot weather like you can in cooler climes.
Let’s break down what exactly happens when you go all out in the heat so we can understand how you can help your body adapt.
Systems of Checks and Balances
Heat production during exercise is 15 to 20 times greater than at rest, and it is sufficient to increase core body temperature by 1.8°F (1°C) every 5 minutes if no thermoregulatory adjustments are made. But of course our bodies make adjustments. The brain, more specifically the hypothalamus, controls thermoregulation. When the hypothalamus senses that the internal temperature is getting too hot or too cold, it automatically sends signals to the skin, glands, muscles, and organs. For example, when the body produces heat from intense exercise, or when the external ambient temperature is high enough to cause a rise in core temperature, signals to the hypothalamus result in signals to the skin to produce sweat. Sweating is a mechanism by which the body cools down 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 cold or hot, is to increase heart rate to increase cardiac output. This is because there are competing interests for blood flow in your body: first, working muscles need oxygen, but heat and metabolic wastes also need to be removed; Second, there must be enough blood to maintain minimal perfusion pressure for our organs to function. and third, there is a significant shift in skin perfusion – increasing the amount of blood that flows through the superficial vessels to help dissipate 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 larger decreases as training intensity increases. With such decreased blood flow and oxygenation, nutrient uptake, motility, and mucosal integrity of the gastrointestinal tract are altered.
All of this is simply part of your body’s control system when you’re racing at high levels of exertion. This system goes into overdrive when the ambient temperature rises. What does this mean for effectively fueling and hydrating when racing in the heat – especially when you want to avoid gastrointestinal distress while still maintaining performance?
TIED TOGETHER: Hot Stuff: The History and Science of Heat Acclimation
How you (yourself) can help
First and foremost, it is important to pay attention to your hydration. When you sweat, you lose water from your blood (and when plasma volume falls, the body draws water from other spaces to try to maintain blood volume). This means when you’re behind on your hydration, you’re affecting your blood volume, which means impaired blood flow to your muscles and skin – and therefore less water to sweat. Not ideal for racing in the heat.
There are certain things the body needs to create a net water gradient in the small intestine (where 95% of water flow occurs). These important 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 (ie, it slows down as the body tries to find glucose to work with the sodium). But the bottom line is that the carb concentration in your drink can’t exceed 4% (~1.2 grams of carbs per fluid ounce) — especially in the heat. Remember that your gut’s ability to absorb fluid and nutrients is severely affected by the heat and lack of blood flow. The more concentrated your fluid, the longer it stays in the gut, increasing osmotic pressure. The body’s response to all of this is to pull in water in the bowel to relieve pressure. This internal tug of war is a surefire way to aggravate dehydration and gastrointestinal distress.
Create a strategy
Since training in hot conditions (and of course the excitement of race day) will dampen your appetite and thirst, it’s best to set a reminder alarm to instruct you to sip during the many hours you’re racing drink. This becomes even more important the longer the race gets, as the only way to slow down the rate of dehydration is with key hydration strategies, and your sense of thirst really decreases the longer your race goes – meaning you won’t remember drinking when you do need to drink the most!
What to eat in the heat
- Minimize fat and concentrated carbohydrates if your gut is compromised. Fat slows intestinal transit time, and heat exacerbates this. As mentioned above, concentrated carbohydrates can cause osmotic diuresis (water in the intestines) and gastrointestinal upset. Your body needs carbohydrates for fuel, and as we’ve seen in research, amino acids can aid in endurance racing, both as a source of fuel and to support gut integrity.
- Glucose-rich foods reduce the occurrence of GI problems. The best thing you can do is experiment with certain foods during high-intensity workouts, knowing that the more time you spend running, your gut will be less able to handle large amounts of nutrients at once.
- Eat small amounts often. While high-carb fueling is trending and can be tolerable (without GI discomfort), the data in this study comes primarily from just a handful of 25-year-old elite male cyclists in cool conditions — which isn’t true for most of us in the race Heat. In the heat, it’s best to spread your calorie intake over the race.
Train your gut
If you have enough time before your race, you can do targeted heat acclimation of the gut by going to the sauna after your workout or doing short, intense sessions in the heat. These brief but intense heat stress episodes not only improve your thermoregulatory abilities (increased blood volume, earlier onset of sweating, more dilute sweating, better heat tolerance), but also force the expression of heat shock proteins. These small proteins are rapidly synthesized after high loads. They are working to restore the normal function of proteins degraded by heat stress – a significant boon for the gut as heat shock protein expression is a key factor in the gut’s adaptation to heat stress. Once you have adjusted, your body will respond to temperatures above 80 degrees F as if you were racing at 60 degrees.
What about supplements?
Two of the most commonly used endurance supplements are caffeine and nitrates, both of which work well in cool or temperate environments. However, they have been shown to have a negative impact on performance in hot conditions. Caffeine, in particular, is known to increase core temperature with no performance benefits in the heat. Nitrates are a vasodilator, meaning they dilate your blood vessels, but their effectiveness diminishes when the body is under heat stress because the physiological response to vasodilate is greater than what the nitrates can deliver. Basically, your body is already doing its maximum to expand the vessels to dissipate heat, and no supplement can do more.
Beta-alanine is also effective in the heat by reducing muscle fatigue, both through its buffering abilities and its ability to enhance calcium release for muscle contraction. Another supplement you may want to load up on in the three days leading up to your race is curcumin. Recent research has shown that 500 mg per day for three days before a heat stress test improved bowel function and physiological stress responses.
You can race well in the heat, but as with almost anything, it takes some planning and preparation to adjust to the increased stress your body will be subjected to.
Heat tested, Sims approved sports nutrition
ProBalance protein water
$2.40, 17 ounces
protein water? It’s one thing. This light and refreshing drink is packed with 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 extra summery sip.
$20, 16 servings
Nuun Endurance* is designed to deliver only the minerals and nutrients your body needs during hot and hard sessions (in highly absorbable ratios). The easily digested carbs go smoothly and are quickly absorbed, helping you avoid GI discomfort.
$39, 60 servings
A small scoop each morning of this easy-to-mix powder, formulated for proper digestion, absorption and protein synthesis, primes your body to perform at its best in the heat.
*Editor’s note: Sims was hired by Nuun to work on Nuun Endurance