How to Fuel to Compete in the Heat

Staying on top of nutrition becomes more challenging as the temperatures rise.

With much of the US stuck in sweltering late summer temps, the seasons changing in the Southern Hemisphere, and Kona right around the corner, I thought it would be a good opportunity to talk about a training and racing challenge that can undo even the best prepared athlete—fueling in the heat.

When you exercise, your heart rate rises to increase cardiac output. Your working muscles need oxygenated blood; you need blood flow to the skin to offload heat, and you need blood flow to keep your organs functioning. With blood flow demands increasing in all those places, your body decreases blood flow to your gut and liver by almost 80 percent. That leaves your hard-working gut in the precarious position of trying to keep the energy flowing with precious few resources.

That means the same strategies that can work when the temperatures are cool to moderate can leave you with GI distress when it’s hot. And to be clear, it doesn’t need to be 89 degrees F/ 32 degrees C on the Queen K for you to experience these physiological challenges. As soon as temperatures cross the mid-70s F, or about 24C (and of course higher), your fueling strategy should change.

Prioritize Hydration

Your top priority here is hydration to keep your blood plasma levels high and allow for cooling so you don’t overheat. This is especially true for women whose core body temperatures may get hotter at a lower level of dehydration because they start out with a lower volume of body water than men do; and have a more rapid rise of core temperature in the early stages of exercise. (Plus, thermoregulation can be even more challenging for peri and postmenopausal women and for women who are in the luteal phase of their cycle.)

Anyone who follows me knows that I caution against highly concentrated carbohydrate drinks (i.e., “liquid calories”) because they can effectively dehydrate you. That’s especially important when it’s hot. Remember, your gut’s ability to absorb fluid and nutrients is significantly compromised from the heat and the lack of blood flow. The more concentrated your fluid, the longer it sits in the intestines, increasing the osmotic pressure. The body’s response to this is to pull water into the intestines to help reduce the pressure. This is a surefire way to perpetuate dehydration and GI distress. When racing in the heat, the concentration of carbohydrates in your drink needs to be no more than 4 percent, or about 1.2 grams of carbohydrates per fluid ounce.

Set a timer on your watch or computer to remind yourself to drink. It's easy to fall behind on your fluid needs, especially in a hectic race situation where you’re managing a lot of logistics on top of your fueling needs. So, it is best to have a reminder to take a drink every 15 to 20 minutes that you are racing. This becomes more important as the hours pass because you are able to only slow the rate of dehydration, and you’ll always finish a little dehydrated (the goal is to not hit the point of being a lot dehydrated!), and your thirst sensation can really take a hit the longer your race goes on.

In general, you want to aim to drink about 0.16 ounces of water per pound of body weight per hour in temperatures that are 80 F and above. (That’s 22 ounces for a 140 lb./64 kg woman.)

Also, skip the salt tablets. You’ll be getting plenty of sodium from your sports drinks and foods. Taking in high doses of sodium can cause reverse water flux, meaning your body water goes toward the sodium in the digestive tract, which can cause GI issues like gut sloshing as well as dehydration.

Go Easy on Your Gut

For fueling purposes, you want to tax your gut as little as possible. The goal is to take in foods that get energy where you need it when you need it with as little work as possible. That means:

• Take in small amounts of glucose-rich foods often. There’s a trend towards pushing the carb-intake ceiling as high as possible during endurance sports. So you’ll see research and anecdotal evidence on men able to burn through and hit optimum performance at about 78 grams (312 calories), or even higher, an hour of carbohydrates from mixed sources.

That’s not going to work for most women, especially in the heat. A better range for most active women is 0.9 to 1.15 food calories per pound / 2 to 2.5 food calories per kg of body weight per hour running or 1.3 to 1.6 food calories per pound / 3 to 3.5 food calories per kg per hour while cycling or participating in another nonjostling sport. In the heat, err toward the lesser amount. And keep it simple. Concentrated carbohydrates can cause GI distress, as we talked about with gels and liquid calories. Glucose-rich foods like chews or blocks can help reduce the incidence of GI distress. Just remember to test drive all your fuel before using it on race day.

• Minimize fat. Fat slows gut transit time, which is compounded in the heat. So be mindful of Nutella sandwiches and other higher fat race foods.
• Go easy on caffeine. Caffeine can not only increase core temperature, but also promotes GI permeability, paving the way for GI distress.

Putting it All Together

Your race day plan for a day in the heat:

Prerace (for an event 90 minutes or longer):

• Finish your meal at least 2 hours before your race. Eat another 150–250 calories 60 minutes before the race starts.
• Drink 0.12 oz/lb (10 ml/kg) of a prehydration drink 20–30 minutes before the start of the race.

During the race:

• Calorie needs after 45 minutes: 0.9 to 1.15 food calories per pound / 2 to 2.5 food calories per kg of body weight per hour running / 1.3 to 1.6 food calories per pound / 3 to 3.5 food calories per kg per hour while cycling or participating in another nonjostling sport.
  • Fluid needs per hour: 0.12–0.18 oz per lb (8–12 ml/kg) per hour of a very cold hydration drink, with sodium in the fluid (~360mg per 16-20oz).
  • The more cold things you can ingest, the better.

Postrace:

• It is critical to double up on your cold recovery drink. Have a cold recovery drink within 30 minutes of finishing and a real meal within 2 hours. Have another serving of the recovery drink 2 ½ hours later.

Bonus: Curcumin can improve gut function and physiological strain responses during heat stress. Try 500mg a day for three days before a hot-weather event.