Strategies for Training and Competing in the Heat and Humidity
By: Christopher D. Jensen, PhD, MPH, RD
Nutrition & Epidemiology Researcher
Training and competing are usually plenty tough. But add sweltering heat or the drenching humidity of summer to the equation, and the degree of difficulty rises to a whole new level. Heat and humidity are probably the most common performance-impairing environmental challenges you'll encounter as a high school athlete. So it definitely pays to know how to safely and effectively train — and compete — in the heat.
Working your muscles makes you faster, stronger — and hotter
You'd like to think that all the energy your muscles are churning out during exercise is going towards making you faster or stronger. In truth, about 75% of that muscle energy is turned into heat. If that heat were to remain in your body, your core temperature would quickly rise, and your performance would rapidly plummet. You'd also be at risk for heat-related illnesses, such as heat stroke, caused by hyperthermia.
Fortunately, your body has powerful physiological mechanisms for eliminating muscle-generated heat. In addition, there are important strategies you can employ to maximize your athletic performance and reduce the dangers of exercising in the heat and humidity.
Temperature regulation in action
The physiological term for managing your core body temperature is thermoregulation, and there are two important mechanisms for keeping your cool:
As your muscles crank out the heat, blood flowing through muscle tissue absorbs the heat and carries it away and toward the skin. Triggered by an uptick in core body temperature, your brain sends a signal to the blood vessels in your skin to dilate or expand. This allows the skin to hold a larger volume of warmed blood. If the ambient air is cooler than the temperature of your skin, a heat exchange occurs, and heat from the blood circulating in the skin is released to the cooler surroundings. You cool off in the process.
Heat loss is further accelerated by sweating. As you exercise, your core temperature rises and your sweat glands receive a signal from the brain to begin secreting sweat onto your skin. If the ambient air is sufficiently dry, sweat evaporates from your skin and a cooling effect takes place.
Double whammy: heat and humidity
When the air temperature exceeds 96.8°F (36°C), instead of heat flowing from your skin to the outside environment, heat exchange is reversed, and your body begins to absorb heat from the environment. So when it's scorching outside, one of your two key thermoregulatory mechanisms has essentially been knocked out of commission.
When this occurs, sweating becomes the primary means for eliminating the heat generated by muscle contraction. When you're training or competing hard outdoors in the heat, a sweat rate of 34–85 fl oz per hour (1.0–2.5 liters per hour) is typical. In sweltering conditions, sweat rates of greater than 85 fl oz per hour (2.5 liters per hour) can occur.
What about when you face both heat and humidity? This formidable duo poses a serious challenge to thermoregulation during exercise because when you add humidity to the mix, the air is heavy with moisture. If the air were dry, sweat would evaporate readily from the skin, and a cooling effect would result. But when the surrounding air is saturated with moisture, sweat doesn't effectively evaporate from your skin during exercise. Instead, it builds up and then drips off your body. As a result, you lose the cooling effect of sweat evaporation.
Thus, the combination of high heat and humidity puts a serious damper on your ability to thermoregulate. In these extreme conditions, you risk having the heat load build up to a critical level at which your body can't function optimally. At that point, you experience very strong signals from your body, asking you to stop exercising in order to lower your core temperature. However, some athletes have been known to labor on and fight through these strong biological cues, only to end up suffering serious heat illnesses from the very high core body temperatures that result.
Dehydration: another insult
Dehydration is another critical factor that influences your ability to cool yourself during exercise. When it's hot out, sweating is the primary means for eliminating heat generated by contracting muscles, and sweat rates of 34–85 fl oz per hour (1.0–2.5 liters per hour) or more are common. However, for this cooling mechanism to function optimally, the fluids you're losing as sweat need to be replaced. The problem is that during exercise, you typically don't feel a sensation of thirst until after you've lost 1–2% of your body weight as fluid. That equates to 1.5–3.0 lbs. of fluid for a 150-lb athlete (0.68–1.36 kg for a 68-kg athlete). With that amount of fluid loss, you're already in the throes of dehydration; your body's ability to cool itself is undermined because dehydration results in decreased blood flow to the skin and a lower sweat rate. Thus, both mechanisms for thermoregulation are compromised when you're running low on fluids.
Navigate heat and humidity: acclimate first
Fortunately, there are strategies you can employ to help you stay cool when training or competing in hot, humid weather. Acclimating to the heat is an important one.
You can acclimate to the heat by regular exposure to hot environments. A key adaptation that occurs with heat acclimation is an increase in the volume of fluid that circulates in your body. With more fluid available, the heart pumps more fluid with each beat, and this leads to a lower heart rate during exercise. In addition, less sodium is excreted in sweat and urine. The extra sodium retained in your body is useful in maintaining an appropriate sodium concentration in the blood when the fluid volume expands. Interestingly, a low-sodium diet seems to impair the body's ability to expand fluid volume. So if you're trying to acclimate to the heat, make sure you're consuming adequate sodium. Two other critical adaptations include the onset of sweating at a lower core temperature and a higher sweat rate.
The increase in fluid volume and lower heart rate occur within about 3–6 days of daily heat exposure. The decrease in sweat and urine sodium takes about 5–10 days, and the increase in sweat rate and the lower temperature threshold for the onset of sweating and dilation of blood vessels in the skin occur in 1–2 weeks.
Training sessions of about 100 minutes in hot conditions are most effective for inducing heat acclimation, and there is no advantage to spending additional time in the heat. Also, exercising in the heat every third day for 30 days is the acclimatization equivalent of exercising every day for 10 days.
Keep in mind that heat acclimation is not permanent. Effects gradually disappear if they are not maintained by repeated exposure to heat. Adaptations start to disappear in about a week and are mostly gone within 30 days. Also, adaptations to dry heat seem to endure longer than adaptations to the combination of heat and humidity. Finally, the better trained you are endurance-wise, the faster acclimation occurs and the longer the effects are sustained. So if you have a big competition coming up in extreme weather conditions, plan your training accordingly.
Other Important Heat/Humidity Strategies
Timing of competitions and workouts
If there is a lower-temperature or lower-humidity time of day to train or compete, take advantage of it. If a competition is scheduled for smack-dab in the heat of the day, talk to event organizers or coaches and see if the time can be changed to early morning or evening when conditions are more bearable. Encourage your fellow athletes to support you.
Clothing worn while exercising becomes a layer of insulation that interferes with heat transfer from your skin to the environment. It can also hinder the evaporation of sweat, which is the most important route for eliminating heat when it's really hot outside. So as a practical matter, minimize the amount of clothing you wear in hot weather conditions, and make sure it poses the least amount of interference to evaporation.
You might guess that adapting to the heat would decrease your need for fluids, but in reality, the opposite is true. Because you sweat sooner and at a faster rate when you're acclimated to the heat, your fluid needs are higher. Researchers have found that after dehydration takes hold, core body temperatures are the same, whether or not you've acclimated to the heat beforehand. So all those hard-earned advantages of heat acclimation are wiped out if you become dehydrated.
To stay hydrated during exercise, consume fluids at a rate that closely matches sweat rate. This typically requires in the range of 13–26 fl oz (400–800 ml) for every hour of exercise, preferably taken in smaller amounts every 15 minutes or so. However, fluid needs can vary considerably, so determining your sweat rate is the best approach. It's really quite simple, and it's important to calculate your sweat rate for the various environmental conditions you will encounter, including hot and humid conditions. To calculate your sweat rate and to obtain a personalized plan to meet your unique hydration needs, click on the Sweat Rate Calculator at PowerBar.com.
Opt for a sports drink
A sports drink such as Ironman PERFORM™ sports drink offers critical advantages over plain water when it comes to keeping hydrated in the heat and humidity, because a well-designed sports drink contains sodium, carbohydrates, and flavor. Research shows that athletes voluntarily consume more fluids when their beverages are flavored. Sodium and carbohydrates are important because both are actively transported into cells after ingestion, which helps speed fluid absorption. The sodium in a sports drink also encourages you to keep drinking fluids when you're exercising, which is vital to staying hydrated, and it helps you retain the fluid that you've consumed. Pure water, on the other hand, tends to turn off your sensation of thirst, even before your fluid needs have been met. And instead of helping you retain consumed fluids, plain water tends to promote fluid elimination, even when you’re still dehydrated.
Beating the heat
In summary, if heat and humidity are in your training or competition forecast, plan to acclimate to the conditions. Do that by training in a hot environment for about 100 minutes daily for 10 days. Full adaptation usually takes place within about a week or two. Wear minimal clothing in the heat and make sure the clothes you do wear don't interfere with the evaporation of sweat. Avoid dehydration by consuming fluids at a rate that closely matches your sweat rate. And finally, stay hydrated with a well-designed, good-tasting sports drink that features sodium and carbohydrates. Following these strategies gives you the best shot at beating the heat.
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