Learn how exercising in the heat can influence your training, for better or worse.
Between marathon training, recreational pickleball games, and weekend hikes, exercising outdoors in hot and humid conditions is almost inevitable in the summer. And this year is particularly brutal: Hundreds of record-high temperatures have been set in the U.S. this summer, and this past June was the hottest June on record across the globe, according to a NASA analysis.
Not only can excessive heat lead to feelings of anger, irritability, stress, and fatigue, but it can also have serious physical impacts. Here’s how the heat can affect your body and, in turn, your performance and what you can do to stay safe while training in scorching environments.
How Heat Affects Your Body
As homeotherms, humans are constantly working to keep their body temperature around 37°C, or 98.6°F, says Orlando Laitano, Ph.D., an exercise physiologist and an assistant professor in the department of applied physiology and kinesiology at the University of Florida. Your cells have a very small tolerance for shifts in temperature, and any slight change can alter their functioning; an increase to 38°C can cause dysfunction, and by 40°C or 104°F, your cells begin to die, he explains.
To prevent your cells from reaching those dangerous temperatures, your body relies on a handful of mechanisms to regulate your core temperature in hot environments. The most important on dry land is sweat, which cools your body as it evaporates, says Laitano. (There’s also convection — when cool air currents carry heat away from your body — and conduction — when you lose heat through physical contact with a cooler object, but these mechanisms can also make you gain heat, he notes.)
You may not realize it, but sweat is actually made out of blood. “Sweat glands receive a very high density of blood vessels, and the blood is filtrated by the sweat glands,” leaving you with a clear fluid pooling on your skin, says Laitano. “So when you lose sweat, you are actually reducing your blood volume.” As a result of this decline in blood volume, sweating while training in the heat may have potentially harmful or beneficial effects on your body.
Heat’s Impact on Exercise Performance
Remember, your blood carries oxygen and nutrients to your working muscles as you exercise. And through sweating, you’ll have less blood delivering this fuel that’s essential for performance, says Laitano. “If you compare performance in the heat versus performance in a normal environment, you will have a lot [worse] performance in the heat,” he explains.
A compounding problem: humidity, or lack thereof. When humidity is high, your sweat won’t evaporate, preventing your core temperature from cooling and causing you to sweat profusely, he notes. When humidity is low, your sweat evaporates so quickly that you may not notice you’re sweating in the first place. “That is also a risky environment because you are not sensing that you are sweating, so you are less likely to go search for water to minimize that dehydration,” says Laitano. Just like sweating, dehydration reduces the amount of blood available to pump to your muscles, he notes.
Still, there can be some potential benefits to exercising in the heat when the relative humidity is 40 to 60 percent, which allows for sweat to evaporate enough that you maintain your core temperature, says Laitano. It will also be noticeable enough that you’re likely to rehydrate. With less blood to pump to your muscles, your body will increase your heart rate (i.e., pump blood faster) to meet your muscular demands, he explains.
And that increase in heart rate can be used in your favor. “One way to gauge exercise intensity is via heart rate,” says Laitano. “So if you're exercising in the heat, you need a lower external load of exercise to achieve the same internal load of physiological strain.” For example, slow jogging outside on a high-temperature, moderate-humidity day may feel just as challenging as running on a cooler day with the same humidity.
This effect could be particularly beneficial if someone is returning to their training after recovering from an injury or health condition and can’t work at the same external intensity. “You could work with a lower load, but if you are in the heat, that could trigger the same physiological strain to promote adaptations, if you will,” says Laitano.
Lower-intensity heat training can also be useful for athletes preparing for games or competitions in hotter environments. If you live in the Midwest and are training for the Miami marathon, for instance, you might do some of your runs in the hot outdoors to acclimate to the conditions, which can help reduce the heat’s negative impacts on performance on race day, he says.
Regardless, if you’re in a hot environment and sweating isn’t enough to prevent your core temperature from rising, you could experience heat stroke, says Laitano. This serious heat illness can cause symptoms such as confusion or altered mental status, loss of consciousness, and seizures, and it may lead to permanent disability or death without emergency medical care, according to the Centers for Disease Control and Prevention.
How to Safely Exercise in the Heat
To maintain that optimal body temperature while exercising in the heat, wear clothes that are moisture-wicking, lightweight, and loose-fitting, which will allow your sweat to evaporate. You’ll also want to stay on top of your fluid intake to prevent dehydration. There’s no one-size-fits-all recommendation for the amount to consume, as everyone sweats at different rates, says Laitano. However, you can figure out how much you personally need by measuring your weight before and after exercise, he notes.
Roughly 98 percent of the body mass you lose during exercise is from water via sweat, and the goal is to limit the amount of weight you lose to 2 percent of your total body mass, says Laitano. Any more than that, and you’re likely to experience more of the negative effects of dehydration, he adds. Say you weigh 150 pounds before exercise. If you hydrate properly as you work out, you should weigh no less than 147 pounds when you’re finished.
Importantly, continue to drink even if you don’t feel thirsty. If you reach that point while exercising, you’re likely already dehydrated. “You have already compromised delivery of oxygen and nutrients to the muscles,” says Laitano.
As you sweat, you’ll also lose electrolytes, particularly sodium and chloride, says Laitano. Electrolytes are minerals in your body that have an electric charge, and they help balance the amount of water in your body and your pH level, among other functions, according to the National Library of Medicine. In hot environments, this sweat can lead to excessive electrolyte imbalances that may impair performance, research suggests.
Just like water, everyone loses electrolytes in different amounts, but it's recommended that all people exercising in the heat consume sports drinks containing electrolytes to prevent an imbalance, says Laitano. Oftentimes, these beverages are also isotonic, meaning the tonicity is similar to that of your blood, so they’ll assist in re-hydration, too. Hypertonic drinks (which are high in tonicity, such as orange juice or soda) will cause your gastrointestinal system to pull water out of your bloodstream, increasing the risk of dehydration and GI discomfort, he notes.
Even if you take these precautions, there’s no shame in pausing your workout or taking it indoors if the heat seems to be negatively affecting your body or performance. Preserving your health is much more important than finishing an outdoor training session.