One of the highest sweat rates ever recorded was that of marathon runner Alberto Salazar at the 1984 Olympics in Los Angeles. In the months leading up to the games, which were expected to be oppressively hot, the marathoner was put through a regimen of temperature acclimation training with the goal of helping him adapt to running in the heat. While Salazar placed only 15th overall, the program was deemed a success, physiologically speaking—vitals taken after the race found that Salazar’s hormonal and thermoregulatory systems were completely normal. His body had compensated by causing him to sweat at an incredibly high rate—about three liters per hour, compared to the roughly one liter per hour for an average human.
Researchers have been looking at the effects of heat on athletic performance for decades, and their results have been consistently surprising. Studies have found that, in addition to an increased rate of perspiration, training in the heat can increase an athlete’s blood plasma volume (which leads to better cardiovascular fitness), reduce overall core temperature, reduce blood lactate, increase skeletal muscle force, and, counterintuitively, make a person train better in cold temperatures. In fact, heat acclimation may actually be more beneficial than altitude training in eliciting positive physiological adaptations, says Santiago Lorenzo, a professor of physiology at Lake Erie College of Osteopathic Medicine and a former decathlete at the University of Oregon. “Heat acclimation provides more substantial environmental specific improvements in aerobic performance than altitude acclimation,” he says. And in contrast to the live low, train high philosophy, we more quickly adapt to heat stress than we do to hypoxia. In other words, heat training not only does a better job at increasing V02 max than altitude, but it also makes athletes better at withstanding a wider range of temperatures.
Athletes can adapt to heat in one of two ways. The first is through incremental improvements in tolerance over time—work out in the heat a little bit every day, and eventually your body will dissipate heat more effectively. The second way is through thermotolerance, which is a cellular adaptation to an extreme heat experience, like suffering such severe dehydration after a run that you need an IV. Essentially, if you shock your system, your body will be able to withstand greater temperature stresses later on. But successful heat adaptation is difficult—and clearly dangerous—to achieve outside of controlled settings. Lorenzo explains that performance gains are possible only when athletes elevate their core body temperature, and without careful monitoring, it’s possible to elevate your core temperature to lethal levels.
When performed safely, however, heat training can have extraordinary effects. This phenomena fascinates Chris Minson, a professor of human physiology at the University of Oregon, who studies heat acclimation responses in athletes. According to his research, heat training can expand blood plasma volume, but Minson says there also seem to be inexplicable changes to the heart’s left ventricle, which helps to increase oxygen delivery to the muscles. In addition, he says that athletes who train in warm temperatures generally get better at regulating heat by sweating earlier, as Salazar did, or developing a colder resting body temperature.
A 2011 study by a group of researchers in New Zealand also found that overall volume of blood plasma increased at a greater rate when athletes did not drink water during exercise. While some coaches are carefully experimenting with dehydration, Minson and Lorenzo are not because it adds too much additional stress. However, they do say that this type of training can be beneficial because it produces a higher number of “heat shock” protein cells.
Ahead of Western States this June, ultrarunning coach Jason Koop worked on heat training with Amanda Basham and eventual winner Kaci Leckteig. Koop believes this type of acclimating is a good example of blending an academic concept with real-world training. But, says Koop, “at a certain level, you have to compromise training quality for the heat acclimation. Acclimating to the heat is additional stress [on the body], just like more miles or intervals, so you can’t simply pile it on. Something on the training side has to give.”
One method of heat acclimation that Minson uses with his athletes is to do hard workouts on colder days or earlier in the morning, and then start training in hotter conditions with less intensity. He is also looking into adding heat in ways that wouldn’t require an athlete to train in high temperatures at all—using hot tubs, for instance.
All this being said, not everyone responds to heat at the same rate or with the same physiological gains, which makes it similar to altitude training in that it might make a high-performing age grouper, college athlete, or elite a little better, but it won’t compensate for intelligent, consistent training.
How to Incorporate Heat Acclimation into Your Training Schedule
When acclimating to heat, you’ll be forced to compromise training quality, says Koop. While he understands the benefits of heat acclimation, he still prioritizes smart, solid training. But if you want to incorporate heat into your workouts, here’s how he recommends doing it safely.
1. First, pick a protocol (sauna, hot bath, or exercising in the heat) that minimizes the impact on training, both physically and logistically.
2. Koop most commonly recommends that his athletes use a dry sauna immediately after running. “It doesn’t impact training nearly as much as running in the heat, and the effects are similarly positive,” he says. He often tells his athletes to not drink water during these sessions to enhance the effect. Koop recommends spending 20-to-30-minutes in the sauna, depending on tolerance.
3. Koop says that when he has his athletes exercise in the heat—either naturally or by wearing extra clothing to simulate the experience—it will be on a long, slow day for 60 to 90 minutes. The time completely depends on the athlete’s tolerance and previous experience. But he stresses to not do this on a recovery day, because heat training is an added stress on the body. Koop recommends drinking 30 to 40 ounces of an electrolyte drink per hour during these sessions And for safety, he advises using low-traffic sidewalks and bike paths—not trails.
4. Despite the benefits of heat training, Koop reminds his athletes that running in the heat is extremely difficult and usually replaces a hard day. “You are substituting one potential gain for another one,” he says. In other words, use it carefully.