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. 

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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. 

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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.

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Ahead of Western States this June, ultrarunning coach Jason Koop worked on heat training with Amanda Basham and eventual winner Kaci Lickteig. 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.”

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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. 

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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.