Environmental stressors, such as heat or altitude, elicit dissimilar physiological adaptations to endurance training programs. Whether these differences (i.e., increased hemoglobin mass vs plasma volume) differentially influence performance is debated. We review data in support of our novel hypothesis, which proposes altitude as the preferred environmental training stimulus for elite endurance athletes preparing to compete in temperate, sea-level climates (5°C-18°C). Various practices of altitude training have been used by athletes as a means to improve sea-level endurance performance. With recent world stage competitions held in hot ambient climates, debate has developed over whether heat training provides equivalent benefits for endurance performance. Altitude acclimatization primarily enhances oxygen carrying capacity, which has the potential to improve performance among endurance-trained athletes. Rather, the primary adaptation elicited by heat training is plasma volume expansion, which has been associated with improvements in maximal aerobic capacity — one determinant of endurance exercise performance — in untrained, but not trained, individuals. Of note, effective heat and altitude training practices both require careful planning from coaches and athletes and may present logistical and financial challenges that should be considered against the potential benefits for performance.
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