Altitude or heat training to increase haemoglobin mass and endurance exercise performance in elite sport

High-altitude training has been a well-integrated tool in elite endurance sport for decades. The underlying premise is that physiological responses to high-altitude exposure, such as an increase in haemoglobin mass, are correlated with exercise performance. In this review, we highlight that physiological adaptations to a given magnitude of hypoxic exposure induce a highly variable range of responses in humans. For haemoglobin mass, this can vary from near nothing to previously unseen augmentations. At present, however, it remains inconclusive whether responses observed after one high-altitude training camp can be reproduced at the individual level during a subsequent camp. Exercise training performed in a warm environment or by wearing heat-capturing garments in a cooler environment has been termed 'heat training' and has, in recent years, become an increasingly popular alternative to altitude training. After a 3- to 5-week period of heat training, a robust increase in haemoglobin mass has repeatedly been reported, although the associated performance gains are only small. Accordingly, heat training should probably not be implemented before all other steps involved in elite training are in place and, importantly, heat training should not be performed at the expense of other important aspects of elite life. Thus, heat training might serve as a substitute for altitude training. Ultimately, it can be recommended that elite athletes experiment with either approach and that the choice of implementation should be made at the individual level, ensuring that the applied stress (hypoxia or heat) is tolerated adequately by the athlete.