A shift to more fat utilization and muscle glycogen conservation are typical responses to energy deficit. In contrast, energy balance was maintained and weight loss prevented in the studies by Brooks and colleagues to deliberately isolate the impact of hypoxia from energy deficit. The fact that energy deficit can obscure the effects of “true” hypoxia suggests that it is a more potent metabolic stimulus. Taken together, these data have led to novel research directly testing the independent effects of hypoxia and energy deficit54 and also provided key insights that inform practical recommendations for nutrition at altitude.
Several environmental factors also change fuel preferences. It is often the case that with a new Olympic cycle, a new environmental factor gets increased attention. Before the high altitude of Mexico City, research on exercise at altitude was popular, whereas the Sydney and Athens games stimulated research on exercise in the heat. In general, exercise in both heat and cold increases glycogen use. A legitimate concern for the 2008 Olympics in Beijing was air quality. Although it is not clear whether air pollutants alter substrate use, it is possible that medications used to treat asthma, such as Clenbuterol, do change fuel preference.