Altitude and Climate

In response to hypoxia, a shift in substrate use to favor greater dependency on glucose and less utilization of lipid has been reported in a variety of experimental models. In young men, both whole-body and leg glucose uptake rose sharply upon acute exposure to hypobaric hypoxia (4300 meters on Pikes Peak in Colorado) and remained elevated above sea level values even after 18 days of acclimatization. In other studies, however, exposure to hypobaric hypoxia has not resulted in a greater dependency on glucose. In one of the first studies to systematically evaluate substrate utilization at high altitude, Young et al. reported the opposite, i.e., reduced dependence on carbohydrate and greater use of fat after several weeks’ acclimatization. The discrepancy between these results and those from Brooks and colleagues is likely a result of different energy states. Depressed energy intake and elevated resting metabolism, both common responses to hypobaric hypoxia, cause energy deficit and weight loss.

 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.