Nutrition and metabolism

Abstract

Like studies on isolated cells and tissues studied under hypoxic conditions, studies on men and women at altitude show a preference for carbohydrate-derived fuel energy sources (glycogen, glucose, and lactate). When dietary energy is adequate to cover need, at altitude working muscle utilizes little lipid. Classical concepts of a Lactate Paradox invoking a Pasteur Effect of presumed anaerobic metabolism are not supported by studies utilizing contemporary techniques. If anything, lactate shuttling is prominent at altitude, with lactate being produced in diverse tissue beds and serving at least two functions at altitude: lactate is a preferred fuel in working muscle and lactate is the major gluconeogenic precursor in support of glycemia at altitude. A third role of lactate, i.e., that of promoting cellular adaptations at altitude by increasing HIF-1 expression, is also suggested in the literature. While controlled laboratory studies show clear preference for carbohydrate-derived fuels under hypoxic conditions, cachexia is common among mountaineers. Under the stresses of altitude, loss of appetite and dietary energy insufficiency result in body wasting. Relative to energy content, carbohydrate foods are less efficient to carry or transport and CHO foods typically require water for cooking. Consequently, the tendency is to carry energy-dense, high-fat and protein foods at altitude. Because of the disparity between needs of the CNS and peripheral nerves and working muscles for CHO energy sources and dietary CHO supply, at altitude lean tissue is mobilized to supply gluconeogenic precursors and adipose is mobilized for glycerol (a lesser gluconeogenic precursor) and fatty acids (a less-preferred, but available energy substrate). The accomplishments of mountaineers are remarkable in many ways. That mountaineers accept and manage the challenge of working under multiple stresses, while having to rely on less-preferred fuel energy substrates is one more example of their extraordinary accomplishments.