MAMMALIAN FUEL UTILIZATION DURING SUSTAINED EXERCISE

The 'crossover' and 'lactate shuttle' concepts of substrate utilizatio n in humans during exercise are extended to describe metabolic respons es on other mammalian species. The 'crossover-concept' is that lipid p lays a predominant role in sustaining efforts requiring half or less a erobic capacity ((V)over dot O-2max); however, greater relative effort s depend increasingly on blood glucose and muscle glycogen as substrat es. Thus, as exercise intensity increases from mild to moderate and ha rd, fuel selection switches (crosses over) from lipid to carbohydrate dependence. Glycogen and glucose catabolic rates are best described as exponential functions of exercise intensity, but with a greater gain in slope of the glycogen than glucose response. in contrast, plasma fr ee fatty acid flux is described as an inverted hyperbola with vertex a t approximately 50% (V) over dot O-2max. Both endocrine and intra-cell ular factors play critical roles in determining substrate balance duri ng sustained exercise. Moreover, genotypic adaptation for aerobic capa city as well as phenotypic adaptations to short- and long-term chronic activity affect the balance of substrate utilization during exercise. The concept of a 'lactate shuttle' is that during hard exercise, as w ell as other conditions of accelerated glycolysis, glycolytic flux in muscle involves lactate formation regardless of the state of oxygenati on. Further, according to the lactate shuttle concept, lactate represe nts a major means of distributing carbohydrate potential energy for ox idation and gluconeogenesis. In humans and other mammals, the formatio n, distribution and disposal of lactate (not pyruvate) represent key s teps in the regulation of intermediary metabolism during sustained exe rcise. (C) 1998 Elsevier Science Inc. All rights reserved.