Comp Biochem Physiol B Biochem Mol Biol. 1998 May;120(1):89-107
Department of Integrative Biology, University of California, Berkeley 94720-3410, USA. GBrooks@Socrates.Berkeley.Edu
The 'crossover' and 'lactate shuttle' concepts of substrate utilization in humans during
exercise are extended to describe metabolic responses on other mammalian species.
The 'crossover concept' is that lipid plays a predominant role in sustaining efforts
requiring half or less aerobic capacity (VO2max); however, greater relative efforts
depend increasingly on blood glucose and muscle glycogen as substrates. Thus, as exercise
intensity increases from mild to moderate and hard, 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 free fatty acid flux
is described as an inverted hyperbola with vertex at approximately 50% VO2max. Both
endocrine and intra-cellular factors play critical roles in determining substrate balance
during sustained exercise. Moreover, genotypic adaptation for aerobic capacity 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 well as other conditions of accelerated glycolysis, glycolytic flux in
muscle involves lactate formation regardless of the state of oxygenation. Further,
according to the lactate shuttle concept, lactate represents a major means of distributing
carbohydrate potential energy for oxidation and gluconeogenesis. In humans and other
mammals, the formation, distribution and disposal of lactate (not pyruvate) represent
key steps in the regulation of intermediary metabolism during sustained exercise.