Balance of carbohydrate and lipid utilization during exercise: The 'crossover' concept

Abstract

The 'crossover' concept represents a theoretical means by which one can understand the effects of exercise intensity and prior endurance training on the balance of carbohydrate (CHO) and lipid metabolism during sustained exercise. According to the crossover concept, endurance training results in muscular biochemical adaptations that enhance lipid oxidation as well as decrease the sympathetic nervous system responses to given submaximal exercise stresses. These adaptations promote lipid oxidation during mild- to moderate-intensity exercise. In contrast, increases in exercise intensity are conceived to increase contraction-induced muscle glycogenolysis, alter the pattern of fiber type recruitment, and increase sympathetic nervous system activity. Therefore the pattern of substrate utilization in an individual at any point in time depends on the interaction between exercise intensity- induced responses (which increase CHO utilization) and endurance training- induced responses (which promote lipid oxidation). The crossover point is the power output at which energy from CHO-derived fuels predominates over energy from lipids, with further increases in power eliciting a relative increment in CHO utilization and a decrement in lipid oxidation. The contemporary literature contains data indicating that, after endurance training, exercise at low intensities (≤45% maximal O2 uptake) is accomplished with lipid as the main substrate. In contrast, the literature also contains reports that are interpreted to indicate that during hard-intensity exercise (~75% maximal O2 uptake) CHO is the predominant substrate. Seen within the context of the crossover concept these apparently divergent results are, in fact, consistent. Because in their training and competition most athletes perform at intensities that elicit >70-75% of maximum aerobic power, they are dependent on CHO for energy. Furthermore, lipid becomes the predominant fuel during recovery from exercises that result in glycogen depletion.