Balance of carbohydrate and lipid utilization during exercise: the "crossover" concept

and lipid utilization during exercise: the “crossover ” concept. J. AppZ. Phys-ioL. 76(6): 2253-2261,1994.-The “crossover ” concept represents a theoreti-cal 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 en-hance lipid oxidation as well as decrease the sympathetic nervous system responses to given submaximal exercise stresses. These adaptations pro-mote lipid oxidation during mild- to moderate-intensity exercise. In con-trast, 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 pro