Metabolism in exercise

The transition from the resting state to the exercising state is mediated by several important physiological responses designed primarily to meet the increased energy demands on the muscles. These responses involve the cardiovascular, respiratory, central nervous and endocrine systems, and have been reviewed in detail by other authors. For comparative purposes, we will summarize the metabolic and hormonal changes in this paper. In the resting state, the principal metabolic energy source for skeletal muscle results from oxidation of free fatty acid (FFA). With the initiation of exercise, carbohydrate sources become more prominent as important energy fuels. Plasma glucose remains within a remarkably narrow range when exercise is performed at a low to moderate intensity, 1.e., 30 percent VO (2max). This is achieved by a synchronous matching of glucose production with glucose utilization in concert with mobilization of other energy sources. In the first few seconds to minutes of exercise, when increased oxygen transport from the circulation is just beginning, the anaerobic breakdown of muscle glycogen t form lactate provides an immediate source of high energy phosphate compounds necessary for the contractile process. However, as one might anticipate, muscle glycogen provides a limited energy source and, with continued exercise, blood-borne fuels from liver and the adipose organ must be utilized. The principal blood-borne oxidative fuels for contracting muscles are glucose from the liver and free fatty acids from adipose tissue, while the contribution of amino acids and ketone bodies to energy balance is relatively insignificant. The pattern of fuel utilization and the metabolic response to exercise is determined by several variables including the duration and intensity of exercise, the degree of cardiovascular fitness, and the nutritional hormonal status. The contribution of carbohydrate as the primary energy source for muscle contraction increases as the intensity of exercise increases. However, with increasing duration of exercise, free fatty acids are preferentially utilized as an energy-providing substrate. Exercise training results in an increased ability to utilize blood-borne glucose and, to a greater extent, free fatty acid as an energy source for increasing workloads, sparing muscle glycogen (AU)