Regulation of skeletal muscle fat oxidation during exercise in humans.

Fat and carbohydrate are the major energy substrates during aerobic exercise in well-fed humans. The regulation of fat metabolism during exercise has not been as thoroughly studied as carbohydrate metabolism, especially in human skeletal muscle. Traditionally, it was believed that the regulation of skeletal muscle fat metabolism was mainly at the level of the delivery of free fatty acids to the muscle (adipose tissue lipolysis) and transport of the long chain fatty acids into the mitochondria. It is now known that the transport of fatty acids into the muscle cell and the regulation of muscle triacylglycerol lipase activity are also important sites of regulation. New lines of research are currently underway examining the regulation of fat metabolism in skeletal muscle at the level of fat transport across the sarcolemmal and mitochondrial membranes and regulation of TG lipase activity in both rodent and human models. A major goal of this research is to determine the regulatory signals that control the up-regulation of fat metabolism during the transition from rest to low and moderate aerobic exercise (30-65% (.)VO(2max)) and the down-regulation that occurs when exercising at intense aerobic exercise (approximately 85% (.)VO(2max)). Although it is expected that the signals that activate carbohydrate metabolism during exercise (Ca and free ADP, AMP, and P(i)) would also play a role in fat metabolism, this has not been demonstrated to date.