Role of fats in exercise. Types and quality.

Plasma TGs, FFAs, and muscle TG are oxidizable lipid fuel sources for skeletal muscle metabolism during prolonged exercise. Plasma FFAs are a major fuel oxidized by skeletal muscle, and their rate of use by muscle depends on several factors, including plasma FFA availability, transport from plasma to the mitochondria, and intracellular metabolism. Mobilization of FFAs from adipose tissue is the first committed step in FFA metabolism, and it depends on the rate of adipose tissue lipolysis. Adipose tissue lipolysis increases with exercise duration and exercise intensity up to intensities of approximately 60% to 65%. Evidence suggests that FFAs are transported from plasma to the mitochondria by FFA transporter proteins that include the plasma membrane and cytosolic FABPPM and FABPC. Plasma FFA use can also be regulated at the mitochondrial transport step by changing the activity of carnitine palmitoyltransferase (CPT-1). Although results from biopsy and tracer studies indicate that muscle TG contribute to skeletal muscle oxidative metabolism during exercise, their exact contribution is difficult to ascertain. Evidence shows that muscle TG use depends on exercise intensity, duration, and mode. The contribution of plasma TG to skeletal muscle metabolism is small. The rate of use of plasma TG is dependent on lipoprotein lipase activity, which is correlated with the oxidative capacity of the muscle fibers. Dietary manipulations can modulate substrate use during exercise and can potentially affect exercise performance. High carbohydrate availability before exercise is associated with an increase in blood glucose and plasma insulin concentrations, which can ultimately decrease the rate of adipose tissue lipolysis and the availability of plasma FFAs. Increased glucose flux has also been shown to decrease lipid oxidation by directly inhibiting the transport of FFAs across the mitochondrial membranes. High lipid availability can be changed by short-term or long-term exposure to high-fat diets. Because carbohydrate reserves are diminished with exposure to high-fat diets, improvements in exercise performance have been difficult to measure under these conditions.