Free fatty acids and exercise.

Although the great explorers were well aware that eating fat was an efficient way to meet their large energy demand, it was not until some decades into this century that it could be demonstrated that lipids are metabolized directly by contracting skeletal muscles. The 1950s produced the first studies with [14C]-tagged fatty acids (FAs), proving that fat is transported into the cell as FAs. An FA-transporting protein that is present in the sarcolemma and in the cytoplasma has been identified. For FA transport into the mitochondria, carnitine and carnitine transferase are needed. It is still unclear how the use of lipids as an energy source for the muscle during exercise is limited. The supply of free fatty acids (FFAs) far exceeds what is taken up by the muscle. Seldom more than 2-4% of the amount of FFAs delivered to an exercising limb is taken up by the muscles and only part of it is oxidized. Physical training induces changes that enhance the uptake of FAs by the contracting muscles, and a larger fraction of this uptake is oxidized, but it is not yet clear which mechanism is behind this adaptation. What is known is that this uptake occurs despite no elevation in the amount of FA supplied to the limb.