Endurance exercise training increases the muscle's ability to metabolize fat by which mechanism?

• A. Increasing the volume of mitochondria in the muscle fiber
• B. Increasing glycolytic enzyme activity
• C. Decreasing capillary density
• D. Reducing mitochondrial content

Answer: (A)

Endurance training raises fat metabolism mainly by increasing mitochondrial content in muscle fibers. With more mitochondria, the muscle has a larger oxidative capacity, including enhanced beta-oxidation of fatty acids and greater flux through the TCA cycle and electron transport chain. This means fatty acids can be oxidized more efficiently to produce ATP during sustained activity, helping spare glycogen.

Other options don’t fit as well because increasing glycolytic enzymes would favor carbohydrate use, not fat; decreasing capillary density would impede oxygen and substrate delivery needed for fat oxidation; and reducing mitochondrial content would lower oxidative capacity.