Source of fat oxidation in exercising dogs

Dogs with indwelling arterial and venous catheters ran on a treadmill for 30–40 min. Palmitate-1-C 14 was infused intravenously. The rate of FFA uptake increased linearly with FFA concentration during rest and exercise. At rest 20– 22% of the FFA uptake was oxidized; during exercise this figure rose to 80–90%. About 50% of the fat-CO 2 could be ascribed to immediate oxidation of plasma FFA. The radioactivity and specific activity of the plasma EFA showed no marked change during exercise. When a 3-hr infusion was stopped 5 min before exercise, the radioactivity in the plasma FFA fraction decreased rapidly in the first 10 min of exercise, followed by a very low but constant disappearance rate. The disappearance of radioactivity from the plasma (FFA and EFA combined) during exercise was less than 1% of the C 14 O 2 output. Therefore, it was concluded that the intracellular fat pool plays an important role as an energy source, whereas the plasma EFA did not seem to participate appreciably in the 6-fold elevated metabolism.