Human Forearm Muscle Metabolism during Exercise VII. FFA Uptake and Oxidation at Different Work Intensities

Abstract

The influence of the work intensity on the FFA metabolism of the forearm muscle was studied during rhythmic exercise for 30 min in 7 subjects. Each subject exercised at 5 kpm/min and at 10 or 15 kpm/min. The rate of FFA uptake was the same at the two work loads and did, at the lower load, equal the rate of fat oxidation. At the heavier work intensity, fat oxidation exceeded FFA uptake, suggesting utilization of intramuscular lipid stores. Fractional oxidation of FFA was close to 100 per cent for exercise at 5 kpm/min but, on the average, 67 per cent at the heavier work load. The radioactivity not leaving the muscle as [¹⁴C]O₂ was quantitatively recovered in a perchloric acid extract of venous blood, containing labeled acetate, 3-hydroxybutyrate, 2-oxoglutarate, and citrate. The results suggest that partial fatty acid oxidation as well as lactate production by muscle at high work intensity are caused by a breakdown of substrates to two or three carbon fragments in excess of the mitrochondrial oxidative capacity.

Key Words:

• Acetate
• exercise
• fatty acids
• forearm
• metabolism
• muscle
• oleic acid
• palmitic acid