Non-esterified long-chain fatty acid metabolism in fed sheep at rest and during exercise

The role of circulating, non-esterified, long-chain fatty acids (NEFA) as a source of energy for the whole animal and skeletal muscle was investigated in fed non-pregnant sheep at rest and during exercise. Infusion of tracer quantities of [1-14C]oleic or [1-14C]stearic acid was combined with the use of arteriovenous difference studies on fed sheep at rest or during a 2 h period of exercise on a belt treadmill moving at 4.5 km h-1. At rest all parameters of NEFA metabolism indicated a minimal role for oxidation. Thus the concentration in plasma (0.07 +/- 0.01 mmol l-1), entry rate (0.08 +/- 0.02 mmol h-1 kg-1 body wt), contribution to whole animal oxidation (1.2 +/- 0.3%) and utilization of NEFA by skeletal muscle (0.046 +/- 0.008 mmol h-1 kg-1 muscle) were all low. Exercise prompted a shift to lipolysis and accordingly the above parameters increased markedly some 13-24-fold. The circulating concentration of ketone bodies showed only a small increase during exercise and consequently the role of ketone bodies as an energy source during exercise was minimal. Glucose utilization by skeletal muscle was considerable in animals at rest and it represented the most significant potential fuel of skeletal muscle. Exercise resulted in a sustained increase of 3-4-fold in the utilization of glucose by skeletal muscle. Thus the traditional view that NEFA and not glucose is a predominant fuel of skeletal muscle of fed sheep should be appraised.