Free fatty acid metabolism in the forearm at rest: muscle uptake and adipose tissue release of free fatty acids.

Free fatty acid (FFA) metabolism in forearm skeletal muscle at rest was studied during continuous intrabrachial arterial ininfusion of [1-14C] oleic acid in 14 normal subjects in the postabsorptive state. Total oleic acid uptake by muscle accounted for at least 65% and perhaps as much as 86% of the measured O2 consumption. The respiratory quotient (0.74) indicated predominant oxidation of lipid. Glucose was a minor substrate for oxidation. Concentrations of six individual free fatty acids were measured in arterial (A), deep venous (DV), and superficial venous (SV) plasma in 25 subjects. There were positive A-DV differences, reflecting net muscle uptake, for each FFA. The extraction ratio (A-DV)/A was greatest for oleic acid. Output of each free fatty acid from subcutaneous adipose tissue was proportional to its arterial concentration, except for stearic acid. There was no stearic acid output. Correction of (A-DV)/A to account for contribution of perimuscular adipose tissue to FFA in DV led to the conclusion that oleic acid was preferentially extracted from arterial blood by forearm muscle. It accounted for 51% of the total FFA uptake, although it comprised only 43% of total FFA in arterial plasma. Total FFA uptake by muscle was more than sufficient to account for all of the measured O2 consumption, suggesting that some of the extracted FFA may be stored in muscle for oxidation at a latter time.