FAT-METABOLISM IN HUMAN OBESITY

Excessive fat turnover and oxidation might cause the insulin resistance of carbohydrate metabolism in obese humans. We studied the response of free fatty acid (FFA) metabolism in lean and obese volunteers to sequential insulin infusions of 4, 8, 25, and 400 mU.m(-2).min(-1). The insulin dose-response curves for suppression of FFA concentration, FFA turnover ([1-C-14]palmitate), and lipolysis ([H-2(5)]glycerol) were shifted to the right in the obese subjects (insulin concentrations that produced a half-maximal response, lean vs. obese: 103+/-21 vs. 273+/-41, 96+/-11 vs. 264+/-44, and 101+/-23 vs. 266+/-44 pM, all P < 0.05), consistent with insulin resistance of FFA metabolism in obesity. After the overnight fast, FFA turnover per fat mass was decreased in obese subjects (37+/-4 vs. 20+/-3 mu mol.kg fat mass(-1).min(-1), P < 0.01) as the result of suppression of lipolysis by the hyperinsulinemia of obesity and an increased fractional reesterification of FFA before leaving the adipocyte (primary FFA reesterification; 0.14+/-0.03 vs. 0.35+/-0.06, P < 0.05). Nevertheless, FFA turnover per fat-free mass (FFM) was also greater in the obese volunteers (8.5+/-0.7 vs. 11.0+/-1.0 mu mol.kg FFM(-1).min(-1), P < 0.05) but only as the result of increased reesterification of intravascular FFA (secondary reesterification; 1.8+/-0.5 vs. 4.8+/-1.1 mu mol.kg FFM(-1).min(-1), P < 0.01), since FFA oxidation was the same in the two groups throughout the insulin dose-response curve. In conclusion, FFA metabolism is resistant to insulin in obesity. However, the hyperinsulinemia of obesity and increased fractional primary FFA reesterification reduce FFA turnover per fat mass in obese humans. FFA oxidation is not increased in obesity and therefore is not the cause of the insulin resistance of carbohydrate metabolism.