The effect of an increased free fatty acid concentration on thermogenesis and substrate oxidation in obese and lean men

OBJECTIVE: To examine whether a certain increase in plasma free fatty acid (FFA) concentration leads to similar increases in lipid oxidation and energ y expenditure in obese and lean men. DESIGN: The study protocol consisted of a 30 min baseline period after whic h subjects received an i.v. bolus of 1000 IE heparin. Then consecutive infu sions of 4.9, 9.8 and 19.6 mul/kg fat-free mass (FFM) min of a lipid hepari n mixture were started, each infusion for 30 min. SUBJECTS: Eleven obese and 13 lean men with a mean body mass index (BMI) of 34.2 +/- 1.0 (+/- s.e.m.) and 23.9 +/- 0.5 kg/m(2) and age 46.0 +/- 1.0 an d 42.6 +/- 1.5 y, respectively. MEASUREMENTS: Energy expenditure, respiratory exchange ratio (RER) and carb ohydrate and lipid oxidation were continuously measured by indirect calorim etry. At the end of each infusion period, a blood sample was taken for FFA, glycerol, insulin, beta -hydroxybutyrate, noradrenaline and adrenaline det ermination. RESULTS: At baseline, plasma FFA levels were comparable in both groups. Lip id heparin infusion increased plasma FFA concentration by 301 +/- 47 mu mol /l and 332 +/- 27 mu mol/l in obese and lean men. Energy expenditure increa sed similarly in obese and lean men (0.34 +/- 0.08 vs 0.40 +/- 0.08 kJ/min, NS) during lipid heparin infusion, whereas RER decreased similarly in both groups. Lipid oxidation rates were comparable at baseline and increased si milarly in obese and lean men (19 +/- 5 vs 13 +/- 4 mg/min, NS). Baseline p lasma insulin levels were higher in the obese, but did not change during li pid heparin infusion. Plasma beta -hydroxybutyrate concentrations were simi lar at baseline, but increased significantly less in the obese during lipid heparin infusion. Baseline noradrenaline and adrenaline concentrations did not differ significantly between groups. During lipid heparin infusion, pl asma noradrenaline levels decreased significantly, but plasma adrenaline le vels remained unchanged in both groups. CONCLUSION: A certain increase in plasma FFA concentration leads to similar increases in lipid oxidation and energy expenditure in obese and lean men. The accumulation of fat in obese subjects may therefore be more likely to be due to a defect in adipose tissue lipolysis than a defect in lipid oxida tion.