Rates of free fatty acid appearance and fat oxidation in healthy younger and older men

Alterations in the mobilization and oxidation of fat may partially account for age-related alterations in body composition. To investigate age-related alterations in fat metabolism, we compared basal rate of appearance of free fatty acids (FFA(app)) and total body fat oxidation as measured by infusions of [C-14]palmitate and indirect calorimetry, respectively, in 18 younger (23 +/- 1 yr) and 30 older (69 +/- 1 yr) men. We also examined whether age-related differences in body composition, body fat distribution, peak oxygen consumption, dietary intake, and/or fasting insulin levels may explain age-related variation in FFA(app) and total body fat oxidation. The FFA(app) showed a tendency to be higher in older compared with younger men (1,134 +/- 184 vs. 680 +/- 105 mu mol/min; P = 0.07), whereas total body fat oxidation was similar between groups (257 +/- 25 vs. 222 +/- 9 mu mol/min). The estimated rate of nonoxidative disposal of free fatty acids showed a tendency to be higher in the older (913 +/- 182 mu mol/min) than in younger men (423 +/- 103 mu mol/min; P = 0.06). Fat-free mass was the most significant predictor of FFA(app) in younger (r = 0.63; P < 0.01) and older (r = 0.41; P < 0.05) men. These results suggest that older men recruit fatty acids from adipose tissue stores in excess of the energy needs of respiring tissue. However, variation in FFA(app) between the age groups could not be explained by differences in body habitus or fasting insulin levels.