Purpose: The purpose of this study was to compare substrate source use in older, long-term exercising, endurance-trained males with sedentary controls.
Methods: [U-13C]palmitate and [6,6-2H2]glucose tracers were applied to assess plasma free fatty acid (FFA) and glucose oxidation rates, and to estimate muscle- and/or lipoprotein-derived triacylglycerol (TG) and muscle glycogen use. Subjects were 10 long-term exercising, endurance-trained males and 10 sedentary controls (age 57 ± 1 and 60 ± 2 yr, respectively). Muscle biopsy samples were collected before and after exercise to assess muscle fiber type-specific intramyocellular lipid and glycogen content.
Results: During exercise, plasma palmitate Ra, Rd, and Rox were significantly greater in the trained subjects compared with the controls (Ra: 0.36 ± 0.02 and 0.25 ± 0.02; Rd: 0.36 ± 0.03 and 0.24 ± 0.02; Rox: 0.31 ± 0.02 and 0.20 ± 0.02 mmol·min−1, respectively, P < 0.01). This resulted in greater plasma FFA and total fat oxidation rates in the trained versus sedentary subjects (P < 0.001). Muscle- and/or lipoprotein-derived TG use contributed 10 ± 2 and 11 ± 3% in the trained and control groups, respectively (NS). No significant net changes in muscle fiber lipid content were observed.
Conclusions: Older, endurance-trained males oxidize more fat during moderate-intensity exercise than do sedentary controls. This greater total fat oxidation rate is attributed to a higher plasma FFA release, uptake, and oxidation rate. In contrast, intramyocellular triacylglycerol does not seem to represent a major substrate source during 1 h of moderate-intensity exercise in older trained or sedentary men.