Substrate source use in older, trained males after decades of endurance training

BOON, H., R. A. M. JUNKERS, R. KOOPMAN, E. E. BLAAK, W. H. M. SARIS, A. J. M. WAGENMAKERS, and L. J. C. VAN LOON. Substrate Source Use in Older, Trained Males after Decades of Endurance Training. Med. Sci. Sports Exerc., Vol. 39, No. 12, pp. 2160-2170, 2007. 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-C-13]palmitate and [6,6-H-2(2)]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 I 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 nioderate-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.