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.
1Department of Human Biology, Nutrition and Toxicology Research Institute Maastricht, Maastricht University, Maastricht, THE NETHERLANDS; 2Department of Movement Sciences, Nutrition and Toxicology Research Institute Maastricht, Maastricht University, Maastricht, THE NETHERLANDS; and 3School of Sport and Exercise Science, University of Birmingham, Birmingham, UNITED KINGDOM
Address for correspondence: Hanneke Boon, Ms.C., Department of Human Biology, Maastricht University, 6200 MD Maastricht, The Netherlands; E-mail: H.Boon@HB.unimaas.nl.
Submitted for publication January 2007.
Accepted for publication July 2007.