Purpose

The purpose of this study was to compare substrate source use in older, long-term exercising, endurance-trained males with sedentary controls.

Methods

[UC] palmitate and [6, 6-H2] 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, 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.