Purpose: 1) To examine the effect of a 12-wk endurance exercise training program on RMR and 2) to provide insight into the mechanisms responsible for alterations in RMR that may occur after exercise training.
Methods: Male participants (19-32 yr) in an exercise group (EX; n = 9) performed jogging and/or running 3-4 d·wk−1, 25-40 min per session, at 60%-80% V˙O2max, whereas subjects in a control group (CON; n = 10) maintained their normal activity patterns. Body composition, V˙O2max, RMR, epinephrine, norepinephrine, total thyroxine, free thyroxine, insulin, free fatty acids, and glucose were measured before and after the intervention.
Results: Training resulted in a significant increase in V˙O2max in EX (46.2 ± 1.2 to 51.0 ± 1.3 mL·kg−1·min−1, P < 0.001). Absolute and relative values for RMR did not significantly change in EX after training. Mean values for epinephrine, norepinephrine, total thyroxine, insulin, and glucose did not significantly change in either group; however, free thyroxine decreased significantly after training in EX (P = 0.04). Training also resulted in a significant increase in free fatty acid concentration in EX (0.37 ± 0.03 to 0.48 ± 0.04 mmol·L−1, P < 0.001). RMR in CON decreased significantly when expressed as an absolute value (P < 0.01) and relative to body weight (P < 0.01), fat-free mass (P < 0.01), and fat mass (P = 0.04).
Conclusions: The mechanism for the decrease in CON is unknown, but it may be related to seasonal variations in RMR. Training may have prevented a similar decline in RMR in EX and may be related to a training-induced increase in fat oxidation.