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J Appl Physiol 103: 1576-1582, 2007. First published August 23, 2007; doi:10.1152/japplphysiol.01455.2006
8750-7587/07 $8.00
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Changes in 24-h substrate oxidation in older and younger men in response to exercise

Edward L. Melanson,1,2 William T. Donahoo,4 Gary K. Grunwald,2 and Robert Schwartz3

1Division of Endocrinology, Metabolism, and Diabetes, 2Center for Human Nutrition, and 3Center On Aging, University of Colorado at Denver Health Sciences Center, and 4Kaiser Permanente of Colorado, Denver, Colorado

Submitted 26 December 2006 ; accepted in final form 18 July 2007

The purpose of this study was to compare 24-h substrate oxidation in older (OM; 60–75 yr, n = 7) and younger (YM; 20–30 yr, n = 7) men studied on sedentary day (Con) and on a day with exercise (Ex; net energy expenditure = 300 kcal). Plasma glucose and free fatty acids were also measured at several time points during the 24-h measurement. Weight was not different in OM and YM (means ± SD; 84.8 ± 16.9 vs. 81.4 ± 10.4 kg, respectively), although percent body fat was slightly higher in OM (25.9 ± 3.5 vs. 21.9 ± 9.7%; P = 0.17).Values of 24-h energy expenditure did not differ in OM and YM on the Con (means ± SE; 2,449 ± 162 vs. 2,484 ± 104 kcal/day, respectively) or Ex (2,902 ± 154 vs. 2,978 ± 122 kcal/day) days. Under both conditions, 24-h respiratory quotient was significantly lower and fat oxidation significantly higher in OM. Glucose concentrations were not different at any time point, but plasma free fatty acid concentrations were higher in OM, particularly following meals. Thus, under these controlled conditions, 24-h fat oxidation was not reduced and was in fact greater in OM. We speculate that differences in the availability of circulating free fatty acids in the postprandial state contributed to the observed differences in 24-h fat oxidation in OM and YM.

whole room calorimeter; fat oxidation; aging


Address for reprint requests and other correspondence: E. L. Melanson, Univ. of Colorado Health Sciences Center, 4200 East 9th Ave., Center for Human Nutrition, Campus Box C263, Denver, CO 80262 (e-mail: Ed.melanson{at}uchsc.edu)






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