Reduced whole-body fat oxidation in women and in the elderly

OBJECTIVE: To test the hypothesis that the increase in fat mass observed with aging might be related to a decrease in whole-body fat oxidation. SUBJECTS AND MEASUREMENTS: Forty volunteers had measurements of sleeping and 24 h substrate oxidation in calorimetric chambers, body composition with the O-18 dilution technique, VO2max, and fiber composition analysis from a biopsy of vastus lateralis. They were divided into 10 young women, 10 young men, 10 elderly women and 10 elderly men. RESULTS: Sleeping fat oxidation and 24 h fat oxidation were lower in women than in men and in elderly than in young participants. Sleeping fat oxidation was correlated to fat-free mass and energy balance (multivariate analysis). Twenty four hour fat oxidation was correlated to total energy expenditure and energy balance (multivariate analysis). After adjustment for differences in these factors, sleeping and 24 h fat oxidation were no longer different between age and sex groups. None of the parameters of macronutrient metabolism was correlated with muscle fiber composition. CONCLUSION: Our data suggest that fat oxidation is lower in elderly subjects. This difference could favour fat mass gain if fat intake is not adequately reduced. Differences in fat-free mass and in total energy expenditure appear to participate in the reduction in fat oxidation.