Lower resting metabolic rate in the elderly may not be entirely due to changes in body composition

Objective: To investigate whether or not the lower resting metabolic rate (RMR) in the elderly is entirely due to changes in body composition. Design: Cross-sectional data of 132 female (age 69.9 +/- 5.5 y, body mass index (BMI) 26.5 +/- 4.0 kg/m(2)) and 84 male ( age 68.9 +/- 5.1 y, BMI 26.1 +/- 2.8 kg/m(2)) participants of the longitudinal study on nutrition and health status in an aging population of Giessen, Germany, as well as that of 159 young women (age 24.8 +/- 3.0 y, BMI 21.1 +/- 2.5 kg/m(2)) and 67 young men ( age 26.8 +/- 3.4 y, BMI 23.3 +/- 2.4 kg/m(2)) were analysed. RMR was measured by indirect calorimetry after an overnight fast and body composition was estimated by bioelectrical impedance analysis and predictive equations from the literature. Analysis of covariance was used to adjust RMR for body composition, body fat distribution and smoking habits. Additionally, RMR that is to be expected theoretically, was calculated on the basis of the subjects' body composition and the specific metabolic rate of the different organs and was compared to measured RMR. Results: Compared to young subjects adjusted RMR was significantly lower in elderly women (5432 +/- 82 vs 5809 +/- 70 kJ/day, P < 0.01) and men (6971 +/- 99 vs 7558 +/- 121 kJ/day, P < 0.001). In both elderly women and men, measured RMR was markedly lower than calculated RMR (-625 +/- 404, -515 +/- 570 kJ/day). By contrast, measured and calculated RMR were nearly the same in young men (159 +/- 612 kJ/day); in young women the difference between measured and calculated RMR was only -300 +/- 457 kJ/day. In both sexes, these differences are significantly larger in the elderly when compared to young adults. Conclusion: These results support the point of view that the decline in RMR with advancing age cannot be totally due to changes in body composition.