Influence of Aerobic Capacity, Body Composition, and Thyroid Hormones on the Age-Related Decline in Resting Metabolic Rate E.T. Poehlman, E.M. Berke, J.R. Joseph, A.W. Gardner, SM. Katzman-Rooks, and M.I. Goran It zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA has been suggested that changes in fat-free weight may not fully explain the decline of resting meta,bolic rate (RMR) that occurs with aging. We therefore examined the hypothesis that a reduction in maximal aerobic capacity (Vosmax) may partially explain the lower RMR in older men, after accounting for differences in fat-free weight and fat weight. We also considered differences in energy intake and plasma thyroid hormones as possible modulators of the age-related decline in RMR in men. Three-hundred healthy men (aged 17 to 78 years) were characterized for: (1) RMR (kcal/min) from indirect calorimetry; (2) body composition from underwater weighing; (3) maximal aerobic capacity from a test of Vo2max; (4) plasma thyroid hormones (total triiodothyronine fTs], free TO,total thyroxine (T4], and free Ta); and (5) estimated energy intake (kcal/d) from a 3-day food diary. A curvilinear decline of RMR with age was found (P < .Ol), in which no relationship was found in men less than 40 years of age (I = .lO, slope = 0.002 kcal/min/yr), whereas in men older than 40 years, RMR was negatively related to age (r = -.52, slope = -0.008 kcal/min/yr). After statistical control for differences in fat-free weight and fat weight, a negative relationship between age and RMR persisted (partial r = -.30, P < .Ol). It was only after control for fat-free weight, fat weight, and Vozmax (partial zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA r = -.lO, P > .05) that no association between age and RMR was noted. When subgroups of younger and older individuals were paired for age and fat-free weight, but different Vozmax values, a higher RMR was noted in the trained younger (8%) and trained older men (11%) compared with untrained younger and older men, respectively. Thyroid hormones and daily energy intake were negatively related to age on a univariate basis, but no independent association was noted after control for fat-free weight. We conclude that, in addition to the loss of fat-free weight, the decline in Vosmax is an additional factor associated with the decline of RMR in aging men. Age-related changes in energy intake, fat weight, and thyroid ,hormones appear to be less important in explaining the reduction of RMR in aging men. Maintenance of fat-free weight and Vozmax by regular physical activity may attenuate the age-related RMR in healthy men. Copyright 0 1992 by W.B. Saunders Company ESTING zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA R METABOLIC RATE (RMR) comprises approximately 60% to 75% of daily energy expendi- ture, and thus represents a major component in the regulation of energy balance and body composition in humans.’ Earlier studie@ and more recent cross-sectional and longitudinal investigations have identified a decline in RMR with advancing age4-’ that has been primarily attrib- uted to alterations in body composition, specifically the decline in fat-free weight. However, it has recently been demonstrated that the loss of fat-free weight does not fully explain the lower RMR in older individuals,s-*0 suggesting that other physiological and/or lifestyle factors may contrib- ute to the reduction of RMR. Maximal aerobic capacity (Vo,max) declines with age” and has been reported to explain some of the individual differences in RMR in younger and older men. even after differences in body composition were taken into account.’ Although the metabolic link between Vo2max and RMR is unclear, recent findings suggest that changes in Vo,max in response to long-term exercise stimulates sympathetic ner- vous system activity and food intake, which are both related to alterations in RMR.i2 Taken together, these observa- tions suggest the hypothesis that age-related alterations in Vo,max or Vo2max-related factors contribute to the lower RMR in aging males. Because energy intake also declines with advancing age,i3.i4 and RMR is sensitive to alterations in energy intake, primarily by influencing energy balance,i5-lx we also considered the possibility that a decline in energy intake may contribute to the lower RMR in healthy older men. Finally, we considered the influence of age-related changes in circulating concentrations of thyroid hormones to the decline in RMR. Although thyroid hormones are generally considered important regulators of metabolic rate,19 their Metabolism, Vol41. No 8 (August), 1992: pp 915-921 association with RMR in euthyroid individuals has yielded inconsistent results.20.z1 Furthermore, it has been unclear whether healthy individuals exhibit age-related changes in plasma concentrations of thyroid hormones.22J3 Therefore, the purpose of the present study was to examine data from a large cohort of healthy men to determine whether differences in Vo:max, energy intake, and thyroid hormones could explain age-associated changes in RMR after differences in fat-free weight were taken into account. SUBJECTS AND METHODS zyxwvutsrqponmlkjihgfe Subjects Three-hundred white men (aged 17 to 7X years) participated in this study. Their physical characteristics are presented in Table 1. zyxwvutsrq From the Division of Endocrinology, Metabolism and Nutrition, Department of M edicine, College of Medicine, and the Department of Nutritional Sciences, University of Vermont, Burlington, VT E. T Poehlman is supported by a Grant from the National Institute of Aging (AC-07857) the American Association of Retired Persons Andrus Foundation (AARP), and a Research Career and Develop- ment Award from the National Institute of Aging (K04 AG00564). A.G. Gardner is supported by a National Research Service Award from the National Institute of Aging (AC-05564). J.R. Joseph was supported by a summer grant from the M edical Student Research Fellowship Program sponsored by the American Diabetes Association. M I. Goran is supported by a grant from the Ametican Diabetes Association, Supported in part by the General Clinical Research Center (National Institutes of Health Grant No. RR-109). Address reprint requests to E. T. Poehlman, PhD, Division of Endocrinology, Metabolism and Nutrition, Department of M edicine, University of Vermont, Burlington, VT 05405. Copyright 0 1992 by Vi! B. Saunders Company 00260495/92/4108- 00191$03.00/0 915