BACKGROUND: Associations between body composition and the energy expended on basal metabolism and activity are complex and age dependent. OBJECTIVE: The objective was to examine associations between body composition and daily (DEE), basal (BEE), and activity energy expenditure (AEE) throughout the adult life span. DESIGN: A cross-sectional study was conducted in 529 adults aged 18-96 y. DEE was measured by using doubly labeled water, BEE by using respirometry, and body composition by isotope dilution. AEE was calculated as DEE - BEE, and physical activity level (PAL) was calculated as DEE/BEE. RESULTS: Up to age 52 y, fat-free mass (FFM) and fat mass (FM) were positively associated with age in men, but no significant effect was observed in women. No effects of age on DEE and AEE were observed. The average DEE in men (14.1 MJ/d) was 27% greater than that in women (10.7 MJ/d). PAL averaged 1.84 in men and 1.75 in women. Above and including the age of 52 y, FFM, FM, DEE, BEE, and AEE were all negatively associated with greater age. The effect of age on AEE was greater than on BEE; consequently, PAL by the age of 95 y was only 1.36. PAL and AEE were both unrelated to FFM (both age adjusted). CONCLUSIONS: PAL and AEE were not associated with age in subjects aged <52 y. AEE, BEE, and PAL were all negatively associated with age in subjects aged >/=52 y. An absence of a relation between age-adjusted PAL and FFM suggested that greater physical activity was not associated with higher FFM in the elderly.