Vitamin D and Physical Activity Status: Associations With Five-Year Changes in Body Composition and Muscle Function in Community-Dwelling Older Adults

Context: High vitamin D and physical activity (PA) levels are independently associated with improved body composition and muscle function in older adults. Objective: The objective of this study was to investigate the interaction of 25-hydroxyvitamin D (25OHD) and PA status in maintenance of body composition and muscle function in older adults. Design and Setting: This was a 5-year prospective population-based study of Australian community- dwelling older adults. Participants: Participants in the study included 615 community-dwelling volunteers aged 50 years old or older [61.4 +/- 6.9 (mean +/- SD) y; 48% female] randomly selected from electoral rolls and categorized according to baseline serum 25OHD (>= or <50 nmol/L) and PA(>= or <10 000 pedometer determined steps/d) levels as follows: high 25OHD and high PA (VitD + PA +); high 25OHD and low PA (VitD+PA-); low 25OHD and high PA (VitD-PA+); and low 25OHD and low PA (VitD-PA-). A subset of 518 participants completed accelerometer assessments during follow-up. Main Outcome Measures: Changes in dual-energy X-ray absorptiometry-assessed body composition and lower-limb muscle function were measured. Results: VitD+PA+had significantly smaller increases in body fat over 5 years compared with other groups (all P < .05). Higher baseline pedometer-determined PA resulted in declines in total body fat (beta = -.23 kg per 100 steps/d, P = .001) over 5 years for participants with high 25OHD but not those with low 25OHD (P > .05). Among participants with accelerometer data, these associations were generally mediated by higher levels of moderate/vigorous PA. Conclusions: High vitamin D status appears to enhance PA-related declines in body fat during aging, but the mechanism may be greater amounts of outdoor moderate/vigorous PA rather than a direct effect of 25OHD.