Reduced sun exposure does not explain the inverse association of 25-hydroxyvitamin D with percent body fat in older adults

Context: Greater adiposity is associated with lower blood levels of 25-hydroxyvitamin D [25(OH) D]. The extent to which this results from reduced sun exposure among heavier individuals is unknown. Objectives: This analysis was conducted to determine whether sun exposure habits differ according to percent body fat (% FAT) in older adults and to what extent they explain the inverse association of adiposity with 25(OH) D in that population. Design: We performed a cross-sectional analysis of baseline data from a randomized trial of calcium and vitamin D supplementation to prevent bone loss. Setting: The study was performed at the Metabolic Research Unit at the Jean Mayer United States Department of Agriculture Human Nutrition Research Center on Aging at Tufts University. Participants: A total of 381 generally healthy male and female volunteers age 65 and older participated in the study. Exclusion criteria included vitamin D and calcium supplement use, and medical conditions and medications known to affect bone metabolism. Intervention: There were no interventions. Measurements for this analysis were made before participants received trial supplements. Main Outcome Measures: Plasma 25(OH) D, an indicator of vitamin D status, was measured. Results: Sunscreen use, hours spent outside per week, and percent of skin exposed did not differ across quartiles of% FAT(P > 0.43). 25(OH) D decreased across % FAT quartiles (P < 0.05) and was about 20% lower in the highest compared with the lowest quartile of % FAT after adjustments for age, sex, season, and vitamin D intake. Further adjustment for sun exposure habits had little effect on estimates of 25(OH) D. Conclusions: In older adults, sun exposure habits do not vary according to adiposity and do not appear to explain lower 25(OH) D concentrations with increasing adiposity.