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

Abstract

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.