Objective A small randomized controlled trial suggested that vitamin D might increase the production of testosterone in men, which is supported by experimental studies in animals and a cross-sectional study showing positive associations between plasma 25-hydroxyvitamin D [25(OH)D] and testosterone and concordant seasonal variation of both biomarkers.
Design and Measurements We investigated the cross-sectional association of plasma 25(OH)D levels and total and free testosterone measured by immunoassay in 1362 male participants of the Health Professionals Follow-up Study who were selected for a nested case–control study on prostate cancer using multivariate-adjusted linear and restricted cubic spline regression models.
Results 25(OH)D was positively associated with total and free testosterone levels. From the lowest to the highest 25(OH)D quintile, multivariate-adjusted means (95% confidence interval) were 18·5 (17·7; 19·4), 19·4 (18·6; 20·2), 19·6 (18·8; 20·4), 20·1 (19·3; 20·9) and 20·0 (19·1; 20·8; P-trend = 0·003) for total testosterone and 97·7 (93·9; 101·5), 98·2 (94·1; 102·2), 99·2 (95·2; 103·2), 100·7 (96·9; 104·5) and 101·5 (97·6; 105·4; P-trend = 0·03) for free testosterone. The shapes of the dose–response curves indicate that the association between 25(OH)D and total and free testosterone is linear at lower levels of 25(OH)D (below approximately 75–85 nmol/l), reaching a plateau at higher levels. Unlike for 25(OH)D, we did not observe any seasonal variation of testosterone concentrations.
Conclusion This study supports previously reported positive associations between vitamin D and testosterone although we did not observe parallel seasonal variation patterns. Possible causality and direction of the vitamin D–testosterone association deserve further scientific investigation.