Vitamin D insufficiency is increasingly being reported to be prevalent amongst those living in northern latitudes. This is because for six months of the year the angle of incidence is too low to allow effective penetration of the skin by ultraviolet radiation. In addition, dietary sources of the vitamin are not widespread and as a result diet cannot compensate for the lack of sunlight. Insufficiency of vitamin D leads to reduced levels of the metabolites 25-hydroxyvitamin D [25(OH)D] and 1,25-dihydroxyvitamin D [1,25OH)D] which play important roles in cell regulation. Therefore vitamin D insufficiency can lead to widespread and diverse metabolic disturbances. One such metabolic function that may be disrupted relates to sex hormone production and regulation. For example, recent research into the effects of vitamin D supplements have shown some benefits to supplementation in overweight subjects with deficient testosterone production.
In one clinical trial, researchers investigated the effects of vitamin D supplements on the testosterone levels of overweight subjects undergoing a weight reduction diet over a one year period1. Supplementation with 3332 IU of vitamin D resulted in a significant increase in the mean testosterone levels of the subjects (from 10.7 to 13.4 nmol/L) compared to the placebo group who received no vitamin D. These results suggest that vitamin D supplementation may increase testosterone levels in overweight, testosterone deficient, vitamin D insufficient males. This is supported by data from cross sectional studies showing an association between vitamin D and testosterone plasma levels. However, the low baseline levels of vitamin D (50 nmol/L or 20 ng/mL) suggest that the subjects had far from optimum health, and it is questionable if the same results would be expected in healthy male subjects.
In fact more recent research suggests that this is not the case. For example, pooled data from randomised control trials shows that vitamin D supplementation does not cause increases to plasma testosterone in healthy males subjects2. This likely relates to the fact that in this study, vitamin D supplementation did not increase serum levels of 25(OH)D despite being administered at doses of up to 40,000 IU. When the researchers analysed data from subjects who were administered testosterone undecanoate, they found no increase in 25(OH)D serum levels, suggesting that elevations to testosterone can not cause elevations in 25(OH)D. Therefore vitamin D supplementation may only raise testosterone levels in overweight male subjects with depressed plasma concentrations of 25(OH)D. Care should therefore be exercised when interpreting data from studies involving metabolically abnormal subjects with regard the effects of vitamin D supplementation and sex hormone changes.
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