Perspectives
From animal and human studies, it is evident that VD is important for optimal male reproductive function. Some of the VD effects are presumably mediated locally by the presence of VDR and the VD metabolizing enzymes in adult male germ cells, Leydig cells, and male reproductive tract, while other actions may be influenced by the systemic effects of VD serum levels. The low expression of VDR and VD metabolizing enzymes in spermatozoa from infertile men compared with normal men supports a role for the local VD metabolism and may have functional consequences. Especially, as 1,25(OH)2D3 mediates a nongenomic increase in intracellular calcium concentration, which seems to be important for sperm function especially following capacitation in the female reproductive tract. Most of the VD effects in the testes appear to be mediated through estrogen biosynthesis, but other candidate genes involved in calcium homeostasis, endocrine function, and cell cycle control may be involved. An association between serum 25-OHD3 and testosterone production remains to be proven, although it is plausible that the declining serum VD levels with age may be involved in the age-related loss of function of both gonads and bone, but further studies are needed to determine causality. Moreover, both local VD metabolism and serum levels of 25-OHD3 may have a direct effect on sperm motility in both animal and humans even after considering the indirect effect mediated by calcium homeostasis. The effects on sperm motility may be mediated mainly in epididymis rather than the testes, but further studies are needed to determine the exact site and mechanisms of action and to investigate whether supplementation of cholecalciferol improves semen quality in VD-deficient men.
