Vitamin D, reproductive endocrinology, and male reproductive organ function in health and disease

The spectrum of vitamin D–regulated effects has expanded beyond the classical regulation of calcium homeostasis and bone health. Presence of the vitamin D receptor and enzymes involved in the activation and inactivation of vitamin D in testis, human spermatozoa, and the epithelial cells lining the male reproductive tract supports a role in male reproduction. In this chapter, functional and clinical studies investigating the role of vitamin D in male reproduction will be discussed. Direct effects of vitamin D are difficult to separate from indirect effects for instance secondary changes in calcium and phosphate homeostasis, but a few studies can be used to discriminate direct versus indirect effects. Vitamin D seems to exert positive effects on semen quality, sperm function, estrogen signaling, testosterone/luteinizing hormone ratio, and testicular cancer but the effects are mainly found in patients with vitamin D deficiency, while none or few effects are found in vitamin D insufficient men receiving vitamin D supplementation.