Vitamin D (25-hydroxyvitamin D) is actually not an essential dietary vitamin as it can be synthesized in adequate amounts by most mammals exposed to sunlight . The body makes vitamin D when the skin is directly exposed to the sun . Many foods are fortified with vitamin D because very few foods naturally contain it . So, it is a nutrient we eat and hormone that our bodies make. The steroid hormone vitamin D has been historically recognized for its relevance to bone health and calcium homeostasis . Its primary role is to regulate calcium and phosphorus metabolism , however, recently many other actions of vitamin D have been identified.
Vitamin D and its Effect on Testosterone Levels
Vitamin D plays a role in male reproduction and androgen levels, animal studies indicate . However, this relation is mostly unexplored in humans.
In a cross-sectional study by Wehr et al.  vitamin D, testosterone and SHBG (Sex Hormone Binding Globulin) levels were assessed in 2229 men. Men with sufficient vitamin D levels (≥30 μg/l) had significantly higher levels of testosterone.
Scientist from Medical University of Graz  evaluated whether vitamin D supplementation effects testosterone levels in men. Healthy overweight men (n = 54) undergoing a weight reduction program who participated in a randomized controlled trial were analyzed for testosterone levels. Participants received either 83 μg (3,332 IU) vitamin D daily for 1 year (n = 31) or placebo (n = 23). Compared to baseline values, a significant increase in total testosterone levels, bioactive testosterone and free testosterone levels were observed in the vitamin D supplemented group. There was no significant change in any testosterone measure in the placebo group.
Pic 1. – Vitamin D softgels
Katharina Nimptsch and associates  investigated the cross-sectional association of plasma vitamin D levels and total and free testosterone. Their findings support previously reported positive associations between vitamin D and testosterone. However, Hammoud and colleagues  concluded that serum vitamin D levels at high and low levels can be negatively associated with semen parameters. Meaning that too much of it can badly effect your testosterone levels.
Jorde et al.  reported that vitamin D randomized clinical trials show no significant effect on serum total or free testosterone levels.
It’s role in reproductive physiology is almost entirely shaped by ‘associative’ studies. Mechanisms by which vitamin D might participate in the regulation of reproductive physiology remain far from clear.
Vitamin D and Exercise
A systematic review and meta-analysis by Stockton and others  evaluated effect of vitamin-D supplementation on muscle strength. It was concluded that vitamin-D supplementation does not significantly effect muscle strength in vitamin D saturated adults. However, some studies examined demonstrated an increase in proximal muscle strength in vitamin-D deficient adults. Vitamin-D supplementation also failed to improve physical performance .
Some randomized controlled trials suggested that vitamin D supplementation improved muscle function and physical performance among older adults at high risk for deficiency, which was evident by 49% reduction in falls [12, 13]. Muscular strength was not improved . Neuromuscular or neuroprotective function improvement was suggested . This may in part explain the mechanism by which vitamin-D reduces falls and fractures .
Vitamin D Deficiency
Vitamin D deficit is associated with muscle weakness, whereas vitamin D supplementation may improve muscle function . Hypogonadism is also associated with vitamin D deficiency .
(Other common names: 25-hydroxycholecalciferol, 25-hydroxyvitamin D3, Calcipotriol, 1-alpha-hydroxycholécalciférol, Vitamin D3, Alfacalcidol, Cholecalciferol)
- Wolf, George. The Journal of nutrition 134.6 (2004): 1299-1302.
- MedlinePlus. Retrieved from http://www.nlm.nih.gov/ at 28. May 2013
- Luk, Janelle, et al. “Relevance of vitaminD in reproduction.” Human reproduction 27.10 (2012): 3015-3027.
- Parikh, G., et al. “VitaminD regulates steroidogenesis and insulin-like growth factor binding protein-1 (IGFBP-1) production in human ovarian cells.” Hormone and metabolic research 42.10 (2010): 754.
- Wehr, Elisabeth, et al. “Association of vitaminD status with serum androgen levels in men.” Clinical endocrinology 73.2 (2010): 243-248.
- Pilz, S., et al. “Effect of vitaminD supplementation on testosterone levels in men.” Hormone and metabolic research= Hormon-und Stoffwechselforschung= Hormones et métabolisme 43.3 (2011): 223.
- Nimptsch, Katharina, et al. “Association between plasma 25‐OH vitaminD and testosterone levels in men.” Clinical endocrinology 77.1 (2012): 106-112.
- Hammoud, Ahmad O., et al. “Association of 25-hydroxy-vitamin-D levels with semen and hormonal parameters.” Asian journal of andrology (2012).
- Jorde, R., et al. “Supplementation with VitaminD Does not Increase Serum Testosterone Levels in Healthy Males.” Hormone and metabolic research (2013).
- Stockton, K. A., et al. “Effect of vitaminD supplementation on muscle strength: a systematic review and meta-analysis.” Osteoporosis international 22.3 (2011): 859-871.
- Kenny, Anne M., et al. “Effects of VitaminD Supplementation on Strength, Physical Function, and Health Perception in Older, Community‐Dwelling Men.” Journal of the American Geriatrics Society 51.12 (2003): 1762-1767.
- Bischoff, Heike A., et al. “Effects of vitaminD and calcium supplementation on falls: a randomized controlled trial.” Journal of bone and mineral research 18.2 (2003): 343-351.
- Dhesi, Jugdeep K., et al. “VitaminD supplementation improves neuromuscular function in older people who fall.” Age and Ageing 33.6 (2004): 589-595.
- Minasyan, Anna, et al. “Vestibular dysfunction in vitaminD receptor mutant mice.” The Journal of steroid biochemistry and molecular biology 114.3 (2009): 161-166.
- Lee, David M., et al. “Association of hypogonadism with vitamniD status: the European Male Ageing Study.” European Journal of Endocrinology 166.1 (2012): 77-85.