Vitamin D is unique among vitamins because it can be synthesized endogenously by skin exposure to UV-B radiation. Globally, sun exposure is the most important source of vitamin D. At northern latitudes, however, dietary intake by natural food sources, food fortification or dietary supplements is also of importance because exposure to UV-B radiation is low during the winter season. Relatively little is known about genetic influences on vitamin D levels and virtually nothing about seasonal differences in these effects.

 The primary step in endogenous vitamin D synthesis is the formation of pre-vitamin D from activation of -dehydrocholesterol when the skin is exposed to UV-B. Pre-vitamin D and vitamin D and D from food are converted to -hydroxyvitamin D in the liver and thereafter to the metabolic active form,-dihydroxyvitamin D in the kidney. Sufficient levels of,-dihydroxyvitamin D have a negative feedback on the secretion of parathyroid hormone. -hydroxyvitamin D can be stored in body fat and mobilised when needed and thereby avoiding deficiency during the winter season. Serum -hydroxyvitamin D is the indicator of vitamin D status.

 Vitamin D deficiency is common, especially among people in defined risk groups, such as the elderly living in nursing homes, obese persons, those normally veiled and dark-skinned people. Deficiency is principally associated with osteoporosis because,-dihydroxyvitamin D stimulates calcium absorption in the intestines and bone mineralization . Vitamin D and its physiological effects have been extensively investigated in both men and women, particularly in its relationship to osteoporosis. The possibility of preventing low energy fractures with substitution has also been broadly studied but the results are inconsistent, although it is generally accepted that supplementation with vitamin D in combination with calcium significantly reduces the risk of low energy fractures among institutionalised elderly women. Vitamin D deficiency is also negatively related to muscle strength and balance, which would increase the risk of injurious falls. Furthermore, it has been suggested that deficiency contribute to cancer occurrence, diabetes, cardiovascular disease, multiple sclerosis and overall mortality.

Snellman G, Melhus H, Gedeborg R, Olofsson S, Wolk A, et al. (2009) Seasonal Genetic Influence on Serum 25-Hydroxyvitamin D Levels: A Twin Study. PLoS ONE 4(11): e7747. doi:10.1371/journal.pone.0007747