Vitamin D, a fat-soluble prohormone, is endogenously synthesized in response to sunlight or taken from dietary supplements. Since
vitamin D receptors are present in most tissues and cells in the body, the mounting understanding of the role of
vitamin D in humans indicates that it does not only play an important role in the musculoskeletal system, but has beneficial effects elsewhere as well. This review summarizes the metabolism of
vitamin D, the research regarding the possible risk factors leading to
vitamin D deficiency, and the relationships between
vitamin D deficiency and numerous illnesses, including
rickets,
osteoporosis and
osteomalacia,
muscle weakness and falls, autoimmune disorders,
infectious diseases,
cardiovascular diseases (CVDs),
cancers, and
neurological disorders. The system-wide effects of
vitamin D and the mechanisms of the diseases are also discussed. Although accumulating evidence supports associations of
vitamin D deficiency with physical and
mental disorders and beneficial effects of
vitamin D with health maintenance and disease prevention, there continue to be controversies over the beneficial effects of
vitamin D. Thus, more well-designed and statistically powered trials are required to enable the assessment of
vitamin D's role in optimizing health and preventing disease.