Vitamin D is both a vitamin and a hormone. It has pleotropic actions that extend beyond calcium and phosphate homeostasis, regulation of parathyroid hormone, and the prevention of osteomalacia, rickets, falls, and fractures. Over 80% of vitamin D requirement is expected to generate in the skin, following exposure to ultraviolet B in sunlight; globally, majority of people are however, under-exposed to sunlight. Nevertheless, over exposure to sunlight, does not cause hypervitaminosis D but can cause damage to skin cells. However, both extremes of vitamin D concentrations can be harmful. Vitamin D is essential for life, including for reproduction, fetal growth, and immunity, and proper functioning of body systems. Evidence supports wider beneficial effects of vitamin D but to achieve such, maintaining serum 25 dihydroxyvitamin D (25(OH)2D) concentrations of more than 30 ng/mL is necessary. This article reviews biological pathways that are critical for generation of 25(OH)D in the liver and 1,25- dihydroxyvitamin D in the kidney, and key abnormalities of vitamin D metabolism that lead to common diseases, such as obesity, insulin resistance, type 2 diabetes, pregnancy complications, autoimmune disorders, certain cancers, impairment of DNA repair, systemic inflammation, and oxidative stress that potentiates metabolic illnesses such as cardiovascular disorders. Treatment of vitamin D deficiency on average costs less than 0.1% of the cost of investigations and treatment of worsening comorbidities and complications associated with hypovitaminosis D (vary between 0.2% and 0.06%). For example, treatment cost of $12/year versus, average cost for managing complications of $6,000 to 20,000/year per affected person. Despite the high benefits relative to cost, millions of people continue to have vitamin D deficiency. The individual and the population health can be markedly improved by maintaining serum 25(OH)D concentrations of greater than 30 ng/mL (75 nmol/L), which would improve the quality of life and reduce all-cause mortality. However, for prevention of certain diseases and to reduce all-cause mortality, serum 25(OH)D concentrations need to be maintained between 40 and 60 ng/mL.
