Absorption occurs by simple passive diffusion and by a mechanism that involves intestinal membrane carrier proteins. Both forms are well absorbed in the small intestine. In foods and dietary supplements, vitamin D has two main forms, D 2 (ergocalciferol) and D 3 (cholecalciferol), that differ chemically only in their side-chain structures. Many tissues have vitamin D receptors, and some convert 25(OH)D to 1,25(OH)2D. Many genes encoding proteins that regulate cell proliferation, differentiation, and apoptosis are modulated in part by vitamin D. Vitamin D has other roles in the body, including reduction of inflammation as well as modulation of such processes as cell growth, neuromuscular and immune function, and glucose metabolism. Together with calcium, vitamin D also helps protect older adults from osteoporosis. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults. Without sufficient vitamin D, bones can become thin, brittle, or misshapen. It is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts. Vitamin D promotes calcium absorption in the gut and maintains adequate serum calcium and phosphate concentrations to enable normal bone mineralization and to prevent hypocalcemic tetany (involuntary contraction of muscles, leading to cramps and spasms). The first hydroxylation, which occurs in the liver, converts vitamin D to 25-hydroxyvitamin D, also known as "calcidiol." The second hydroxylation occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxyvitamin D, also known as "calcitriol". Vitamin D obtained from sun exposure, foods, and supplements is biologically inert and must undergo two hydroxylations in the body for activation. It is also produced endogenously when ultraviolet (UV) rays from sunlight strike the skin and trigger vitamin D synthesis. Vitamin D (also referred to as "calciferol") is a fat-soluble vitamin that is naturally present in a few foods, added to others, and available as a dietary supplement. For information on vitamin D and COVID-19, see Dietary Supplements in the Time of COVID-19.
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