Levels of 50 nmol/L (20 ng/mL) or more are sufficient for most people. Some people are potentially at risk of inadequacy at 30 to 50 nmol/L (12–20 ng/mL). After reviewing data on vitamin D needs, an expert committee of the Food and Nutrition Board (FNB) at the National Academies of Sciences, Engineering, and Medicine (NASEM) concluded that people are at risk of vitamin D deficiency at serum 25(OH)D concentrations less than 30 nmol/L (12 ng/mL see Table 1 for definitions of "deficiency" and "inadequacy"). Researchers have not definitively identified serum concentrations of 25(OH)D associated with deficiency (e.g., rickets), adequacy for bone health, and overall health. Serum concentrations of 25(OH)D and healthĪlthough 25(OH)D functions as a biomarker of exposure, the extent to which 25(OH)D levels also serve as a biomarker of effect on the body (i.e., relating to health status or outcomes) is not clear. Levels of 1,25(OH)2D do not typically decrease until vitamin D deficiency is severe. In contrast to 25(OH)D, circulating 1,25(OH)2D is generally not a good indicator of vitamin D status because it has a short half-life measured in hours, and serum levels are tightly regulated by parathyroid hormone, calcium, and phosphate. The international Vitamin D Standardization Program has developed procedures for standardizing the laboratory measurement of 25(OH)D to improve clinical and public health practice. As a result, a finding can be falsely low or falsely high, depending on the assay used and the laboratory. One nmol/L is equal to 0.4 ng/mL, and 1 ng/mL is equal to 2.5 nmol/L.Īssessing vitamin D status by measuring serum 25(OH)D concentrations is complicated by the considerable variability of the available assays (the two most common ones involve antibodies or chromatography) used by laboratories that conduct the analyses. Serum concentrations of 25(OH)D are reported in both nanomoles per liter (nmol/L) and nanograms per milliliter (ng/mL). In serum, 25(OH)D has a fairly long circulating half-life of 15 days. It reflects vitamin D produced endogenously and that obtained from foods and supplements. Serum concentration of 25(OH)D is currently the main indicator of vitamin D status. Neither aging nor obesity alters vitamin D absorption from the gut.
The concurrent presence of fat in the gut enhances vitamin D absorption, but some vitamin D is absorbed even without dietary fat. 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.