Vitamin D deficiency is common in Hispanic and African-Americans in the United States, with levels dropping significantly in the winter.  The modified oil cured the sick dogs, so McCollum concluded the factor in cod liver oil which cured rickets was distinct from vitamin A. , There is conflicting evidence about the benefits of interventions with vitamin D, one view purporting an intake of 4,000–12,000 IU/day from sun exposure with concomitant serum 25-hydroxyvitamin D levels of 40 to 80 ng/mL, while another view is that serum concentrations above 50 ng/mL are not plausible. , Vitamin D toxicity is treated by discontinuing vitamin D supplementation and restricting calcium intake.  Its general use for prevention of this disease in those without vitamin D deficiency is thus likely not needed. The evidence for other health effects of vitamin D supplementation in the general population is inconsistent. Health Canada published recommended dietary allowances (RDA) and tolerable upper intake levels for vitamin D in 2012 based on the Institute of Medicine report.  This is due to the levels of melanin in the skin, as it acts as a natural protectant from sun exposure. For vitamin D labeling purposes, 100% of the Daily Value was 400 IU (10 μg), but on May 27, 2016, it was revised to 800 IU (20 μg) to bring it into agreement with the RDA. Vitamin D2 is found in fungi and is produced by ultraviolet irradiation of ergosterol. X-ray crystallography demonstrated the sterol molecules were flat, not as proposed by the German team led by Windaus. , Calcifediol is transported to the proximal tubules of the kidneys, where it is hydroxylated at the 1-α position (lower right of the molecule) to form calcitriol (1,25-dihydroxycholecalciferol, 1,25(OH)2D). The EFSA reviewed safe levels of intake in 2012, setting the tolerable upper limit for adults at 100 μg/day (4000 IU), a similar conclusion as the IOM. Haldane, J.D.  The disorder can continue into adulthood. They had to either ingest it or be exposed to sunlight to photosynthesize it in their skin. The conversion of calcifediol to calcitriol is catalyzed by the enzyme 25-hydroxyvitamin D3 1-alpha-hydroxylase, which is the product of the CYP27B1 human gene.  Land vertebrates have been photosynthesizing vitamin D for more than 350 million years.  Only a few foods, such as the flesh of fatty fish, naturally contain significant amounts of vitamin D. In the U.S. and other countries, cow's milk and plant-derived milk substitutes are fortified with vitamin D, as are many breakfast cereals. , In birds and fur-bearing mammals, fur or feathers block UV rays from reaching the skin.  Plant milks are defined as beverages made from soy, almond, rice, among other plant sources intended as alternatives to dairy milk. ( sometimes l.c.) The conversion of previtamin D3 to vitamin D3 in the skin is about 10 times faster than in an organic solvent.  This reaction is catalyzed by the microsomal enzyme vitamin D 25-hydroxylase, the product of the CYP2R1 human gene, and expressed by hepatocytes.  However, some animals, such as the naked mole-rat, are naturally cholecalciferol-deficient, as serum 25-OH vitamin D levels are undetectable. Vitamin D is produced in the keratinocytes of two innermost strata of the epidermis, the stratum basale and stratum spinosum, which also are able to produce calcitriol and express the VDR.. D or d is the fourth letter of the modern English alphabet and the ISO basic Latin alphabet. Serum 25(OH)D levels above 50 ng/mL (125 nmol/L) may be cause for concern. , Because it found mounting evidence for a benefit to bone health, though it had not found good evidence of other benefits, the US Food and Drug Administration (FDA) has required manufacturers to declare the amount of vitamin D on nutrition facts labels, as "nutrients of public health significance", since May 2016. By binding to vitamin D-binding protein, calcitriol is transported throughout the body, including to the classical target organs of intestine, kidney and bone.  The optimal vitamin D levels are still controversial and another review concluded that ranges from 30 to 40 ng/mL (75 to 100 nmol/L) were to be recommended for athletes. Exposure to light through windows is insufficient because glass almost completely blocks UVB light. Reviews have described the evidence as being "inconsistent, inconclusive as to causality, and insufficient to inform nutritional requirements" and "not sufficiently robust to draw conclusions". , In June 2020, the US National Institutes of Health found insufficient evidence to recommend for or against using vitamin D supplementation specifically to prevent or treat COVID-19.  There is no evidence for vitamin D affecting respiratory infections in children under five years of age. , Click on icon in lower right corner to open. The Swedish National Food Agency recommends a daily intake of 10 μg (400 IU) of vitamin D3 for children and adults up to 75 years, and 20 μg (800 IU) for adults 75 and older. However, some people with serum 25(OH)D between 30 and 50 ng/mL (75 nmol/L-125 nmol/L) will also have inadequate vitamin D., Vitamin D toxicity is rare. , In 2011 an IOM committee concluded a serum 25(OH)D level of 20 ng/mL (50 nmol/L) is needed for bone and overall health. , Published cases of toxicity involving hypercalcemia in which the vitamin D dose and the 25-hydroxy-vitamin D levels are known all involve an intake of ≥40,000 IU (1,000 μg) per day. :5 The tolerable upper intake level (UL) is defined as "the highest average daily intake of a nutrient that is likely to pose no risk of adverse health effects for nearly all persons in the general population.  During the first six months following the January 1, 2020 compliance date, the FDA plans to work cooperatively with manufacturers to meet the new Nutrition Facts label requirements and will not focus on enforcement actions regarding these requirements during that time.
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