Vitamin D3 is really a hormone precursor your skin makes from sunlight
Cholecalciferol, better known as vitamin D3, is manufactured in your skin when sunlight hits it. The liver and kidneys then convert it, in two steps, into calcitriol, the active form that helps the gut absorb calcium. It was first described in 1936 and now fills over 9 million American prescriptions a year.
Strictly speaking, D3 acts as a pro-hormone. After the skin produces it, the liver turns it into calcifediol, and the kidney then converts that into calcitriol. Calcitriol's chief job is pulling calcium in from the intestine, which is why the vitamin is bound up with bone growth and strength. Fatty fish, beef liver, eggs and cheese supply some, and several countries add it to milk, juice, yoghurt, margarine and plant-based products.
The WHO includes it among its essential medicines, and it is sold as a cheap generic; in 2023 it ranked 68th among the most prescribed drugs in the United States. Doctors use it to prevent or correct deficiency and to treat related conditions such as rickets, as well as familial hypophosphatemia and Fanconi syndrome. It may not work well for people with severe kidney disease. Official intake targets differ by country: the United States sets 600 IU a day for ages 1 to 70 and 800 IU beyond that, while the United Kingdom uses 400 IU for most people over four.
Deficiency is more common in those who get little sun: people at northern latitudes, the housebound, frail or elderly, people with obesity or darker skin, and those whose clothing covers most of the body. Supplements are advised for these groups.
Too much can cause vomiting, constipation, weakness, confusion and kidney stones, although high blood calcium generally needs more than 40,000 IU daily. In 2010 the Institute of Medicine set an upper limit of 4,000 IU a day. Whether D3 beats the plant-derived D2 is contested: some studies find D2 weaker, others no difference, though evidence usually favours D3 for raising blood levels. D3 also seems to switch on type I interferon signalling, part of the body's defence against microbes, which D2 does not.
Source: Cholecalciferol
