Vitamin D
cholecalciferol
A unique hormone our body produces thanks to sunlight, essential for bone and muscle health. The benefit of supplementing is proportional to the deficiency.
The AIS classification is specific to the sports context.
AIS Sports Supplement Framework, “Vitamin D” entry (Group A). Australian Institute of Sport.
The hormone we make from light
What is vitamin D?
Vitamin D is a substance your body makes in the skin when it gets sunlight, and that you also obtain, in smaller amounts, from some foods. Its main job is to help you absorb calcium properly and keep your bones and muscles in good shape. It is taken as a supplement when levels are low, which is more common than it seems.
What is it for?
Its best-demonstrated role is in bones and muscles: with sufficient vitamin D levels, the body absorbs dietary calcium and bone stays strong. In older people or those with a deficiency, correcting it helps bone and muscle health.
There is also solid evidence on one point: when someone starts from a significant deficiency, correcting it is associated with fewer respiratory infections. The benefit lies in filling the gap, not in stockpiling more.
What vitamin D does not do, according to the large trials, is as important as what it does. In people who already have normal levels, taking supplements has not been shown to prevent cancer, heart attacks, fractures, falls, depression or diabetes. It sounds like a brochure promise, but the large, well-conducted studies do not support it.
Who is it for?
It makes most sense for those with a real chance of running low: older people, especially if they rarely leave the house; people with dark skin living at high latitudes; people with obesity; those who barely get any sun or always stay well covered; and those with intestinal absorption problems.
And here comes the counterintuitive part: living in a sunny country does not protect you as much as you think. In southern Europe deficiency is more frequent than in the north, despite more hours of sunshine. There is an explanation (we develop it further on) and it means that “I get sun and eat well” does not guarantee your levels are where they should be.
The sunshine paradox
Dosage
The maintenance doses commonly used range between 800 and 2,000 a day. European and U.S. agencies set 4,000 IU/day as the maximum level they consider free of appreciable risk for adults; above that figure, the decision belongs to a professional.
One warning that does matter: more is not better. Very widely spaced megadoses (those annual or monthly ampoules taken all at once) not only offer no advantage but, in a trial with a very high annual dose, increased falls and fractures. The regimen with the best profile is the daily one (or weekly), in moderate amounts.
Absorption improves when it is taken with some fat, because vitamin D dissolves in fat, such as with the main meal of the day.
Can I take it without medical supervision?
Within the usual maintenance doses (up to 4,000 IU/day in adults), vitamin D has a wide safety margin. Toxicity is rare and appears with very high intakes sustained over a long time, not with a normal dose.
That said, there are situations where the prudent thing is to consult a professional: if you take certain medications (such as thiazide diuretics or digoxin), if you have kidney problems, kidney stones, sarcoidosis or calcium disorders. Also if you want to know your real level: that is measured with a blood test, not guessed. None of this is medical advice; it is a list of cases where asking first makes sense.