Vitamin D
Vitamins
Less a vitamin than a hormone, made by the skin in sunlight.
Mushrooms are the only plant-side source, and only if they have seen the sun or a UV lamp — the content varies fiftyfold. At our latitudes, from October to March, the plate is not enough anyway.
Effects and benefits
Less a vitamin than a hormone, made by the skin in the sun. It fixes calcium in bones, maintains muscles and defenses; and after 60, 20 µg a day helps to reduce the risk of falling associated with muscle weakness.
The sun's UVB turn a cholesterol derivative in the skin into vitamin D, which the liver then the kidneys activate. It controls calcium absorption by the gut: without it, only 10 to 15% is absorbed, with it 30 to 40% (Holick, 2007).
It contributes to normal absorption and use of calcium and phosphorus, to the maintenance of normal bones, teeth and muscle function, and to the normal function of the immune system (Regulation 432/2012); and, after 60, it helps to reduce the risk of falling associated with postural instability and muscle weakness, falling being a risk factor for fractures (Regulation 1228/2014).
By age and skin: over the years, the skin makes much less for the same sun; darker skin makes less too, as melanin filters UVB. EFSA sets the adequate intake at 15 µg a day from age one (EFSA, 2016), and in France, infants routinely receive a supplement.
Sources: Holick, NEJM, 2007 · EU Regulations 432/2012 and 1228/2014 · EFSA, 2016