Vitamin D is the one vitamin the body is built to make for itself. Ultraviolet light on the skin turns a relative of cholesterol into cholecalciferol, vitamin D3, the molecule drawn here. The arrangement works beautifully in the latitudes our species grew up in. It works poorly in a German winter, in an office, or under a sunscreen, which is to say in most of the lives our formulas are made for.
What it does
Vitamin D is less a vitamin than a hormone the body produces when it can. Its best-known job is calcium: it is what lets the gut take calcium from food and the bones keep it, which is why a shortage in children was once called rickets and in adults shows as weak bones and muscles. Its receptors, though, sit in nearly every tissue, immune cells included, and the immune system is where its second well-established role lies.
The winter problem
From about October to March, the sun over Germany stands too low for the skin to make any vitamin D at all, whatever the weather. The body lives on what it stored in summer and on what it eats, and the diet carries little: oily fish, egg yolk, a few fortified foods.
The numbers show it. In the national examination survey of German adults, more than half had blood levels below the 50 nmol/L that counts as adequate, and in the winter months a quarter were below 30 nmol/L, the line for deficiency. This is not a rare condition in a few people; it is the ordinary state of the population for half the year.
Why it is in our formulas
Because it is the gap a daily cup can actually close. A vitamin that the body cannot make for months, that food barely supplies, and that is needed every day belongs in something that happens every day. Vitamin D is in The Daily and The Longevity: for the immune system, for bones, for muscles.
The form, and the amount
We use D3, cholecalciferol, the form the skin itself makes and the one the body raises its levels with most reliably; D2, from fungi, is the other and works less well. Vitamin D is fat-soluble and stable to heat, so it survives brewing; it is sensitive to oxygen and light, so it is microencapsulated like everything else we add. The amount per cup, as brewed, is stated on each formula’s page. The reference intake in Europe is 5 µg a day; the adequate intake the European Food Safety Authority set in 2016 is 15 µg; and the safe upper level for adults, 100 µg a day, is far above what any cup of coffee could carry. A formula is a supplement to daylight, not a replacement for it.