[discostu]

Vitamin D. It controls how much calcium your body can process, with D deficiency you can consume nothing but pure calcium and you’ll still have brittle bones. And with the way modern America fears the sun we don’t get enough D. It’s why most calcium pill also have D, that’s actually the part most folks need.

[MaxMax]

I use my arm to collect my "D" surplus. When driving it's always out the window.
When I hit the beach my left arm is always outside the umbrella, and in the
tanning booth I just close the lid on it.