Sunlight and vitamin D

UVB also does a useful job in your skin: it starts the making of vitamin D, which your body needs to take calcium from food and build strong bones.

The outer layer of your skin holds 7-⁠dehydrocholesterol, C₂₇H₄₄O, a close relative of cholesterol. Its name says how they differ: dehydro- means two hydrogen atoms fewer, and in their place is a second C=C double bond, starting at carbon 7. The two double bonds in its second ring take turns with a single bond, like buta-1,3-⁠diene. Held in a ring and surrounded by carbon groups, this run absorbs further toward red than buta-1,3-⁠diene does, and reaches into the UVB, with peaks at 282 and 293 nm. Cholesterol, with one C=C, absorbs no UVB.

A photon of UVB breaks one bond of that ring, and the ring springs open, giving previtamin D₃. Over the next hours, the warmth of your skin moves a hydrogen atom from one end of the run to the other, the double bonds shift along, and previtamin D₃ becomes vitamin D₃. Every one of these molecules is C₂₇H₄₄O: the same atoms, joined in new ways. They are isomers.

Vitamin D₃ travels in the blood to the liver and then the kidneys, which each add an -⁠OH group. The finished molecule is a hormone, a chemical messenger, and it tells your gut to take up calcium from food. Sunlight keeps the amount in check: once plenty of previtamin D₃ has built up, more UVB turns the extra into other molecules, such as lumisterol, so sunshine alone cannot give you too much vitamin D.

Seasons and skin

When the Sun is low in the sky, its light crosses more air, and the ozone absorbs more of the UVB on the way. Far from the equator in winter, skin makes little or no vitamin D: in Boston, 42° north, almost none from November to February. Darker skin needs longer in the sun to make the same amount, because its melanin absorbs more of the UVB first. Oily fish and eggs hold some vitamin D, and the NHS advises everyone in the UK to consider a daily 10 microgram vitamin D supplement in autumn and winter.