Many people don’t get enough vitamin D in their food. They still get enough vitamin D, because ultraviolet radiation radiation creates it—usually.
If you eat a lot of fish, eggs, and mushrooms, you might get the vitamin D you need from your diet. If you don’t, you have to rely on the sun. Specifically, you have to rely on intense ultraviolet radiation. When ultraviolet rays hit a compound known as 7-dehydrocholesterol in the lower layers of the epidermis, it converts to a substance called “previtamin D,” which is not vitamin D, but at least has the potential to become vitamin D in the body.
The heat in the body causes previtamin D to re-scramble itself to a new form of vitamin D, which still can’t be used. It gets taken to your liver where it is reworked into a new form of vitamin D—still unusable. Lastly, it goes to the kidneys, which turn it into the active form of vitamin D, called 1,25-dihydroxyvitamin D.
A lack of vitamin D prevents the body from taking in calcium, which causes bones to bend out of shape—a condition called rickets. The runaround that vitamin D gets in the body confused scientists for years. The breakthrough came in the 1920s, when researchers noticed that rats low on vitamin D recovered when fed hog skin—but only when the skin from the hog had been irradiated with ultraviolet light when it was fed to them. You only need a little sun to get your recommended daily allotment of vitamin D, so there’s no need to sunbathe. If you feel low on D, and also don’t want to expose yourself to ultraviolet rays even for a second ... eat some irradiated hog skin. Or fish.