And while one eight-ounce serving of orange juice is all you need to get your daily fill of vitamin C, to get enough D you’d need to drink ten tall glasses of milk or eat seven pounds of wild salmon—every single day. “There is essentially no vitamin D in your diet,” says Michael F. Holick, M.D., Ph.D., a professor of medicine at Boston University. His surprising solution? Embrace the sun again—sans sunscreen.
Our body synthesizes vitamin D naturally when exposed to sunlight, but high-SPF lotions block the ultraviolet rays needed for its production. The UV triggers a form of cholesterol in our skin that is converted into D in our kidneys. This is where Kastor weakened her system. A skin cancer survivor, she is religious about sun protection. “I was so surprised that my effort to be healthy caused damage in another way,” she says. Now recovered, Kastor relies on daily D supplements—and hasn’t been injured since.
“Different skin types and diets make everyone’s sun needs unique,” says Robyn Lucas, M.D., Ph.D. But generally, “short sun exposure of more skin is a lot safer than longer exposure of just your face and hands.” Even the American Cancer Society is getting on board, recently issuing a joint statement with the Canadian Cancer Society that concluded “supplementation and small amounts of sun exposure are the preferred methods of obtaining vitamin D.”
Still, time in the sun is never safe for some. If you have a fair complexion or a family history of skin cancer, stick to nonsolar sources. Also, if you have dark skin (which, like sunscreen, impedes vitamin D synthesis), aim to get 1,000 international units (IU) a day through supplements. Most multivitamins contain only 400 to 600 IU of D, so add an extra tablet of pure vitamin D to your daily intake. Popping pills is definitely easier than finding new ways to cook up seven pounds of salmon every day.