Vitamin D helps maintain a strong skeleton and regulate your body's calcium and phosphate. You’ll find it naturally in fatty fish and as an additive in fortified dairy products, cereals, and juices. But most of the vitamin D in your body—about 90 percent—is manufactured internally in response to sun exposure. Since D is fat-soluble, your body can store up the vitamin when it’s sunny and then release it on rainy days.

The Evidence

“This fat-soluble vitamin is necessary for bone health, especially in postmenopausal women and in people with osteopenia and osteoporosis,” says Darcy McConnell, M.D., a functional medicine physician at the Blum Center for Health in Rye Brook, New York. “It’s also useful for immune support, autoimmune disease, depression, fibromyalgia, and weight loss.”

Research has also shown that vitamin D can help treat and prevent rickets, osteoporosis, bone loss, and bone pain. Doctors prescribe it to ease high blood pressure, high cholesterol, multiple sclerosis, rheumatoid arthritis, and premenstrual syndrome, and in one study, taking a supplement with 800 international units (IU) of vitamin D was shown to reduce the risk of falls in elderly people by up to 22 percent.

While sunlight is undeniably the best way to charge your vitamin D battery, you don’t need to take up sunbathing. Skin cancer is no fun, but according to the Skin Cancer Foundation, it will affect one in five people at some point in their lives. Plus, in one study, six days of “casual sunlight exposure” (read: your normal day, minus sunscreen) generated enough vitamin D for 49 days of zero sunlight.

Still, D deficiency is surprisingly common, especially in northern states and in people with darker skin. 

How to Use It

Not sure if you need a supplement? Ask the white-coats. McConnell and other doctors can run a simple blood test to determine vitamin D levels. “It’s rare to see very high levels of D in the blood, but it happens—and it can be toxic,” she warns. So testing before you begin taking a supplement is advised. In the meantime, you can always reach for D-fortified foods to keep your skeleton strong.

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