In today’s world of designer diets and high-priced supplements, we can still get something that our body needs for free.

Our skin produces vitamin D, which our bodies need to absorb calcium and keep our bones strong. But you can’t just assume your body is making enough vitamin D to meet your needs.

One way to tell is to look at your daily multivitamin, says Eileen Engle, MD, Medical Director of Women’s Services at Doylestown Hospital.

Unfortunately, it’s not always easy. “Even if you’re taking a multivitamin, you don’t always know what type of D you’re taking, so you may not be maintaining adequate levels,” she says.

The Federal Drug Administration recently changed the minimum daily requirement of vitamin D from 400 to 800 international units. Dr. Engle recommends women take 1,000 IU of vitamin D3 daily: the form that the body absorbs best.

Unlike sunbathing, vitamin D3 supplements have no risk of skin damage. They can even reduce your risk of breast and colon cancer, according to some studies. Despite these benefits, Dr. Engle admits being surprised by the low vitamin D levels of some of her patients.

“We don’t get a lot of sun in the winter,” Dr. Engle says, “and more people wear sunblock daily, which prevents conversion of vitamin D.” If you’re concerned about getting enough vitamin D, talk to your doctor about getting a simple blood test that can check your levels and ensure your body is at its best.