Sources of vitamin D
Vitamin D is called “the sunshine vitamin,” and for good reason. Your skin cells use sunlight to produce a precursor chemical that the liver and kidneys then convert into active vitamin D. Some people make all the vitamin D they need by going outside for a few minutes a day with bare arms and legs. (Don’t wear sunscreen during this short time, except on your face to avoid the photoaging effects of the sun.) Keep your exposure time short—just 10 minutes or so a day— to guard against skin cancer. And if you’re out longer than that, do cover up or apply sunscreen.
However, sunlight alone may not generate adequate amounts of vitamin D for most Americans during much of the year. For example, if you live far ther north than 40° latitude (the latitude of Denver, Indianapolis, and Philadelphia), the winter sunlight isn’t strong enough to produce significant amounts of vitamin D. Sunscreen, glass, and clothing also interfere with your ability to produce the vitamin. People with dark skin produce less vitamin D than those with fair skin. And no matter who you are, as you age, your skin can’t produce vitamin D as readily, your intestines have more difficulty absorbing this vitamin from food or supplements, and your kidneys convert less vitamin D to the active form that your body uses.
For this reason, many vitamin D experts would recommend that you not only follow the Bone Health and Osteoporosis Foundation’s recommendations for vitamin D intake but also have your blood tested for vitamin D, especially if you have osteopenia or osteoporosis, to gauge how much is actually getting into your system.