Harvard Women's Health Watch

By the way, doctor: Is it safe to take high doses of vitamin D?

Q. On the basis of a blood test, my doctor wants me to take high doses of vitamin D for three months. I hear there are wide variations among laboratories performing these tests. Should I trust my first results, or take the test again? What are the risks of taking high doses of vitamin D?

A. Vitamin D is essential to bone metabolism; below-normal blood levels can increase your risk for osteoporosis, falls, and possibly fractures. There are three main forms of vitamin D. The active form found in our bodies is 1,25-dihydroxyvitamin D; the body makes it from two precursors, vitamin D2 and vitamin D3. Foods and supplements are the main sources of vitamin D2. Vitamin D3 is produced in the skin through exposure to ultraviolet light (as in sunlight); it's also found in foods and supplements. Both vitamin D2 and vitamin D3 are converted into active vitamin D in the liver and the kidneys.

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