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.

Vitamin D levels decline when sun exposure is reduced, especially during the winter in northern latitudes. In addition, we absorb vitamin D less efficiently with age. So older people and those with limited sun exposure (including those who are housebound) are especially at risk for low blood levels of vitamin D. Other risk factors are malabsorption syndromes (like Crohn's disease), dark skin (which blocks vitamin D production through sunlight), and excess fat, which can hinder the release of vitamin D into the blood. Too little vitamin D in the blood triggers the release of parathyroid hormone (PTH), stimulating bone resorption (breakdown). When blood levels of vitamin D rise above 30 nanograms per milliliter (ng/mL), PTH returns to normal and bone resorption slows. Most experts believe that a vitamin D level below 30 ng/mL can cause excessive bone resorption.

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