Celeste Robb-Nicholson, M.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.

For most women, getting 800 to 1,000 international units (IU) of vitamin D per day provides enough vitamin D for normal bone metabolism. If a vitamin D level is below 30 ng/mL, the deficiency can be corrected by taking high doses -- 50,000 IU of vitamin D as a single dose once a week for six to eight weeks.

The safe upper limit for vitamin D supplementation is 2,000 IU per day. But over limited time periods, even very high doses are safe. The chief adverse effects, which are due to high levels of calcium in the blood and urine, do not occur unless vitamin D levels reach 88 ng/mL -- a concentration that is unlikely to result from either high-dose treatment or regular supplementation.

It's true that laboratory test results can vary considerably. Still, if your blood level is hovering around 30 ng/mL, you could take the high doses for six to eight weeks. It's safe -- and less expensive than being tested a second time. After you've completed the high-dose treatment, take a daily vitamin D supplement of 800 to 1,000 IU. And be sure to get 1,200 to 1,500 milligrams (mg) of calcium a day through diet or supplements.-- Celeste Robb-Nicholson, M.D., Editor in Chief, Harvard Women's Health Watch

© Harvard Health Watch, Celeste Robb-Nicholson, M.D.

 

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Health - Is it Safe to Take High Doses of Vitamin D