The Truth About Vitamin D (cont.)

Too much of any good thing is a bad thing. Too much vitamin D can cause an abnormally high blood calcium level, which could result in nausea, constipation, confusion, abnormal heart rhythm, and even kidney stones.

It's nearly impossible to get too much vitamin D from sunlight or from foods (unless you take way too much cod liver oil). Nearly all vitamin D overdoses come from supplements.

The Institute of Medicine's Food and Nutrition Board's old 1997 recommendations suggested that 2,000 IU per day of vitamin D is safe for adults and that 1,000 IU per day is safe for infants up to 12 months of age. Many observers expected a drastic increase in the IOM's 2010 update.

That didn't exactly happen. The IOM committee did increase its "upper level intake" -- that is, the boundary at which it feared vitamin D would become unsafe. That dose is 4,000 IU/day for adults, 3,000 IU/day for kids ages 4-8, 2,500 IU/day for kids ages 1-3, 1,500 IU/day for infants ages 6-12 months, and 1,000 IU/day for infants ages 0-6 months.

But some recent studies suggest that healthy adults can tolerate more than 10,000 IU of vitamin D per day. John Jacob Cannell, MD, executive director of The Vitamin D Council, notes that the skin makes 10,000 IU of vitamin D after 30 minutes of full-body sun exposure. He suggests that 10,000 IU of vitamin D is not toxic.

According to the National Institutes of Health, 25-OHD levels that are consistently over 200 ng/mL are "potentially toxic."

The IOM committee found no conclusive evidence that increased vitamin D levels confer increased health benefits, "challenging the concept that 'more is better.'"

What kind of vitamin D is best?

The recommended form of vitamin D is vitamin D3 or cholecalciferol. This is the natural form of vitamin D that your body makes from sunlight. Supplements are made from the fat of lambs' wool.

However, a clinical study reported in 2008 suggested that vitamin D2 works as well as vitamin D3.

Many supplements contain vitamin D as vitamin D2 or calciferol. It's derived from irradiated fungus. Because this is not the form of vitamin D naturally made by your body, WebMD nutritionist Kathleen M. Zelman, MPH, RD, recommends using the D3 form for those taking vitamin D supplements.

Because of its potency, different forms of vitamin D are used in prescription medications. If you have a prescription for one of these medications, do not switch to another form of vitamin D without checking with your doctor.

Does vitamin D interact with other medications?

Yes. Steroid medications such as prednisone can interfere with vitamin D metabolism. If you take steroid drugs regularly, discuss vitamin D with your doctor.

The weight loss drug orlistat -- brand names include Xenical and Alli -- may cut absorption of vitamin D. So does the cholesterol-lowering drug cholestyramine (sold as Questran, LoCholest, and Prevalite). People taking these drugs should discuss vitamin intake with their doctors.

The seizure drugs Phenobarbital and Dilantin (phenytoin), affect vitamin D metabolism and affect calcium absorption. So do anti-tuberculosis drugs.

On the other hand, cholesterol-lowering statin drugs and thiazide diuretics increase vitamin D levels.

SOURCES:

Cannell, J.J. and Hollis, B.W. Alternative Medicine Review, March 2008; vol 13: pp 6-20.

Holick, M.F. Journal of Clinical Endocrinology and Metabolism, March 2008; vol 93: pp 677-681.

Autier, P. and Gandini, S. Archives of Internal Medicine, Sept. 10, 2007; vol 167: pp 1730-1737.

Holick, M.F. and Chen, T.C. American Journal of Clinical Nutrition, 2008; vol 87: pp 1080S-1086S.

Bordelon, P. American Family Physician, Oct. 15, 2009; vol 80: pp 841-846.

Rovner, A.J. and O'Brien, K.O. Archives of Pediatric and Adolescent Medicine, June 2008; vol 162: pp 513-519.

Pepper, K.J. Endocrinology Practice, 2009; vol 15: pp 95-103.

WebMD Health News: " Vitamin D Deficiency Worsens Breast Cancer?"

WebMD Feature: " Are You Getting Enough Vitamin D?"

WebMD Health News: " Vitamin D Deficiency May Hurt Heart."

WebMD Health News: " Calcium/Vitamin D Slows Weight Gain."

WebMD Health News: " Vitamin D Fights Colon Cancer."

WebMD Health News: " Vitamin D Compounds May Fight Prostate Cancer."

National Institutes of Health Office of Dietary Supplements, Dietary Supplement Fact Sheet: Vitamin D, updated Nov. 13, 2009.

The Vitamin D Council web site.

Reviewed on December 17, 2009

© 2009 WebMD, LLC. All rights reserved.


Last Editorial Review: 12/17/2009 7:46:41 PM



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