Questions & answers about vitamin D supplementation

Townsend Letter for Doctors and Patients, Nov, 2006 by John Cannell

How Much Vitamin D Should I Take?

Before I answer, I have a few questions for you. Where do you live (latitude)? How much do you weigh? What skin type do you have? How much do you go into the sun? How much vitamin D do you get in your diet? How much do you get in supplements? What is your 25-hydroxy-vitamin D level? During what season of the year was it obtained?

You see, the answer is different for everyone. The question also assumes we know the ideal vitamin D blood level. We don't know for sure; around 50ng/ml is the best current guess. However, an easy answer is 2,000 units a day. Life Extension Foundation sells an inexpensive pharmaceutical grade of vitamin D. Two hundred and fifty of the 1,000 unit capsules cost about ten bucks. Your local health food store should also carry 1,000 unit capsules. Just be sure they don't contain any vitamin A. Beta-carotene is okay, but preformed retinols (vitamin A) interfere with vitamin D's function. I do not advise cod liver oil; if you just have to take it, don't take more than a teaspoon per day. Get your omega-3s from molecularly distilled fish body oils (available at Trader Joe's) or by eating wild salmon or sardines. Get your vitamin D by taking vitamin D, by going into the sun, or by combining both approaches (J Nutr. 1999 Dec;129(12):2246-50; J Bone Miner Res. 2001 Oct;16(10):1899-905; J Nutr. 2005 Jul;135(7):1647-52).

The government says 2,000 units of vitamin D a day is safe for all adults to take without a doctor's supervision. Two thousand units a day will get about 80% of Americans to a vitamin D level of 35 ng/ml or higher with no risk of toxicity. The 20% of the Americans with levels still lower than that are almost all African-American (J Steroid Biochem. Mol Biol. 2005 Oct;97(1-2):13-9).

For now, if you take more than 2,000 units a day, you should do so either because you are under the care of a knowledgeable health care practitioner (good luck finding one) or because you know what you are doing. I personally take no vitamin D in the late spring, summer, or early fall because I frequently go into the sun. I take 5,000 units a day in the winter, but I know my vitamin D level, because I get it checked several times a year. If I had cancer or heart disease or Alzheimer's disease or multiple sclerosis or a hundred other common diseases, I might take more than 5,000 units a day, but I'd check my calcium and vitamin D levels even more often. I'd also follow my doctor's advice about standard medical treatment.

Keep in mind that if you use a sunlamp or a tanning parlor once a week during the winter, you don't have to worry about blood tests, because your body will maintain adequate vitamin D blood levels. Just don't get burned. Also, remember to take calcium tablets if you don't get adequate amounts of calcium in your diet; most people don't. Trader Joe's sells a good Calcium/Magnesium/Zinc combination product for next to nothing.

What Blood Test Should I Have?

The only blood test that can determine vitamin D adequacy is a 25-hydroxy-vitamin D. Whenever I say "vitamin D level," I'm talking about a 25-hydroxy-vitamin D. Ask your doctor to order a 25-hydroxy-vitamin D blood test. Unfortunately, many doctors order an "activated vitamin D" level, thinking it better to measure the most active form of vitamin D. It is not. Activated vitamin D, also known as 1,25-di-hydroxy-vitamin D or calcitriol, should never be obtained to determine vitamin D sufficiency. Calcitriol is often elevated in vitamin D deficiency. You cannot tell anything about your vitamin D nutrition by measuring a calcitriol level. If your doctor insists on ordering a calcitriol level to determine your vitamin D nutrition, find another doctor.

However, serious problems exist with the technology used by some laboratories to measure vitamin D levels. Different labs will report different results when given the exact same specimen of blood. Furthermore, the same lab often reports significantly different numbers when sent the same specimen of blood at different times. In general, low numbers are more reliable than high numbers because interfering substances can easily give falsely elevated results. Prominent scientists have issued urgent calls for standardization. Dr. Graham Carter, one of my personal heroes, has dedicated his life to obtaining standardized--and accurate--vitamin D levels. His outfit in London is the Vitamin D External Quality Assessment Scheme (DEQAS). Call a reference lab, such as Quest Diagnostics (800-222-0446), and insist they participate in DEQAS. DEQAS's phone in England is 44 (0)20 8383 3645. (J Clin Endocrinol Metab. 2004 Jul;89(7):3152-7; J Clin Endocrinol Metab. 2004 Jul;89(7):3149-51.)

If you take ergocalciferol, or "vegetarian" vitamin D, be warned. Ergocalciferol is not vitamin D, but a vitamin D-like patent drug whose patent has expired. It does not normally occur in the human body and is probably a weak agonist at the receptor site, meaning it may actually partially block vitamin D actions. Ergocalciferol is the villain in most of the reported cases of toxicity in the world's literature. All bets are off in terms of measuring blood levels if you take ergocalciferol. Some of the labs can pick it up, and some can't. Don't take ergocalciferol; it is not vitamin D.