The Truth About Vitamin D

WebMD
The hottest topic in medicine isn't the newest drug or the latest surgical device: It's vitamin D.

What brought the simmering debate to a boil was a 2007 study showing that people taking normal vitamin D supplements were 7% less likely to die than those who didn't take the daily supplements.

A year later, a major study found that when women with low vitamin D levels get breast cancer, they have a much higher chance of dying from their cancer than women with normal vitamin D levels.

That was surprising news. But just as surprising is the fact that many men, women, and children have insufficient blood levels of this important vitamin.

How many? Data suggest many of us don't get the vitamin D we need. For example, a 2007 study of childbearing women in the Northern U.S. found insufficient vitamin D levels in 54% of black women and in 42% of white women.

These findings led the American Academy of Pediatrics to double the recommended amount of vitamin D a child should take -- and have led many doctors to advise their adult patients to up their vitamin D intake.

Why do I need vitamin D?

Your body must have vitamin D to absorb calcium and promote bone growth. Too little vitamin D results in soft bones in children (rickets) and fragile, misshapen bones in adults (osteomalacia). You also need vitamin D for other important body functions.

Vitamin D deficiency has now been linked to breast cancer, colon cancer, prostate cancer, heart disease, depression, weight gain, and other maladies. These studies show that people with higher levels of vitamin D have a lower risk of disease, although they do not definitively prove that lack of vitamin D causes disease -- or that vitamin D supplements would lower risk.

The Vitamin D Council -- a scientist-led group promoting vitamin D deficiency awareness -- suggests vitamin D treatment might be found helpful in treating or preventing autism, autoimmune disease, cancer, chronic pain, depression, diabetes, heart disease, high bloodpressure, flu, neuromuscular diseases, and osteoporosis. However, there have been no definitive clinical trials.

The best known benefit of vitamin D is its role in helping calcium build strong bones. But that's far from the whole story. Vitamin D helps regulate the immune system and the neuromuscular system. Vitamin D also plays major roles in the life cycle of human cells.

Vitamin D is so important that your body makes it by itself -- but only after skin exposure to sufficient sunlight. Dark skin absorbs less sunlight, so people with dark skin do not get as much vitamin D from sun exposure as do light-skinned people.

How can I get enough vitamin D?

Thirty minutes of sun exposure to the face, legs, or back -- without sunscreen -- at least twice a week should give you plenty of vitamin D.

But this much direct sun exposure might also expose you to potentially dangerous levels of cancer-causing UV radiation. The American Academy of Dermatology recommends against getting vitamin D from unprotected exposure to sunlight. It's probably a better idea to get vitamin D from foods or from supplements.

Will a vitamin D test tell me if I need more vitamin D?

Yes. As part of your regular blood test, your doctor should order a test for 25-hydroxyvitamin D (25-OHD).

Everyone agrees that anyone with a 25-OHD level of less than 15 ng/mL or 37.5 nmol/L (depending on the units reported by a lab) needs more vitamin D. The U.S. National Institutes of health notes that 25-OHD levels over 30 ng/mL are optimal, and that there is "insufficient data" to support recommendations for higher levels. The Vitamin D Council considers the ideal 25-OHD level to be between 40 ng/mL and 70 ng/mL.

Which foods contain vitamin D?

Surprisingly few foods contain vitamin D -- unless it's added to the food. That's because your body is built to get vitamin D through your skin (from sunlight) rather than through your mouth (by food). But once your body has enough, it doesn't matter whether you got it through your skin or through your stomach.

There are three vitamin D super foods:

• Salmon (especially wild-caught)
• Mackerel (especially wild-caught; eat up to 12 ounces a week of a variety of fish and shellfish that are low in mercury)
• Mushrooms exposed to ultraviolet light to increase vitamin D

Other food sources of vitamin D include:

• Cod liver oil (warning: cod liver oil is rich in vitamin A; too much may be bad for you)
• Tuna canned in water
• Sardines canned in oil
• Milk or yogurt -- regardless of whether it's whole, nonfat, or reduced fat -- fortified with vitamin D
• Beef or calf liver
• Egg yolks
• Cheese

How much vitamin D do I need?

The current recommended daily dose of vitamin D is 200 IU for people up to age 50, 400 IU for people aged 51 to 70, and 600 IU for people over age 70.

That's not enough, Boston University vitamin D expert , MD, PhD, tells WebMD. Holick recommends a dose of 1,000 IU a day of vitamin D for both infants and adults -- unless they're getting plenty of safe sun exposure.

In 2008, the American Academy of Pediatrics recommended that breastfed infants receive 400 IU of vitamin D every day until they are weaned. This doubled the AAP's previous recommendation.

The AAP also recommends 400 IU/day of vitamin D for children and teens who drink less than a quart of vitamin D-fortified milk per day.

The Vitamin D Council recommends that healthy adults take 2,000 IU of vitamin D daily -- more if they get little or no sun exposure.

There's evidence that people with a lot of body fat need more vitamin D than lean people.

Can I get too much vitamin D?

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 1997 recommendations -- scheduled for a May 2010 update -- suggest 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.

However, the relatively small doses of vitamin D in daily vitamin pills are not enough to correct serious vitamin D deficiency. A 2009 study suggested that the best regimen for treating vitamin D insufficiency is 50,000 IU of vitamin D3 taken three times a week for six weeks. This time-limited regimen did not result in vitamin D toxicity.

How much vitamin D is too much? That's controversial. According to the National Institutes of Health, the maximum upper limit for vitamin D is 25 micrograms (1,000 IU) for children up to age 12 months and 50 micrograms (2,000 IU) for everyone else.

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."

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.