Get answers to commonly asked questions about vitamin D.
By Daniel J. DeNoon
Reviewed by Laura J. Martin, MD
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.
That was surprising news. But just as surprising are assertions 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.
Not so fast, says an expert panel convened by the prestigious Institute of Medicine. In it's long-awaited November 2010 report, the IOM committee expressed horror at the idea that many North Americans are vitamin D deficient.
"Of great concern recently have been the reports of widespread vitamin D deficiency in the North American population," the committee wrote. "The concern is not well founded. In fact, the cut-point values used to define deficiency, or as some have suggested, 'insufficiency,' have not been established systematically using data from studies of good quality."
The IOM committee put its emphasis on what science has proved, not on what studies may suggest. Using this conservative approach, the committee found no proof that vitamin D has health effects beyond building strong bones.
"While the current interest in vitamin D as a nutrient with broad and expanded benefits is understandable, it is not supported by the available evidence," the IOM committee concluded.
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 blood pressure, flu, neuromuscular diseases, and osteoporosis. However, there have been no definitive clinical trials.
That's why the Institute of Medicine expert committee's November 2010 review found no conclusive evidence that vitamin D, by itself, offers wide-ranging health benefits.
"Despite the many claims of benefit surrounding vitamin D in particular, the evidence did not support a basis for a causal relationship between vitamin D and many of the numerous health outcomes purported to be affected by vitamin D intake," the IOM committee concluded.
The only proven 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. This is a problem for people in northern climates. In the U.S., only people who live south of a line drawn from Los Angeles to Columbia, S.C., get enough sunlight for vitamin D production throughout the year.
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. This is a particular problem for African-Americans in the northern U.S.
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. And unless you live in the South or Southwest, you probably won't get enough sunlight during the winter months for your body to make enough vitamin D. 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?
That depends on whom you ask. As part of your regular blood test, your doctor can order a test for 25-hydroxyvitamin D (25-OHD).
The problem is not with the test. The problem is how to interpret the results. An expert committee convened by the Institute of Medicine in November 2010 concluded that "the cut-point values used to define deficiency, or as some have suggested, 'insufficiency,' have not been established systematically using data from studies of good quality."
Even so, most experts agree 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. A 2002 study found that 42% of African-American women of childbearing age had vitamin D levels below 15 ng/mL.
The IOM committee says that people are at risk of vitamin D deficiency at 25-OHD levels below 30 nmol/L (12 ng/mL), and that some people -- but not everyone -- may be at risk of vitamin D deficiency at 25-OHD levels from 30 nmol/L up to 50 nmol/L (12-20 ng/mL).
The Vitamin D Council considers the ideal 25-OHD level to be between 40 ng/mL and 70 ng/mL. But the IOM says there is no evidence of increased benefit at levels above 30 ng/mL, and that "there may be reason for concern" at levels above 50 ng/mL.
"There is a critical public health and clinical practice need for consensus cut-points for serum 25-OHD," the IOM committee states.
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
Nearly all milk in the U.S. is fortified with vitamin D. So are many brands of orange juice, yogurt, margarine, and ready-to-eat breakfast cereals.
How much vitamin D do I need?
In November 2010, the Institute of Medicine's expert committee set a new "dietary reference intake" for vitamin D.
Assuming that a person gets virtually no vitamin D from sunshine -- and that this person gets adequate amounts of calcium -- the IOM committee recommends getting the following amounts of vitamin D from diet or supplements (Note that the IOM's upper limit is not a recommended intake, but what the IOM considers the highest safe level):
- Infants age 0 to 6 months: adequate intake, 400 IU/day; maximum safe upper level of intake, 1,000 IU/day
- Infants age 6 to 12 months: adequate intake, 400 IU/day; maximum safe upper level of intake, 1,500 IU/day
- Age 1-3 years: adequate intake, 600 IU/day; maximum safe upper level of intake, 2,500 IU/day
- Age 4-8 years: adequate intake, 600 IU/day; maximum safe upper level of intake, 3,000 IU/day
- Age 9-70: adequate intake, 600 IU/day; maximum safe upper level of intake, 4,000 IU/day
- Age 71+ years: adequate intake, 800 IU/day; maximum safe upper level of intake, 4,000 IU/day
That's not enough, says Boston University vitamin D expert Michael Holick, MD, PhD, professor of medicine, physiology, and biophysics, Boston University Medical Center. 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.
But it's clear that the IOM's conservative recommendations will stir debate in the scientific and medical communities. Here's a rule of thumb: If you're considering taking more vitamin D than the IOM committee recommends, first check with your doctor or pediatrician.
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 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.'"
Quick GuideVitamin D Deficiency: How Much Vitamin D Is Enough?
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.
On the other hand, cholesterol-lowering statin drugs and thiazide diuretics increase vitamin D levels.
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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
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