The vitamin D-cision brings surprises

New dietary standards are more cautious than many expected.

After a decade of glowing reports for vitamin D, a panel of experts has seemingly thrown the dimmer switch on the sunshine vitamin. In revising the 1997 dietary guidelines, a panel convened by the Institute of Medicine (IOM) concluded that there wasn't enough solid evidence to support taking vitamin D to stave off a host of disorders from colon, breast, and prostate cancer to heart disease, arthritis, and autoimmune conditions. The panel's report, released on Nov. 30, 2010, also throws cold water on the assumption that most North Americans are deficient in vitamin D. The new recommended intake of the vitamin is higher, but not nearly to the level some advocates had anticipated and called for.

Many researchers had argued that an update of the vitamin D guidelines was long overdue because of new evidence for the vitamin's expanded health benefits. In response, the United States and Canadian governments asked the IOM, an independent body of medical scientists, to review the research and, if needed, change the recommendations for vitamin D and calcium. Calcium was included because the two nutrients are inseparable; vitamin D is essential to the metabolism of calcium.

The IOM selected 14 scientists from a range of disciplines and institutions to serve on an expert panel, including Dr. JoAnn E. Manson, a member of the Health Letter's editorial board and chief of the Division of Preventive Medicine at Harvard-affiliated Brigham and Women's Hospital. The panel was asked to do three things: determine what health conditions are influenced by calcium and vitamin D, set the amount of each necessary to achieve good health in most people, and establish safe upper limits for both. The experts reviewed surveys on nutritional intake to determine how much calcium and vitamin D North Americans are getting, used mathematical models to estimate how much they need, and studied the scientific literature for evidence of toxic levels of both nutrients.

The panel's recommendations for calcium were similar to the 1997 guidelines — 1,000 milligrams (mg) a day for women through age 50 and men through age 70, and 1,200 mg a day for older women and men. They found only two segments of the population whose calcium intake was of concern — teenage girls, who may not get enough, and women over 50, some of whom get too much. They advised women in the latter age group to limit calcium consumption from food and supplements to 1,200 mg a day to avoid kidney stones and other complications of calcium overload.

Deciding what D doesn't do

Evaluating vitamin D was more challenging. Manufactured in the skin in response to ultraviolet B radiation, D is the only vitamin that humans can make for themselves. From a blood test, it's impossible to tell whether people are getting it through sun exposure, diet, or supplements. And because it functions as a hormone that facilitates calcium metabolism, it is difficult to separate vitamin D's effects from those of calcium.

The panel set high standards for evidence of disease prevention, giving the most weight to randomized controlled clinical trials, which leave little doubt that the effect produced is due to the substance being tested. By those standards, the panel found ample evidence for vitamin D's bone-building prowess, but determined that data on the vitamin's role in preventing other conditions were often conflicting, inconclusive, or otherwise inadequate. Thus, the panel decided to base its vitamin D recommendations solely on the amount necessary for healthy bones.

To estimate whether North Americans are getting enough vitamin D, the IOM panel reviewed data on blood levels of the vitamin in several populations. Most of the blood tests for vitamin D measure a form of the vitamin called 25- hydroxyvitamin D, or 25OHD, and levels reflect both vitamin D produced by sun exposure and that ingested orally, either in food or from supplements. The IOM panel calculated that half the population is protected by 25OHD levels as low as 16 nanograms per milliliter (ng/ml) and that 20 ng/ml is adequate for 97.5% of the population. In contrast, many of the reports of widespread vitamin D deficiency have assumed that blood levels of 30 ng/ml and above were desirable.

The new RDA and upper limit

The panel relied on national nutrition surveys in the United States and Canada to determine the average vitamin D intake. It discovered that most North Americans' diets don't include the 400 IU that the 1997 recommendations advised middle-aged adults to get on a daily basis. Yet the panel also noted that representative samples indicate that the average blood level of 25OHD of Americans and Canadians is safely above the 20 ng/ml level that it deemed adequate. The panel concluded that most people in the two countries seem to compensate for dietary deficiencies of vitamin D through either sun exposure or supplements.

The new Recommended Dietary Allowances (RDA) for vitamin D are 600 IU a day for everyone ages 1 to 70 and 800 IU a day for those 71 and older. They are based on the amount of ingested vitamin D needed to produce blood levels of 20 ng/ml and assume that people aren't getting any of the vitamin from sun exposure. They are designed to protect people with dark skin, those living in upper latitudes, the homebound, and scrupulous sunscreen users.

It's impossible to overdose on vitamin D from sun exposure (the body inactivates the excess), and difficult to do so from food. But you can get too much through supplementation. The panel acknowledged that symptoms of vitamin D toxicity don't occur until intake reaches 10,000 IU a day, but set 4,000 IU as the daily upper limit for most age groups based on evidence linking high doses and blood levels to possible increases in risk for cardiovascular disease, pancreatic cancer, and premature death from all causes.

What should you do?

If you are generally healthy and taking vitamin D for your bones, supplements supplying a total of 600 to 800 IU a day will cover you.

If you have been taking vitamin D for other reasons, you may want to have a talk with your doctor. The IOM panel gave more weight to randomized controlled trials and called for more of them to be done (Dr. Manson is the lead investigator of one such trial, www.vitalstudy.org). However, it didn't entirely dismiss observational evidence linking vitamin D supplementation with a reduction in the risk of certain cancers and chronic degenerative conditions. Some doctors believe there's already enough evidence for taking 1,000 IU, even 2,000 IU, of vitamin D a day — which is still well under the new upper limit of 4,000 IU.

You may also want to revisit vitamin D testing with your physician. The panel cited a need to standardize vitamin D test methods and ways of reporting results, if only because so many laboratories seem to be at odds with the new guidelines deeming 25OHD levels of 20 ng/ml sufficient. In fact, the National Institutes of Health's own MedlinePlus Web site identifies the "normal" blood levels of 25OHD as ranging from 30 to 74 ng/ml, and several major medical centers use similar reference values. The test can cost $100 to $200. Several Canadian provinces have refused to reimburse healthy patients for vitamin D tests, and American insurers may follow suit.