Published: August 16, 2008 11:06 pm            

Get your daily dose of D

A whole lot of Americans are D-ficient – lacking adequate levels of vitamin D circulating in the bloodstream, according to research that increasingly suggests shortfalls contribute to numerous health woes.

Several recent studies suggest vitamin D plays an important role in keeping a lot more than our bones healthy, affecting everything from the immune system to circulation to development of the brain and the risk of developing certain cancers.

People take in vitamin D either through supplements or in their diets from milk, some fish, meat and fortified foods, along with exposure to sunlight.

The latest evidence on vitamin D and human health was published in the Archives of Internal Medicine.

Tracking 13,000 initially healthy men and women through a national health survey for almost nine years, researchers from Johns Hopkins University found that those with the lowest blood levels of vitamin D faced a 26 percent greater risk of death compared with people with greater amounts of the vitamin.

Other recent studies have suggested a link between vitamin D shortages and increased rates of breast cancer and depression in the elderly, lung function and hypertension, along with better-known risks for bone weakness and fractures.

“Our results make it much more clear that all men and women concerned about their overall health should more closely monitor their blood levels of vitamin D, and make sure they have enough,” said Dr. Erin Michos, co-investigator for the study.

Blood testing is necessary because doctors need to know how much of the vitamin is circulating in a form called 25-hydroxyvitamin D. This is converted in the kidney – and several other parts of the body, including the brain – into its active form. Then, it functions as a hormone that controls levels of calcium and phosphorus in the blood and acts as a cell growth regulator.

The current “healthy” threshold for vitamin D in the bloodstream is 25 to-30 nanograms per milliliter of blood. This level is supposed to be achieved if people are taking in 200 to 400 international units of the vitamin through diet or pills each day (with twice that much for the elderly and other high-risk groups).

Surveys show 41 percent of adult men and 53 percent of women have D levels lower than 28 nanograms, and other research indicates 40 percent of toddlers may be deficient.

That’s prompting calls from some scientists for the D bar to be raised to 60 monograms or higher, with intake from diet or supplements in the 2,000-unit a day range. Most researchers agree that the toxic threshold for the vitamin is around 10,000 units a day – more than that can damage the kidneys, among other things.

But there’s still a lot of debate among scientists about how much D is enough, how we use it and how it’s measured. For instance, levels of the vitamin are known to fluctuate in direct proportion to how physically active a person is.

There’s also the sunlight issue. Just 15-20 minutes of daily skin exposure to the sun is thought to produce sufficient amounts of vitamin D. But people living north of 40 degrees latitude – above a line running from Philadelphia to Omaha to Northern California – don’t get enough year-round sun to cover their D needs. Neither do many elderly people elsewhere, people with dark skin color, even breastfed infants who don’t get supplements.

Experts who reviewed recent vitamin D and health studies last fall at a conference sponsored by the National Institutes of Health concluded it’s too early to revise daily intake guidance upward.

“It’s easy to see why people are so enthusiastic about the potential power of vitamin D, but we must recognize the limitations of any study and exercise caution when making any broad public health recommendations,” said Mary Picciano, a senior nutrition scientist at the NIH who co-authored a report on the conference published this month in the American Journal of Clinical Nutrition.

The report notes, among other things, that many studies have not taken into account other health factors or nutrients that might accompany low or high vitamin D intake, that there’s not enough reliable data on the D content of many foods, and that blood tests used to measure levels of the vitamin are not consistent.

“There’s still a lot we don’t know about how vitamin D levels affect health, especially across different age groups and ethnic populations,” Picciano said.

On the Web: http://archinte.ama-assn.org

        

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