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Update: Our Rickety Understanding of Vitamin D


by Bill Chameides | December 7th, 2010
posted by Erica Rowell (Editor)

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Should you be taking vitamin D supplements? A leading U.S. advisory group weighs in.

In September TheGreenGrok ran a post on vitamin D that featured more questions than answers. In short, even though it’s been known for nearly a century [pdf] that vitamin D is essential in preventing rickets and other bone ailments, there’s still a healthy debate about how much of the stuff we need to stay healthy.

Last week a panel of the independent Institute of Medicine (IOM), the health arm of the National Academy of Sciences, released a report with new Recommended Dietary Allowances (RDA) for vitamin D (as well as calcium because of “its close inter-relationship with vitamin D“). Finally, a simple answer to the question, How much vitamin D do I need every day. Right? Hardly.

Competing Headlines on the Report

If, like me, you first learned about the report in the New York Times and Wall Street Journal, you were probably highly confused by competing headlines: “Report Questions Need for 2 Diet Supplements” in the NYT versus the WSJ’s “Triple That Vitamin D Intake, Panel Prescribes.”

So who got it right?

The IOM did recommend a tripling of Americans’ dietary intake of vitamin D: for Americans aged 1-70 years, the RDA increased from 200 International Units (IUs) to 600 IUs — a recommendation that assumed other dietary sources of vitamin D but minimal sun exposure.*

As noted in a post on the NYT’s Freakonomics blog last week, the information in the two articles is less contradictory than the headlines suggest. To its credit, the NYT also reported that “people need only 600 international units a day” (although one has to wonder if “only” is the appropriate way to describe a factor-of-three increase).

But Are the Panel’s Recommendations Sufficient?

The IOM’s RDA will bring your blood concentration of 25-hydroxyvitamin D to about 20 nanograms per milliliter (ng/mL). And there’s the problem — lots of medical folks consider that insufficient to maintain good health. Citing a host of studies statistically tying low blood levels of vitamin D to a wide variety of ailments, including cancer, many experts recommend that one’s 25-hydroxyvitamin D blood concentration should be at least 30 ng/ml, and that means ramping up one’s daily intake of vitamin D well above the IOM’s latest RDA.

So what gives? The IOM panel’s recommendation was narrowly confined to the amount of vitamin D needed to maintain bone health. It did not consider other ailments that might be related to one’s vitamin D intake. Why? Because the studies linking low levels of vitamin D to bad health outcomes are based on statistical correlations alone; they fall short of establishing causality. And the IOM panel only considered modifying dietary recommendations on the basis of a clear cause-and-effect relationship between the nutrient and disease. The report states:

“a relationship between cancer incidence and vitamin D (or calcium) nutrition is not adequately and causally demonstrated at present; indeed, for some cancers, there appears to be an increase in incidence associated with … higher vitamin D intake. The role of vitamin D related to falls and physical performance, cardiovascular disease, autoimmune disorders, and immune functioning has also received considerable attention, and remains unresolved. These potential roles of vitamin D are currently best described as hypotheses of emerging interest, and the conflicting nature of available evidence cannot be used to establish health benefits with any level of confidence.”

But the report’s authors left themselves an out with their conclusion that “there is not sufficient evidence to establish a relationship between vitamin D and health outcomes other than bone health does not mean that future research will not reveal a compelling relationship between vitamin D and another health outcome.”

The bottom line for now: the IOM recommends tripling your vitamin D intake to 600 IUs. Many will argue that is not enough, and as new data comes in, the IOM could very likely agree. If you ask me what you should do, I have a very simple answer: not my job.

______________________

Notes

*For those older than 70 years of age, the recommended amount increased from 600 IUs to 800 IUs. The upper intake threshold beyond which the risk of adverse risks increases also jumped from 2000 IU to 4000 IU for those 9 years of age and older.

The IOM panel took into account food sources of vitamin D but assumed minimal sun exposure in developing the RDA. So if you are getting some sun, maybe you don’t need to supplement as much. Vitamin D synthesis from the sun was one of the variables the IOM had trouble evaluating. For example, the IOM found that daily intake of vitamin D from food was well below the estimated average requirement (EAR) — a measure of the level needed to meet roughly half of the population’s needs—implying supplementation was necessary. Yet, on average, U.S. blood levels of 25-hydroxyvitamin D were above the EAR (equivalent to 16 ng/mL 25-hydroxyvitamin D).

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