Vitamin D to Prevent Cancer: How Compelling Is the Evidence?

JoAnn E. Manson, MD, DrPH


April 08, 2011

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Hello, this is Dr. JoAnn Manson, Professor of Medicine at Harvard Medical School, Brigham and Women's Hospital. I'd like to talk with you today about vitamin D and prevention of cancer: Is it ready for prime time? We addressed this subject in a recent perspective article in the New England Journal of Medicine.[1] We [myself and my colleagues who were members of the Institute of Medicine (IOM) Committee on the dietary guidelines for vitamin D] provided a behind-the-scenes look at how the IOM committee evaluated the evidence that vitamin D can prevent cancer.

Of course, these benefits have been widely touted in the popular press, and many of our patients are taking mega-doses of vitamin D (4000-6000 IUs a day) with the hope and expectation that these high doses will prevent cancer. It is important that we are able to share with them some of these findings from when the evidence is reviewed comprehensively and rigorously.

The conclusion of the IOM committee, and of several systematic reviews on this subject, is that the evidence is, in fact, inconsistent and inconclusive with respect to a cause-and-effect relationship for vitamin D in the prevention of cancer. Very importantly, no randomized clinical trial that was designed to test whether vitamin D can prevent cancer, and has shown the relationship, has been completed. Many of the trials have been too small to test this association, and some have used a very low dose of vitamin D, so the hypothesis has not really been fully tested. We definitely need large-scale randomized trials testing adequate doses of vitamin D to see if, in fact, vitamin D has a role in preventing cancer.

The observational research also has been inconsistent. For example, there appears to be an initial association between blood levels of 25-hydroxy vitamin D and breast cancer, which disappears after adjusting for body mass index and physical activity in the Women's Health Initiative. The studies for prostate cancer also have been inconsistent. A large pooling project looking at less common cancers found no association between vitamin D and a large number of individual cancers and even suggested a potential increased risk for pancreatic cancer at high-dose levels of 25-hydroxy vitamin D.[2]

For colorectal cancer, the observational research evidence has been strongest and most consistent. However, in observational studies, association cannot prove causation (a cause-and-effect relationship), and correlations could be related to other confounding factors, such as obesity or physical activity levels, that may be related to outdoor activity, more sun exposure, or just general nutritional status.

Overall, the conclusion, in looking very carefully at the literature -- literally hundreds and hundreds of studies on the subject -- is that the evidence is inconsistent. It's not ready for prime time. The evidence is most compelling for bone health, and that's why the recommended dietary allowances for vitamin D of 600 IUs a day for ages 1-70 years, and 800 IUs a day for ages 70 years and older, are set to cover the needs for bone health of at least 97.5% of the population.

Keep in mind that the recommended dietary allowance is a population recommendation. It is for long-term or chronic use and certainly does not preclude individual decision-making on the part of a clinician that a specific patient may require a higher dose of vitamin D.

Thank you very much. This is Dr. JoAnn Manson.


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