Do Vitamin D Supplements Reduce the Risk of Dying from Cancer?


It all started with this famous study, published in 1980. Johns Hopkins researchers were trying to figure out why states like New Mexico and Arizona have only about half the colon cancer rates of states like New York, New Hampshire, and Vermont. Maybe it’s because they got so much sun. And so, they proposed that maybe the sunshine vitamin—vitamin D—is a protective factor against colon cancer. Since then, sun exposure has been associated with lower rates of 14 other types of cancer, too.

Vitamin D may also affect cancer survival. Higher blood levels of vitamin D were associated with lower mortality of patients with colorectal cancer. How much lower? Like nearly half the mortality. And, the higher the D levels, the lower the death rate appeared to fall. This may explain why the survival rate from colon cancer may depend, on part, on the season of diagnosis—the reason the risk of a rapid death is lowest if you’re diagnosed in the fall, after you’ve spent the summer building up your vitamin D stores.

But look; there are other risk factors that could be seasonal, too. Maybe people are taking advantage of the fall harvest, and eating healthier. Maybe that’s why the lower risk in the fall season. Or, maybe there’s more drinking in the winter. And, in the summer, running around outside, not only are you getting more sun; you’re running around outside, getting more exercise—which may itself be protective.

So, these kinds of studies just provide circumstantial evidence. Establishing a cause-and-effect relationship between colon cancer and vitamin D deficiency using observational studies is challenging, because of confounding factors like the exercise—so-called “lurking variables.” For example, there may be a tight correlation between ice cream sales and drowning deaths, but that doesn’t mean ice cream causes drowning. A more likely explanation is that there is a lurking third variable—like hot weather, summertime—that explains why drowning deaths are highest when ice cream consumption is highest.

That’s kind of a trivial example. But, this actually happened with hormone replacement therapy. Women taking drugs like Premarin appeared to have 50% less risk of heart disease. And so, doctors prescribed it to women by the millions. But, if you dig a little deeper into the data, yes, women taking estrogen had 50% lower risk of dying from heart disease. But, they also had a 50% lower risk of dying from accidents and homicide. So, it probably wasn’t the drug. See, the only way to know for sure is to put it to the test, in a randomized clinical trial, where you give half the women the drug, and see what happens.

And, a decade later, they did. And, instead of having a 50% drop in risk, within a year of being given the hormone pills, heart attack and death rates shot up 50%. In retrospect, the lurking variable was likely socioeconomic class. Poor women are less likely to be prescribed hormone replacement therapy, and more likely to be murdered, and die of heart disease. Because of the lurking variable, a drug we now know to be dangerous had appeared protective.

Besides lurking variables, there’s also the possibility of reverse causation. Maybe low vitamin D levels didn’t worsen the cancer. Maybe the cancer worsened the vitamin D levels. This may be unlikely, since tumors don’t appear to directly affect vitamin D levels. But cancer treatment might. Even simple knee surgery can dramatically drop vitamin D levels within hours, thought to be because of just the inflammatory insult of cutting into somebody. So, maybe that could help explain the link between lower D, and lower survival. And hey, if you’ve got cancer, maybe you’re spending less time running around at the beach.

So, yes, higher vitamin D levels are associated with improved survival in colorectal cancer, and in breast cancer. In fact, about double the risk of breast cancer recurrence and death in women with the lowest vitamin D levels. And, vitamin D levels also associated with longer survival with ovarian cancer, and other cancers, like lymphoma. But, bottom-line, as we learned with hormone replacement, is that you have to put it to the test. But, there weren’t a lot of randomized controlled trials on vitamin D supplements and cancer—until now.

We now have a few randomized controlled trials, and vitamin D supplements do indeed appear to reduce the risk of dying from cancer. What dose? The researchers suggest maybe getting blood levels up to at least around 75 nanomoles per liter; levels not reached by as many as three-quarters of women with breast cancer, or a striking 97% of colon cancer patients.

Getting up to these kinds of levels, 75, or perhaps even better, 100, might require about 2,000 to 4,000 international units of vitamin D a day—levels of intake for which there appear to be no credible evidence of harm. Regardless of what the exact level is, the findings of these kinds of studies may have a profound influence on future cancer treatment.

Doctor’s Note

What about just getting sun, instead? Be sure to check out my recent six-part video series:

Better, of course, to prevent colon cancer in the first place. See, for example:

For more on that extraordinary story about Premarin and hormone replacement therapy, see How Did Doctors Not Know About the Risks of Hormone Therapy?

In health,
Michael Greger, M.D.

Michael Greger, M.D., is a physician, New York Times bestselling author, and internationally recognized professional speaker on a number of important public health issues. Dr. Greger has lectured at the Conference on World Affairs, the National Institutes of Health, and the International Bird Flu Summit, testified before Congress, appeared on The Dr. Oz Show and The Colbert Report, and was invited as an expert witness in defense of Oprah Winfrey at the infamous “meat defamation” trial. Currently Dr. Greger proudly serves as the Director of Public Health and Animal Agriculture at the Humane Society of the United

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