There’s no question that vitamin D is essential for keeping bones strong. But whether having low levels of the vitamin increases your risk of getting or dying from cancer is a lot less certain. Researchers have wrestled with that question for decades.
Three recent studies add to what we know. One study, published in July 2014 in the Journal of Clinical Endocrinology & Metabolism, analyzed 25 previous studies that had looked at vitamin D levels in a total of 17,332 cancer patients. Researchers found the strongest link between vitamin D levels and survival in breast cancer, colorectal cancer and lymphoma. There was less evidence of a connection between vitamin D and survival for leukemia, melanoma, Merkel cell skin cancer and gastric, lung and prostate cancer.
A second study, published in the May 1, 2014, issue of Clinical Cancer Research, found that men with a severe vitamin D deficiency were more likely to be diagnosed with an aggressive type of prostate cancer than men with only a mild deficiency or adequate vitamin D levels. The third, published June 17, 2014, in the BMJ, analyzed eight studies that looked at vitamin D levels and deaths from various causes, including cancer. It found that men and women with a history of cancer and the lowest vitamin D levels were more likely to die from their cancer than were those with the highest levels.
Like many vitamin D studies, these three examined associations—whether levels of vitamin D were associated with cancer outcomes. These types of studies can show that two things happen at the same time but can’t prove that one caused the other.
Vitamin D levels may be a marker for other aspects of health that influence disease risk and outcomes, says Joyanna Hansen, a dietitian at Oregon Health and Science University in Portland. For example, people who spend more time being active outside in the sun tend to have higher vitamin D levels than those who do not. And people who are obese tend to have lower levels of circulating vitamin D than lean people because the body stores the vitamin in fat tissue. Another limitation often cited by researchers is the possibility of reverse causality: that an illness such as cancer is the cause of a vitamin D deficiency rather than its result.
To pinpoint cause and effect, randomized controlled studies are needed—and four large-scale clinical trials looking at vitamin D and cancer risk are now underway. The U.S. trial, called VITAL (The VITamin D and OmegA-3 TriaL), comprises more than 25,000 women and men; half are taking vitamin D and omega-3 supplements, and the other half are taking placebos. “I think VITAL and other large-scale randomized trials will provide informative and even definitive answers to many of these questions in terms of cause-and-effect relationships,” says JoAnn Manson, an epidemiologist at Brigham and Women’s Hospital in Boston, who is co-leading the study. Manson says she and her colleagues expect to publish their initial findings in late 2017 or early 2018.
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