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.

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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 1​7, 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.​