It’s well established that black men are more than twice as likely to die from prostate cancer as white men. A new study (paywall) suggests that differences in the way African-Americans process vitamin D could explain some of these worse outcomes. For several reasons not completely understood, black men with prostate cancer appear to be more sensitive to vitamin D deficiency—and less able to create and use vitamin D—than men of other races.

In the study, the researchers compared 275 white men and 273 black men with recent prostate cancer diagnoses, sampled from clinics in the Chicago area. They found that black men with low levels of vitamin D were twice as likely to have highly-aggressive tumors as white men with similar vitamin D levels.

For at least 25 years, scientists have known that vitamin D has some association with prostate cancer, and that people with African origins generally have lower vitamin D levels than those with European origins. But addressing low vitamin D levels (which are associated with various other health problems, including rickets, diabetes, and cardiovascular disease) has been easier said than done.

The findings suggest that black men are more sensitive to vitamin D deficiency when it comes to prostate cancer—and therefore more likely to develop aggressive tumors even at the same vitamin D levels as their white counterparts. And because black men are less able to synthesize vitamin D from sunlight or absorb it from food, they are also more likely to suffer from such deficiencies.

“This association being stronger in blacks for higher grades of the disease suggests that vitamin D deficiency does explain some of the disparity in aggressiveness between blacks and whites,” says co-author Adam Murphy, an urologist from Northwestern University’s Feinburg School of Medicine in Chicago.

Humans get most of their vitamin D from sunlight. When a sufficient amount of it strikes their outer layer of skin, it triggers a reaction with a substance called 7-dehydrocholesterol, which naturally synthesizes vitamin D. Studies have suggested that for those with darker pigmented skin, it takes more exposure to sunlight to start synthesizing vitamin D.

But it might not just be skin tone that makes it harder for black men to get enough vitamin D. The vitamin also can be taken in through food or supplements, but in that form the vitamin D is inert and the body has to do some chemistry before it can use it. Other studies seem to show that, because of their genetics, people with African ancestry are less able than other races to convert this inert vitamin D into a usable form.

Doctors have long recommended that African-Americans get more vitamin D. Murphy hopes that testing for a deficiency of the vitamin can also help diagnose aggressive forms of prostate cancer, and reduce invasive over-treatment of the disease. His research may lead to something called a biomarker validation study, which could be used by doctors to help diagnose aggressive forms of prostate cancer, and save men with relatively benign tumors from costly, and often painful, treatments.

If successful, this would mean a vitamin D test could be added to the existing tests for prostate cancer diagnosis: the digital rectal exam, and prostate specific antigen (PSA) test. “The limitations of PSA and the digital rectal exam is that they only predict outcome 25 percent of the time,” Murphy says. Inadequacies in diagnosing prostate cancer has led to a lot of overtreatment, and the medical community is looking for ways to better calibrate their diagnoses.

For vitamin D to successfully become a biomarker, research would need to prove that the test is safe, consistent, and a provable substitute for clinical endpoint. Testing this would take around five to seven years and require a sample of several thousand men.

But lest this research prompts you to go on a sunbathing binge to increase your vitamin D levels, just remember to lather up with sunscreen: Men are more likely to get skin cancer than tumors on their prostate.