The relationship between melanin and vitamin D – the nutrient that sunlight provides – may explain why African American, Caribbean, and men of African ancestry have the highest rates of prostate cancer than anyone in the world, according to a new study.
The study by a team of researchers at Northwestern University, which appears in this month's issue of Clinical Cancer Research, finds that vitamin D deficiency is associated with increased risk of diagnosis among black men – but not among white men.
"Our report is the first to describe the association of vitamin D deficiency and outcomes of prostate biopsies in high-risk men with an abnormal (blood test or clinical exam)," the study states. "If vitamin D is involved in prostate cancer initiation or progression, it would provide a modifiable risk factor for primary prevention and secondary prevention to limit progression, especially in the highest risk group of African-American men."
Among American men, prostate cancer is the most common cancer, and the second leading cause of cancer deaths. One in seven American men will develop it in their lifetime. However, African-American men are 60 percent more likely than whites to be affected, according to the American Cancer Society. Although the mortality rate is among the lowest of all cancers, it is more than twice as high for African-American men than white men. (The incidence of prostate cancer is low among Latino and Asian men).
It's especially a concern for men over 50, as the risk of onset rises steadily over time; cancer (in general) is the number one cause of death for African-American men age 65 to 84 in the United States, according to the Centers for Disease Control and Prevention.
The study tested the vitamin D levels of nearly 700 men in the Chicago area undergoing their first prostate biopsies, which is the usual recommendation after an abnormal test result or clinical exam. Researchers found that while severely low vitamin D levels were associated with more aggressive tumors, across race, African-American men with even moderately low vitamin D levels had higher odds of being diagnosed after that initial biopsy. There was no similar link among the white men studied.
Vitamin D primarily allows the body to absorb calcium, but it also plays a role in regulating cell growth and creation. Although the nutrient can be found in a handful of foods – most significantly in fatty seafood, such as wild-caught salmon – the body primarily creates its own vitamin D by absorbing sunlight. Melanin, which naturally blocks the sun's ultraviolet (UV) rays, alters this process.
"The darker the color of the skin, the less effective sunlight is in producing vitamin D in skin," says Dr. Donald Trump, president and CEO of the Roswell Park Cancer Institute, the first cancer center in the nation. (Trump was not involved in this particular study). "An African-American person is more likely to have lower levels of vitamin D than a European person, because the same amount of sun exposure doesn't generate the same amount of vitamin D for darker skin as it does for lighter skin."
Additionally, people who are overweight are more likely to have low vitamin D levels. According to 2011 data from the Office of Minority Health, 70 percent of African-American men 20 years and older are overweight or obese. The National Cancer Institute asserts that studies have shown obese men to be at greater risk for aggressive prostate cancer than men at a healthy weight.
"The fatter I get, the lower my vitamin D level goes, because it gets absorbed into body fat instead of my blood. That could be one possible explanation for the (racial) disparities in data," Trump said. "So maybe vitamin D is just a surrogate or marker for obesity. You see a few of these confounding factors in the vitamin D literature."
Although the association between vitamin D and cancer has already been discovered and is still being explored, this study takes a targeted look at how this link manifests differently between African Americans and whites. There is still controversy in the medical community regarding how significant this link is, or if it has real-world treatment orprevention implications.
Further complicating matters, a study released last year in the New England Journal of Medicine asserts that African-American people generally do have sufficient vitamin D levels – it's just a different, more readily-available form than the one measured by the standard test.
"We know a lot about the fact that in a lab test tube or animal, the active form of vitamin D can moderate, slow, or stop prostate tumor cells, and at high doses can even kill them. We don't know yet whether treating people with vitamin D will reduce the chance of getting (cancer)," Trump said.
He recommends a vitamin D-level test for his patients who are diagnosed with prostate cancer. In his experience, at least 70 percent diagnosed men are deficient, and he does prescribe supplements.
"We don't know for sure that it makes a difference, but I believe it does" Trump said. "I think there is a distinct possibility that low vitamin D levels might contribute to the severity of prostate cancer in African-American men – but we don't have proof of that at the moment."