By Brenda Goodman
The study, published May 1 in the journal Clinical Cancer Research, suggests that vitamin D may play an important role in how prostate cancer starts and spreads, although it does not prove a cause-and-effect relationship. Researchers aren't yet sure exactly how it comes into play or even if taking extra vitamin D might keep prostate cancer in check.
"There are still many questions about this relationship that have to be answered," said Dr. Len Lichtenfeld, deputy chief medical officer at the American Cancer Society. He was not involved in the research.
"We really don't know, for certain, what role vitamin D plays in cancer -- either the genesis or beginning of cancer -- or in defining how aggressive the cancer may be," he said. "Further research has to be done."
What is known is that vitamin D plays several critical roles in how cells develop and grow.
"It seems to regulate normal differentiation of cells as they change from stem cells to adult cells. And it regulates the growth rate of normal cells and cancer cells," said study author Dr. Adam Murphy, an assistant professor of urology at Northwestern University's Feinberg School of Medicine, in Chicago.
Vitamin D is also known as the "sunshine vitamin" because skin makes it when exposed to sunlight. Vitamin D levels tend to drop with advancing age, and deficiency is more common in seasons and regions that get less sunlight and in people with darker skin, which naturally blocks the sun.
What about the vitamin's possible relationship to cancer?
"When you squirt vitamin D on prostate cells in a petri dish, their rate of growth slows down," Murphy said.
The idea is that too little of this critical vitamin in the body may cause cell growth to go awry, leading to cancer.
To test that idea, researchers checked vitamin D levels in 667 Chicago men between the ages of 40 and 79 who were having prostate biopsies because they'd recently had an abnormal prostate specific antigen (PSA) test or because a doctor felt changes to the prostate during a physical exam.
Normal vitamin D levels are in the range of 30 to 80 nanograms per milliliter (ng/ml).
Vitamin D deficiency, or a level under 20 ng/ml, was relatively common among all the men tested.
About 44 percent of the men with positive biopsies and 38 percent of those who tested negative for cancer had low vitamin D levels.
Among men who tested positive for cancer after their biopsies, those who also had very low levels of vitamin D -- under 12 ng/ml -- had greater odds of more advanced and aggressive cancers than those with normal levels.
The connection between vitamin D and cancer seemed to be even stronger in black men.
Black men with vitamin D levels under 12 ng/ml were far more likely than those with normal levels to test positive for prostate cancer in the first place.
In general, black men are also more likely to be diagnosed with prostate cancer. On average, men have about a one-in-seven lifetime risk of getting prostate cancer. That risk rises to one in five for black men, according to the U.S. Centers for Disease Control and Prevention.
Researchers aren't sure whether lower vitamin D levels may help to explain why black men are at higher risk for prostate cancer.
They say longer and larger studies are needed to sort out the connection.