April 4, 2000 (San Francisco) -- Men who consume more than two-and-a-half servings of dairy products a day may be at a slightly increased risk of prostate cancer, say Harvard researchers. A serving is an 8 ounce glass of milk, a slice of cheese, or a scoop of ice cream. But researchers say the issue is far from clear.
June M. Chan, ScD, one of the study authors, says there are conflicting reports on the role of calcium in prostate cancer. She says that there is no reason to recommend dietary changes. The research was presented here at the annual meeting of the American Association for Cancer Research.
Two previous studies have pointed to a link between prostate cancer and the use of calcium supplements. "At the same time," says Chan, "there were a handful of other studies that have found no association.
"At this time, we would only recommend caution in the use of dietary supplements by men who have no medical indication for supplement use." She says that calcium supplements are "heavily promoted, and so the caution should just be that there may be a downside to their use." Chan is an epidemiologist at the Harvard School of Public Health.
Chan says that men with prostate cancer who are undergoing treatment are often advised to take calcium supplementation to protect against the osteoporosis associated with treatment. "There is no need to change that recommendation," she says, adding that those supplements are often given with "vitamin D supplementation as well."
Another reason to withhold on any dietary recommendations is the fact that it is still unknown whether calcium itself is a factor in prostate cancer or whether calcium is causing some other change that may promote the cancer.
Chan says that men who consume high levels of dairy products decrease the blood levels of a type of vitamin D. This type of vitamin D may be an "active suppressor of prostate cancer growth," according to some researchers. So it may be that calcium acts by this suppressor action, or "calcium itself may promote prostate cancer growth," says Chan.
The researchers tracked more than 20,000 men over 11 years, using dietary questionnaires to estimate how much dairy they consumed.
Donald Coffey, PhD, professor of urology, oncology, molecular sciences, and pathology at Johns Hopkins University in Baltimore, says there has been a "growing interest surrounding vitamin D and prostate cancer ... but this is about a 30% increase in risk, so we have to ask, when is a risk a risk?" He says, for example, that cigarette smoking increases the risk of lung cancer by about 30-fold, or 3,000%.
Nonetheless, Coffey says that the concept of a dietary link to prostate cancer is very attractive, because there are very low rates of prostate cancer in Asian nations, and "do you ever see cheese on the menu in a Chinese restaurant? Here in America we put cheese on everything; we put it on ice cream."