Circumcised men may have a slightly lower risk of developing prostate cancer than those who still have their foreskin, according to a U.S. study.
The World Health Organization already recommends the controversial procedure based on research showing it lowers heterosexual men's risk of contracting HIV, the virus that causes AIDS.
Last year scientists also reported that wives and girlfriends of circumcised men had lower rates of infection with human papillomavirus or HPV, which in rare cases may lead to cervical and other cancers. And last week, researchers reported that African men who were circumcised were less likely to be infected with a particular herpes virus.
The new study, published in the journal Cancer, jibes with those findings but falls short of actually proving that circumcision will reduce a boy's future cancer risk, said Jonathan Writer, at Fred Hutchinson Cancer Research Center in Seattle, who led the study.
"I would not go out and advocate for widespread circumcision to prevent prostate cancer," Wright said. "We see an association, but it doesn't prove causality."
Although most U.S. men are circumcised, the procedure has become less popular over the past decade, and various groups have spoken out against it. In September, the Royal Dutch Medical Association discouraged the practice, calling it a "painful and harmful ritual."
For their study, Wright and his colleagues compared two groups of more than 1,600 men who had answered questions about their medical history, sex life and whether or not they were circumcised. Half had prostate cancer, the others didn't.
In the group with cancer, 69 percent of the men have been circumcised, compared to 72 percent of those without cancer -- suggesting a small protective effect.
Even accounting for a host of other factors, such as age, race and whether or not the men had been screened for prostate cancer, those who were circumcised still had a 15 percent lower risk of the disease.
"Circumcision before first sexual intercourse is associated with a reduction in the relative risk of prostate cancer in the study population," Wright and his colleagues wrote.
The foreskin is prone to tiny tears during sex, which may help bacteria and viruses enter the bloodstream.
Wright said that some viruses can trigger cancer when they are incorporated into human DNA. Another possibility is that sexually transmitted microorganisms could lead to cancer by causing chronic inflammation.
One in six U.S. men will get prostate cancer during his lifetime, although only a minority of them will die from the disease.