"Men who have sex with more than 20 women during the course of their lifetimes are less likely to be diagnosed with prostate cancer, new research shows.
But for homosexuals who have had sex with at least 20 men and virgins, the risk of being diagnosed is doubled, the researchers from the University of Montreal have found.
Compared with men who have had just one sexual partner, men who have slept with 20 or more women have a 28 per cent lower risk of being diagnosed, and are 19 per cent less likely to develop the most aggressive form of prostate cancer.
However, the researchers found that homosexual men who have had at least 20 sexual partners, were twice as likely to to develop prostate cancer compared with those in monogamous relationships.
Homosexual men were also five times more likely to develop a less serious form of the disease than those with only one male partner.
The "highly speculative" hypothesis to explain this increased risk for homosexual men is that there is a heightened risk of picking up a sexually transmitted disease and "physical trauma" to the prostate from anal intercourse.
Between 2005 and 2009, 3208 men participated in the Prostate Cancer & Environment Study and were asked to give information about their lifestyles and sexual experiences.
In this time frame, 1590 men who participated in the study were diagnosed with prostate cancer..
The research, published in the journal Cancer Epidemiology, is the first of its kind to make a link between the number of sexual partners and the risk of developing cancer.
Lead researcher Marie-Elise Parent said that, despite the results, she did not see a need for public health authorities to encourage men to have sex with as many women as possible.
"It is possible that having many female sexual partners results in a higher frequency of ejaculations, whose protective effect against prostate cancer has been previously observed in cohort studies," Dr Parent said
The research found that men who had never had sexual intercourse, were twice as likely to be diagnosed with prostate cancer.
Prior studies have indicated sexual intercourse might help reduce the risk of prostate cancer because it lowers the concentration of carcinogenic crystal-like substances in the fluid of the prostate.
"We were fortunate to have participants from Montreal who were comfortable talking about their sexuality, no matter what sexual experiences they have had, and this openness would probably not have been the same 20 or 30 years ago," Dr Parent said.
"Indeed, thanks to them, we now know that the number and type of partners must be taken into account to better understand the causes of prostate cancer."