The University of Montreal has found that men who had sex with more than 20 women lower their prostate cancer risk
Sleeping with more than 20 women protects men against prostate cancer, a study has suggested.
Men who had slept with more than 20 women lowered their risk of developing cancer by almost one third, and were 19 per cent less likely to develop the most aggressive form.
In contrast, men who slept with 20 men doubled their risk of developing prostate cancer compared with men who have never had sex with another man.
Researchers at the University of Montreal believe that intercourse protects men, and men who are more promiscuous have more sex than those in monogamous relationships.
However, for homosexual men the benefit is lost because of the increased risk of picking up a sexually transmitted disease, and the damage to their bodies from intercourse. However gay men with just one partner are at no greater risk.
"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," said lead researcher Dr Marie-Elise Parent.
Sleeping with more than 20 women reduces risk of prostate cancer by nearly one third.
But when asked whether public health authorities should recommend men to sleep with many women in their lives Dr Parent added: "We're not there yet."
The study looked at more than 3,200 men over a four year period between 2005 and 2009.
Overall, men with prostate cancer were twice as likely to have a relative with cancer. However, the researchers were surprised to find that the number of sexual partners also affected the development of their cancer.
Men who said they had never had sexual intercourse were almost twice as likely to be diagnosed with prostate cancer as those who said they had.
When a man has slept with more than 20 women during his lifetime there was a 28 per cent reduction in the risk of having prostate cancer, and a 19 per cent reduction for aggressive types of cancer.
On the other hand, those who have slept with more than 20 men are twice as likely to be diagnosed with prostate cancer of all types compared to those who have never slept with a man.
And their risk of having a less aggressive prostate cancer increases by 500 per cent compared to those who have had only one male partner.
Dr Parent said that she could only formulate "highly speculative" hypotheses to explain the association.
"It could come from greater exposure to STIs, or it could be that anal intercourse produces physical trauma to the prostate," she said.
Previous studies have found that sexual intercourse may have a protective effect against prostate cancer because it reduces the concentration of carcinogenic crystal-like substances in the fluid of the prostate.
The study, published in the Journal Cancer Epidemiology is the first to find a link between the number of sexual partners and the risk of developing cancer.
"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," said lead researcher Dr Marie-Elise Parent.
"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."
By Sarah Knapton, Science Editor. Telegraph.co.uk