To protect against prostate cancer, take a lover – or 20.
According to a new study, men who sleep with multiple women are almost a third less likely to develop the disease.
Researchers found men who have more than 20 notches on their bedpost slashed their risk of prostate cancer by 28 per cent.
And the study also revealed that men who have slept with more than 20 women reduced their chances of getting the most aggressive tumours by 19 per cent.
Celibacy, on the other hand, doubles the risk of the disease.
Prostate cancer is the most common male cancer in the UK, with 41,700 new cases diagnosed and 10,800 deaths each year.
The findings add to evidence that regular intercourse may flush out cancer causing chemicals as the prostate secretes the bulk of the fluid in semen.
It is the first study to suggest the number of female partners is what matters, rather than the amount of sex, or even masturbation.
But asked if it meant public health authorities will be encouraging men to sleep with many women in their lives to protect against prostate cancer, Professor Marie-Elise Parent replied: ‘We are not there yet.’
Another theory for the protective effect of sex is it reduces calcifications in the prostate, a gland located between the bladder and the penis, just in front of the rectum.
Professor Parent, of the University of Montreal, said: ‘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.’
Men who said they had never had sexual intercourse were almost twice as likely to be diagnosed with the disease as those who said they had.
According to some studies, the underlying mechanism of the preventative effect is in reducing the concentration of cancer causing substances in prostatic fluid, or lowering the production of crystal-like balls in the gland.
The age at which the participants lost their virginity, or the number of sexually transmitted infections (STIs) they had contracted, had no effect.
Moreover, just 12 per cent reported having had at least one STI.
The results were obtained as part of the Montreal study PROtEuS (Prostate Cancer & Environment Study), in which 3208 men responded to a questionnaire on, amongst other things, their sex lives.
Of these, 1,590 were diagnosed with prostate cancer between September 2005 and August 2009, while the other 1,618 formed a control group.
Overall, the prostate cancer patients were twice as likely to have had a relative with the disease. But the evidence suggests the number of sexual partners affects the development of the tumour.
On the other hand, gay men who have had more than 20 male partners in their lifetime suffered a twofold higher risk of getting prostate cancer compared to those who had never slept with a man.
And their risk of developing less aggressive prostate cancer – which is more likely to respond to treatment and has a lower likelihood of spreading – increased sixfold compared to those who have had only one male partner, which had no effect on overall risk.
Professor Parent said: ‘It could come from greater exposure to STIs, or it could be anal intercourse produces physical trauma to the prostate.’
She said having participants who were open about talking about their sexuality allowed the study to take place.
She said: ‘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 twenty or thirty years ago.
‘Indeed, thanks to them, we now know the number and type of partners must be taken into account to better understand the causes of prostate cancer.’
The research was published in the journal Cancer Epidemiology.