As compared to men who have had only one partner during their lifetime, having sex with more than 20 women is associated with a 28 percent lower risk of being diagnosed with prostate cancer, research shows.

However, having more than 20 male partners in one's lifetime is associated with a two-fold higher risk of getting prostate cancer compared to those who have never slept with a man, according to researchers from the University of Montreal and Institut Armand-Frappier Research Centre (INRS) in Quebec.

To reach this conclusion, Marie-Elise Parent and Marie-Claude Rousseau, professors at University of Montreal's school of public health, and their colleague Andrea Spence, analysed the Montreal study PROtEuS (Prostate Cancer and Environment Study).

In the study, 3,208 men responded to a questionnaire on, amongst other things, their sex lives.

Of these men, 1,590 were diagnosed with prostate cancer between September 2005 and August 2009 while 1,618 men were part of the control group.

"Overall, men with prostate cancer were twice as likely as others to have a relative with cancer. However, evidence suggests that the number of sexual partners affects the development of the cancer," researchers said.

Consequently, 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 is a 28 percent reduction in the risk of having prostate cancer (all types) and a 19 percent reduction for aggressive types of cancer.

"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," Parent said.

According to some studies, the underlying mechanism of this protective effect is in reducing the concentration of cancer-causing substances in prostatic fluid or lowering the production of intraluminal crystalloids.

The data also indicated that having only one male partner does not affect the risk of prostate cancer compared to those who have never had sexual intercourse with a man.

"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," researchers said.

Does this mean public health authorities will soon be recommending men to sleep with many women in their lives?

"We are not there yet," Parent added.

The findings were published in the journal Cancer Epidemiology.