Men who have had many female sexual partners during their life have a reduced risk of developing prostate cancer, but those who have had many male sexual partners have an increased risk, new research suggests.

The results were based on a Canadian study of over 3,200 men, 1,590 of whom were diagnosed with prostate cancer.

The study found that men who had never had sex were almost twice as likely to be diagnosed with cancer of the prostate. However, if a man had slept with more than 20 women during his lifetime, his risk of developing the disease fell by 28%. Having this many female partners also reduced the risk of more aggressive types of the disease by 19%.

"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," noted Prof Marie-Elise Parent of the University of Montreal.

Meanwhile, the study also found that while having one male sexual partner did not affect the risk of developing prostate cancer, men who had at least 20 male sexual partners were more than twice as likely to be diagnosed with the disease compared to those who had never had a male sexual partner.

The researchers could only suggest a ‘highly speculative' theory for this.

"It could come from greater exposure to STIs (sexually transmitted infections), or it could be that anal intercourse produces physical trauma to the prostate," Prof Parent said.

She thanked those who participated in the study for being so open about their sexual experiences, noting that ‘this openness would probably not have been the same 20 or 30 years ago'.

"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," she added.

Details of these findings are published in the journal, Cancer Epidemiology.