Men who had slept with more than 20 women throughout their lives were 28% less likely to develop prostate cancer, according to researchers.
The research team, from the University of Montreal's School of Public Health in Canada, publish their findings in the journal Cancer Epidemiology.
According to the researchers, the link between sexual activity and prostate cancer risk remains controversial. Some studies have associated high sexual activity with a greater risk of the disease, while others have suggested the opposite.
In this study, the team set out to determine whether the number of sexual partners men have throughout their lifetime influences their risk of prostate cancer.
They analyzed 3,208 men who were part of the Prostate Cancer & Environment Study (PROtEuS) in Montreal, Canada. Of these, 1,590 were diagnosed with prostate cancer between 2005 and 2009, while the remaining 1,618 men were free of the disease.
As part of this study, all men were required to complete a questionnaire that asked about their sexual activity, as well as sociodemographic, environmental and lifestyle factors.
28% lower prostate cancer risk among men who slept with more than 20 women
The team was not surprised to find that men who had a relative with prostate cancer were twice as likely to have the disease themselves. A family history of prostate cancer is a well-established risk factor.
However, the researchers found that a man's prostate cancer risk also appeared to be influenced by the number of sexual partners he had in his lifetime.
Men who reported never having a sexual partner were twice as likely to develop prostate cancer than those who reported having sexual partners.
However, the team found that men who had slept with more than 20 women in their lives had a 28% lower risk of developing all types of prostate cancer and a 19% lower risk of developing aggressive prostate cancer, compared with those who had slept with fewer than 20 women.
Hypothesizing the reasons behind this finding, study co-author Marie-Élise Parent says:
"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."
The researchers explain that some studies have suggested that the more a man ejaculates, the lower the concentrations of cancer-causing substances in his prostatic fluid. Other studies have indicated that a higher frequency of ejaculations may also reduce the production of intraluminal crystalloids in the prostate, which have been linked to higher risk of cancer.
The number of sexually transmitted infections (STI) participants contracted in their lifetime did not appear to influence the risk of prostate cancer, the researchers say, noting that only 12% of men reported having at least one STI in their lives.
In addition, the team found no association between the age at which the men first had sexual intercourse and prostate cancer risk.
Men with more than 20 male sexual partners twice as likely to develop prostate cancer
It was not good news for men who had slept with more than 20 male partners in their lifetime, however.
The team found that these men were twice as likely to develop all types of prostate cancer, compared with men who had never had a sexual partner. Furthermore, men who had slept with more than 20 male partners had a 500% increased risk of developing a less aggressive prostate cancer, compared with those who had only slept with one man.
The researchers are unable to accurately identify the reasons behind these findings, but they speculate that it could be a result of higher exposure to STIs among this group. Furthermore, they note that physical trauma to the prostate may be caused by anal intercourse, which could raise the risk of prostate cancer.
Speaking with Medical News Today, Parent said the team was surprised by their findings:
"To the best of my knowledge, previous studies have not made the distinction between the gender of sexual partners. We were surprised to observe that the association with several sexual partners goes in opposite directions depending on the gender of the partners one had."
Commenting on their overall results, the researchers say:
"Our findings are in support of a role for the number of sexual partners in prostate cancer development. The gender of sexual partners should be taken into account in future studies investigating this association."
Parent told us that larger studies investigating populations engaging in sex with men are warranted. "And information on masturbation, use of condoms, types of sexual activities would be useful," she added. "We are missing those in our study."
MNT recently reported on a study suggesting that men with a specific baldness pattern may be at higher risk of prostate cancer.