Sex with multiple women during a lifetime has proven to help protect men against prostate cancer, according to a new study by the University of Montreal. The study showed how men who slept with more than 20 women were 28% less likely to develop the disease, and 19% less likely to develop an aggressive cancer compared to those who had only slept with one person. The results were tested on more than 3,000 men during a Prostate Cancer & Environment Study, where the participant’s lifestyle and sexual habits were examined.  Lead researcher Professor Marie-Elise Parent, from 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." According to one theory, large numbers of ejaculations may reduce the concentration of cancer-causing substances in prostatic fluid, a constituent of semen. The results also indicated that men who were virgins were almost twice as likely to be diagnosed with prostate cancer as those who were sexually experienced. The tests also found that the age men first lost their virginity, and the number of STIs they had carried had no effect on whether they would develop prostate cancer. Although the higher number of partners benefitted a heterosexual male, having more than 20 male partners saw the chance of a male being diagnosed with prostate cancer doubling. The research has now been published in the journal Cancer Epidemiology.