Sleeping with numerous women protects a man from prostate cancer, a study has found.

Research showed that Lotharios with a lot of notches on their belts are significantly less likely to develop the disease.

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Compared with men who have had only one sexual partner during their lifetime, those with a score of more than 20 conquests have a 28 per cent reduced risk of being diagnosed.

But the same is not true for gay encounters, according to the Canadian scientists. In fact, having more than 20 male partners doubled the risk of prostate cancer.

The findings are from the Prostate Cancer & Environment Study, in which 3208 men answered questions about their lifestyle and sex lives.

“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,” lead researcher Professor Marie-Elise Parent said.

The study found that men who were virgins were almost twice as likely to be diagnosed with prostate cancer as those who were sexually experienced.

Compared with those who had only had one partner, men who had slept with more than 20 women were 28 per cent less at risk of prostate cancer of all types.

They were also 19 per cent less likely to develop an aggressive type of cancer.

According to one theory, large numbers of ejaculations may reduce the concentration of cancer-causing substances in prostatic fluid, a constituent of semen.

They may also lead to fewer crystal-like structures in the prostate that have been associated with prostate cancer.

The research is published in the journal Cancer Epidemiology.

On the question of whether promiscuity might now be recommended in health advice to men, Prof Parent said: “We’re not there yet.”