Scientists claimed yesterday that frequent masturbation could protect men from prostate cancer.

Men between the ages of 20 and 50 were encouraged to partake in the activity regularly. Researchers believe that ejaculation can flush cancer-triggering chemicals out of the prostate gland, or induce prostate cells to mature fully, making them less susceptible to carcinogens.

Australian scientists discovered the link after questioning 1,079 men with prostate cancer about their sexual habits. Their responses were compared with those of 1,259 healthy men of the same age. Men who had ejaculated more than five times a week in their 20s were a third less likely to develop aggressive prostate cancer later in life.

The results contradict those of previous studies which suggest that having many partners or frequent sex increases the risk of prostate cancer by up to 40%.

But those studies focused on sexual intercourse and ignored masturbation.

The new research, led by Graham Giles, of the Cancer Council of Victoria in Melbourne, recorded the number of ejaculations, whether or not intercourse was involved.

The most likely explanation for the discrepancy is that infections caused by intercourse promote prostate cancer, say the scientists.

But ejaculations without intercourse appear to be protective, it was reported in New Scientist magazine. Prof Giles believes that the association would be even more striking if masturbation was studied on its own.

Together with the seminal vesicles, the prostate produces most of the fluid in semen. The fluid is rich in potassium, zinc, fructose and citric acid. Some experts think this build-up of chemicals helps to trigger cancer in vulnerable men.