A team in Australia led by Graham Giles of The Cancer Council Victoria in Melbourne asked 1079 men with prostate cancer to fill in a questionnaire detailing their sexual habits, and compared their responses with those of 1259 healthy men of the same age. The team concludes that the more men ejaculate between the ages of 20 and 50, the less likely they are to develop prostate cancer.
The protective effect is greatest while men are in their twenties: those who had ejaculated more than five times per week in their twenties, for instance, were one-third less likely to develop aggressive prostate cancer later in life (BJU International, vol 92, p 211).
The results contradict those of previous studies, which have suggested that having had many sexual partners, or a high frequency of sexual activity, increases the risk of prostate cancer by up to 40 per cent. The key difference is that these earlier studies defined sexual activity as sexual intercourse, whereas the latest study focused on the number of ejaculations, whether or not intercourse was involved.
The team speculates that infections caused by intercourse may increase the risk of prostate cancer. "Had we been able to remove ejaculations associated with sexual intercourse, there should have been an even stronger protective effect of other ejaculations," they suggest. "Men have many ways of using their prostate which don't involve women or other men," Giles adds.
Giles accepts the possibility that the men who completed the questionnaires could have lied about their habits. But he doubts this skewed the results, since questions about masturbation are unlikely to evoke the same macho exaggeration as questions about, say, number of sexual partners.
But why should ejaculating more often cut the risk of prostate cancer? The team speculates that ejaculation prevents carcinogens building up in the gland. The prostate, together with the seminal vesicles, secretes the bulk of the fluid in semen, which is rich in substances such as potassium, zinc, fructose and citric acid. Generating the fluid involves concentrating these components from the bloodstream up to 600-fold- and this could be where the trouble starts. Studies in dogs show that carcinogens such as 3-methylcholanthrene, found in cigarette smoke, are also concentrated in prostate fluid. "It's a prostatic stagnation hypothesis," says Giles. "The more you flush the ducts out, the less there is to hang around and damage the cells that line them."
His findings suggest an intriguing parallel between prostate cancer and breast cancer, as recent studies indicate that lactating reduces a woman's risk of breast cancer, perhaps because this also flushes out carcinogens. Alternatively, ejaculation might induce prostate cells to mature fully, making them less susceptible to carcinogens. "All these mechanisms are totally speculative," cautions breast cancer expert Loren Lipworth of the International Epidemiology Institute in Rockville, Maryland.
But if the finding is confirmed, future health advice from doctors may no longer be restricted to diet and exercise. "Masturbation is part of people's sexual repertoire," says Anthony Smith, deputy director of the Australian Research Centre in Sex, Health and Society at La Trobe University in Melbourne. "If these findings hold up, then it's perfectly reasonable that men should be encouraged to masturbate."
Douglas Fox, Adelaide
New Scientist issue: 19th July 2003
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