Prostate cancer strikes 232,000 American men annually and kills 30,000, numbers similar to the toll of breast cancer on women. Could sex boost men's risk? That disturbing possibility has been raised by several recent studies:

* Italian researchers found that compared with men who never married, those who did--and presumably had more sex--had significantly greater risk of prostate cancer. For men married more than twice, risk was three times that of never-married men.

* University of Illinois researchers correlated prostate cancer risk with men's age at first intercourse and estimated lifetime number of women sex partners. The younger the men became sexually active, the greater their risk. And the more sex partners they reported, once again, the greater their risk.

* And University of Iowa researchers found that as number of women sex partners increased, so did risk of prostate cancer, with men who reported sex more than three times a week showing the greatest risk.

On the other hand, the largest study shows just the opposite--that frequent sex protects against prostate cancer. Researchers at the National Cancer Institute asked 29,000 men, aged 46 to 81, to estimate their number of weekly ejaculations during their twenties, forties, and during the past year. Compared with men who reported seven or fewer ejaculations per month, men who experienced 21 or more were significantly less likely to develop prostate cancer.

What's going on?

The smart money says is protective. In medical research, the larger the study, the more valid the results are likely to be. The studies showing that sex increases prostate cancer risk involved a few hundred men. The study showing that sex reduces risk involved 29,000.

But if sex reduces risk of prostate cancer, it's only protective if men avoid sexually transmitted infections (STIs). Many studies show that a history of STIs, especially gonorrhea and syphilis, approximately doubles prostate cancer risk. It's not entirely clear how STIs spur development of prostate cancer, but these infections cause inflammation, which apparently triggers or accelerates cancerous cell changes.

Many studies have linked frequency of sex and an increasing number of sex partners to increased risk of STIs. It now seems likely that it's the STIs, not sex per se, that increases prostate cancer risk in the studies showing that finding. It seems increasingly likely, that like cervical cancer, prostate cancer is sexually transmitted.

"Hey, babe, wanna help me prevent prostate cancer tonight?" Women might be hearing more of that line if subsequent studies confirm that sex is protective. Or maybe not. The large study tracked ejaculations, but did not distinguish between those that occurred during partner sex and masturbation. If a woman is not in the mood to help her man prevent prostate cancer, men can do it themselves.

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