Men who are very sexually active in their twenties and thirties are at greater risk for developing prostate cancer, a new study suggests, especially if they are frequent masturbators.

British researchers asked more than 800 men - about 400 cancer patients and 400 who do not have cancer - about their sexual history, including the number of partners they have had, how frequently they have sex and how often they masturbate.

Among other findings, the team learned that men with prostate cancer were among the subjects who reported having sex with the greatest frequency.

Of the subjects who had been diagnosed with cancer:

• Forty per cent of men in their twenties fell in the highest frequency category, reporting that they had sex 20 or more times per month, compared to 32 per cent in the control group.
• Thirty-four per cent of men in their twenties reported that they masturbated frequently, compared to 24 per cent in the control group.

One theory to explain the link could be that having higher sex hormone levels not only leads to a higher sex drive, but also to a higher risk of cancer, the researchers said.

"Hormones appear to play a key role in prostate cancer and it is very common to treat men with therapy to reduce the hormones thought to stimulate the cancer cells," lead study author Dr. Polyxeni Dimitropoulou, of the University of Nottingham, said in a statement. "A man's sex drive is also regulated by his hormone levels, so this study examined the theory that having a high sex drive affects the risk of prostate cancer."

The level of the men's sexual activity when they were in their forties and fifties did not increase their prostate cancer risk.

In fact, even a low level of sexual activity in their fifties appeared to offer a small amount of protection against the disease.

Dimitropoulou said further research is needed to determine how sexual activity may offer protection for older men.

The findings are published in the journal BJU International.

Prostate cancer is the most common form of cancer in Canadian men.

The Canadian Cancer Society estimated that more than 24,000 men would be diagnosed with the disease in 2008, and 4,300 would die of it.

Prostate cancer has no single cause, but a man's risk increases as he gets older. Recent research has linked obesity, low levels of physical activity and eating a high-fat diet to an increased risk of developing the disease.