Aug 19, 2007
Researchers found the cancer was much less likely to develop in people who are insulin resistant - a pre-diabetes condition linked to obesity.
But they were more likely to develop an aggressive form of the disease likely to spread to other parts of the body.
The study involving 784 men, by Sweden's Umea University, is published in the International Journal of Cancer.
Every year in the UK nearly 35,000 cases of prostate cancer are diagnosed in the UK - about one in eight of all cases of cancer.
The disease causes 10,000 deaths in the UK each year.
The researchers compared 392 men who had been diagnosed with prostate cancer and an equal number of healthy men.
Sex hormone theory
Lead researcher Dr Par Stattin said: "The suggestion that obese people are less likely to develop prostate cancer is provocative.
"Possibly, low levels of testosterone, the male sex steroid that is low in obese men, may explain why these men are less likely to develop prostate cancer in the first place.
"But they may be at a higher risk of a more aggressive form of cancer that is less reliant on testosterone."
Dr Stattin said more research was needed to confirm the findings, and pin down the reasons.
Dr Greg Martin, of the World Cancer Research Fund, which funded the study, said the findings demonstrated just how closely obesity was related to cancer.
He said: "While this study suggests that obese people could be less likely to develop prostate cancer in the first place, it is important to remember that being overweight significantly increases your risk of developing a number of different cancers, and is bad for your overall cancer risk."
And Dr Chris Hiley, of the Prostate Cancer Charity, said: "Men should not get the impression that there is an up side to obesity because it looks as if it might prevent prostate cancer. This is not the story at all.
"Cancer and heart disease are the biggest causes of preventable deaths and getting your weight under control with a healthy diet and lifestyle cuts the risk of both."