Obesity can double a man's risk of dying from prostate cancer, a National Cancer Institute study shows.

Moreover, the more weight a man gains during adulthood, the higher his risk of prostate cancer death.

Ironically, the study also shows obese men have a lower risk of early prostate cancer than lean men.

But that may be because obese men in the study reported fewer prostate exams -- and because early prostate cancer is harder to detect in obese men.

The study, by Margaret E. Wright, PhD, of the U.S. National Cancer Institute and colleagues, started with an in-depth questionnaire filled out by 287,760 male members of AARP, the association for older and retired Americans, who were aged 50 to 71.

During five years of follow-up, Wright's team linked this information to state cancer registries and to the National Death Index.

Their finding: Obesity "was associated positively with fatal [prostate cancer]," Wright and colleagues conclude.

"Increasing weight gain from age 18 years to [the start of the study] also was associated with greater risk of dying from prostate cancer," the researchers write.

The higher a man's body mass index (or BMI, a measure of weight relative to height), the higher his risk of prostate cancer death. Men with a BMI of 35 or higher had twice the risk of dying from prostate cancer as men with a BMI of less than 25.

BMI of 25.0-29.9 is considered overweight. A BMI of 30 and above is considered obese.

The findings held up for men of all ages and races and for men with and without a family history of prostate cancer.

A possible explanation?

Wright and colleagues note that obese men tend to have low testosterone levels. Recent studies suggest that low testosterone may increase a man's risk of dangerous prostate tumors.

Obesity also increases the body's production of growth factors that speed tumor growth.

The findings appear in the Feb. 15 issue of Cancer, a journal of the American Cancer Society.

By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCE: Wright, M. Cancer, Feb. 15, 2007; manuscript received ahead of publication.