Men who put on a significant number of pounds after their 20s face a higher risk of prostate cancer than those who remain close to their youthful weight - but the effects vary by race, a new study indicates.

Researchers found that among nearly 84,000 middle-aged and older U.S. men followed for about a decade, white and African-American men who had gained weight since the age of 21 had a higher risk of developing prostate cancer.

Compared with white men who gained fewer than 10 pounds, those who gained more had twice the risk of being diagnosed with advanced or aggressive prostate cancer.

Among black men, the risks began increasing after a 25-pound weight gain - though the link was seen only with early-stage and less-aggressive prostate tumors, and not advanced cancer.

In contrast, men of Japanese descent actually saw their prostate cancer decline with weight gain.

These differences may have something to do with racial and ethnic differences in the way people tend to put on fat as they age, Dr. Brenda Y. Hernandez of the University of Hawaii in Honolulu and her colleagues report in the journal Cancer Epidemiology, Biomarkers and Prevention.

Regardless, the researchers say, the findings do not change the general advice that people try to maintain a normal weight throughout life.

“These results do not warrant a change in the current public health messages about obesity,” Dr. Elizabeth A. Platz, another researcher on the work and an associate professor of epidemiology at Johns Hopkins University in Baltimore, said in a written statement.

“Men of normal weight in all racial/ethnic groups should be encouraged to avoid weight gain,” she said, “and men who are overweight and obese should be encouraged to lose weight for good health in general.”

The study included 83,879 men between the ages of 45 and 75 living in California or Hawaii. Over an average follow-up of 10 years, 5,554 were diagnosed with prostate cancer.

There was only weak evidence that men who were heavier at the start of the study had a higher risk of the disease than thinner men. Weight gain since young adulthood, on the other hand, showed a stronger link - at least in white and black men.

The findings appear to be the first to find ethnic differences in the relationship between body size and prostate cancer, according to the researchers.

It’s possible, they say, that differences in body fat distribution help explain the findings. White men, for example, have been found to have more visceral fat - deep fat surrounding the abdominal organs - than African-American men, even with total body fat taken into account.

Excess body fat may, in theory, raise prostate cancer risk by altering levels of various hormones, including testosterone and insulin, or through other metabolic effects. It’s thought that visceral fat, in comparison to body fat elsewhere, is more likely to spur such physiological changes.

More studies, the researchers conclude, are needed to see how and why weight gain may have varying effects in men of different races and ethnicities.

SOURCE: Cancer Epidemiology, Biomarkers and Prevention, September 2009.