Men with type 2 diabetes seem to be less likely to develop prostate cancer, overall, a new study indicates.

"One previous study has suggested that diabetes may decrease risk of prostate cancer but only several years after diagnosis of diabetes," Dr. Carmen Rodriguez and colleagues from the American Cancer Society in Atlanta note in the American Journal of Epidemiology.

The researchers examined the relationship between the time of diabetes diagnosis and the risk of prostate cancer among some 72,000 men enrolled in the Cancer Prevention Study II.

Subjects completed a mailed questionnaire regarding information on diabetes at the beginning of the study in 1992 and at follow-up in 1997 and 1999.

By 2002, a total of 5318 men had been documented with prostate cancer, while a total of 10,053 men reported a physician-diagnosis of diabetes.

"Diabetes was associated with lower prostate cancer incidence rates after adjustment for age, race, education, and prostate-specific antigen testing," the researchers report.

Men who were diagnosed with diabetes within the last three years had slightly higher rates of prostate cancer compared to non-diabetic men, but those who had diabetes for at least four years had a one-third lower rate of prostate cancer.

"Our results are consistent with the hypothesis that risk of prostate cancer differs by time since diagnosis of diabetes," Rodriguez's team concludes.

As for why this occurs, they suggest several possibilities. Insulin seems the be strongest candidate, since insulin levels rise in the early stages of diabetes then drop to abnormally low levels later in the course of the disease. This pattern, along with related fluctuations in other hormones, might first promote then deter prostate cancer.

SOURCE: American Journal of Epidemiology, January 15, 2005.