SAN DIEGO — A new study by the University of California, San Diego casts doubt on the belief of many scientists and the public that a diet rich in vegetables might slow the progression of prostate cancer, a disease that kills about 33,000 Americans a year.

The findings were based on a study in which more than 200 men who had been diagnosed with early-stage prostate cancer significantly increased their consumption of vegetables over a two-year period.

“We were hoping the diet would slow and maybe reverse the cancer,” said Dr. J. Kellogg Parsons, the UCSD urology professor who led the research. “However, we found that even eating up to seven servings a day of vegetables did not have that affect.

“We still believe that a diet like this can help people with prostate cancer better tolerate treatments like surgery, radiation or chemotherapy because it goes to their overall health.”

The study was published Tuesday in the Journal of the American Medical Association and involved collaborators from the Roswell Park Comprehensive Cancer Center in Buffalo, N.Y.

Parsons said, “This is one study and no study, by itself, is definitive.” But it is the largest study of its kind and is likely to stir discussion among researchers because the results seem counterintuitive.

Scientists have found evidence that diets rich in fruits and vegetables can play a significant role in preventing or treating heart disease, obesity and diabetes. The data isn’t as strong, broadly speaking, when it comes to cancer. But many medical organizations, physicians and researchers urge people with prostate cancer to include lots of vegetables in their diet.

“The findings of this study are certainly unexpected given the known role of vegetable nutrients and fiber in cancer prevention,” said Dr. Mariana C. Stern, a professor of preventive medicine and urology at the University of Southern California.

“However, vegetables may still play a role as part of a larger set of factors including increased physical activity, lifestyle factors, and the reduced consumption of red meat and processed meat, which were not evaluated here,” she said.