WEDNESDAY, Oct. 17 (HealthDay News) -- Prostate cancer patients who, before their diagnosis, routinely consumed hefty helpings of the flavonoid compounds found in plant-based foods and drinks may be at lower risk for the most aggressive form of the disease, new research suggests.

But the research has significant limitations, the study authors noted, so it's too soon to say that a plant-based diet protects against prostate cancer.

Flavonoids are found in vegetables and fruits, as well as in tea, wine, juices and cocoa. Researchers have long theorized that these particular antioxidants may help reduce cancer risk by fighting inflammation, oxidation, cell death and tumor cell growth.

The new study did not assess the ability of flavonoids to prevent the onset of cancer as a whole. But the investigation, involving about 1,900 patients newly diagnosed with prostate cancer, found that those whose diets included the highest amount of flavonoids were 25 percent less likely to have been diagnosed with the fastest-moving and harshest form of the disease compared to those who had been taking in the fewest flavonoids.

"We compared men with low-aggressive disease to high aggressive," said study lead author Susan Steck, associate professor at the University of South Carolina's Arnold School of Public Health. "We did not have a healthy comparison group. So while we think that consuming more fruits and vegetables will improve the odds of not getting prostate cancer altogether, we can't say that based on our study results."

"But what we are seeing here is the impact of flavonoids on reducing the risk for aggressive prostate cancer," she added. "They may not affect your risk for getting the cancer, but it may mitigate against the kind of cancer you would get."

Steck and her colleagues are scheduled to discuss their findings Wednesday at the annual cancer prevention meeting of the American Association for Cancer Research, in Anaheim, Calif.

The authors looked at self-reported dietary habits already in place at the time of diagnosis among their patient pool, which included 920 black men and 977 white men. No dietary intervention was imposed after diagnosis.

All the men had been enrolled in the North Carolina-Louisiana Prostate Cancer Project.

The new study found that smokers and men younger than 65 appeared to receive the most protective benefit from fruit and vegetable consumption.

The authors identified green and black tea, as well as orange and grapefruit juice, as the prime sources of flavonoids consumed by study participants. Strawberries, onions, cooked greens, kale and broccoli also were popular flavonoid-rich foods.

No single class of plant-based food was linked on its own to the observed protective effect, leading the team to conclude that the benefit was rooted in a dietary mix of flavonoids.

Dr. Lionel Banez, assistant professor of urologic surgery at Duke University Medical Center in Durham, N.C., said the study design makes it hard to read much into the findings.

It is "difficult to be confident about the conclusions," he said, given that the current study was a backward look at patients' recollections of their pre-diagnosis diets.

Banez suggested that the findings of a flavonoid benefit would be more reliable if they had stemmed from a highly controlled study of risk levels among patients who were proactively placed on a specific dietary plan, and then tracked for the future onset of cancer.

"These results are not enough to warrant recommending plant-based diets as regimens to treat prostate cancer or prevent aggressive prostate cancer," he said.

Although the study found an association between flavonoids and risk for a form of prostate cancer, it did not prove a cause-and-effect relationship. Because the study was presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.

More information

For more on prostate cancer risk, visit the American Cancer Society.