A diet high in flavonoids, which are found in plant-based foods such as oranges, grapefruits, strawberries, greens, and tea, might lower the risk for aggressive prostate cancer, according to a new study.

Men with the highest intake of flavonoids had a 25% lower risk for aggressive prostate cancer than men with the lowest flavonoid intake.

"We focused on aggressive prostate cancer, which makes us unique among other observational studies," said study author Susan E. Steck, PhD, MPH, RD, associate professor at the Arnold School of Public Health, University of South Carolina, in Columbia.

Dr. Steck spoke at a press briefing press briefing held in advance of the 11th Annual American Association for Cancer Research International Conference on Frontiers in Cancer Prevention Research.

"In terms of nutrition, there have been very few definitive conclusions with which to make dietary recommendations for prostate cancer prevention," she said. "Fruits and vegetables have been implicated, but the evidence isn't as strong as it is for other cancers, such as colorectal cancer."

"This is also the first study to use the updated 2011 flavonoid database," she said. It needs to be replicated in other populations, but this finding supports the move toward a more plant-based diet in an effort to increase the intake of flavonoids "as one possible means of preventing aggressive prostate cancer," she noted.

Flavonoids are ubiquitous in the food supply. Many of the more than 4000 identified flavonoids can be found in fruits, vegetables, and beverages (such as tea, coffee, wine, and fruit drinks). Experimental studies have suggested that the chemopreventive effect of flavonoids on prostate cancer are related to anti-inflammatory action, the induction of apoptosis, antioxidation, cell-cycle regulation, and the impairment of angiogenesis.

However, few epidemiologic studies have evaluated the association between the intake of flavonoids and human prostate cancer, the researchers note.

Younger Men and Smokers Reap Benefits

Dr. Steck and colleagues used data from the North Carolina–Louisiana Prostate Cancer Project (PCaP), a multidisciplinary study of the social, individual, and tumor-level causes of racial differences in disease aggressiveness.

The cohort involved 920 black and 977 white men who were newly diagnosed with prostate cancer. Patient diet was assessed using a modified version of the National Cancer Institute Diet History Questionnaire.

Prostate cancer is a significant public health issue, but we know that black men bear a disproportionate burden of the disease, Dr. Steck noted. "They are more likely to be diagnosed at younger ages, with a more virulent form of the cancer, and with more advanced disease," she added.

The researchers used the 2011 US Department of Agriculture (USDA) Database for the Flavonoid Content of Selected Foods to help them calculate total flavonoid intake in this study.

Study participants were deemed to have highly aggressive disease if their Gleason score was at least 8, if their prostate-specific antigen level was above 20 ng/mL, or if their Gleason score was at least 7 and their clinical disease stage was T3c to T4c. Those deemed to have prostate cancer with low or intermediate aggressiveness comprised the comparison group.

After adjustment for confounders such as age, race, and smoking status, an inverse association was seen between total flavonoid intake and prostate cancer aggressiveness. Men with the highest flavonoid intake had the least aggressive disease (odds ratio [OR], 0.75; 95% confidence interval [CI], 0.54 to 1.04).

This association between flavonoid intake and disease aggressiveness was strongest in men younger than 65 years who were in the middle flavonoid-intake tertile (OR, 0.62; 95% CI, 0.40 to 0.98) and in the highest tertile (OR, 0.67; 95% CI, 0.42 to 1.06), and in current smokers who were in the middle tertile (OR, 0.25; 95% CI, 0.11 to 0.54) and in the highest tertile (OR, 0.48; 95% CI, 0.22 to 1.06).

The results did not differ by race, Dr. Steck pointed out, and orange and grapefruit juices and tea were the main contributors to total flavonoid intake for both the black and white men.

11th Annual American Association for Cancer Research (AACR) International Conference on Frontiers in Cancer Prevention Research: Abstract A103. Presented October 17, 2012.