Broccoli and tomato -- two vegetables known to help fight cancer -- are more effective against prostate cancer if they're eaten together as part of a daily diet than if they're eaten alone, a new study with rats suggests.

University of Illinois researchers fed a diet containing 10 percent broccoli powder and 10 percent tomato powder to a group of rats that had been implanted with prostate cancer cells. Other groups of rats received either tomato powder or broccoli powder alone; a supplemental dose of lycopene (the red pigment in tomatoes believed to be an anti-cancer agent); or finasteride, a drug prescribed for men with enlarged prostates. Another group of rats was castrated.

After 22 weeks, the researchers found that the combined tomato/broccoli diet was the most effective at prostate tumor reduction. Of the other treatments, castration was the only one that came close to being as effective.

"When tomatoes and broccoli are eaten together, we see an additive effect. We think it's because different bioactive compounds in each food work on different anti-cancer pathways," study co-author John Erdman, a professor of food science and human nutrition, said in a prepared statement.

"Older men with slow-growing prostate cancer who have chosen watchful waiting over chemotherapy and radiation should seriously consider altering their diets to included more tomatoes and broccoli," added study co-author and doctoral candidate Kirstie Canene-Adams.

"To get these effects, men should consume daily 1.4 cups of raw broccoli and 2.5 cups of fresh tomato, or 1 cup of tomato sauce, or 1/2 cup of tomato paste. I think it's very doable for a man to eat a cup and a half of broccoli per day or to put broccoli on a pizza with 1/2 cup of tomato paste," Canene-Adams said.

The study was published in the Jan. 15 issue of the journal Cancer Research.