Tomatoes and soy foods may be more effective in preventing prostate cancer when they are eaten together than when either is eaten alone, U.S. researchers reported Wednesday.

"Eating tomato, soy, and the combination all significantly reduced prostate cancer incidence," said John Erdman, a professor of food science and nutrition at the University of Illinois. "But the combination gave us the best results."

In their study, the researchers used mice that were genetically engineered to develop an aggressive form of prostate cancer. They found that only 45 percent of mice fed both foods developed the disease at study's end compared to 61 percent in the tomato group, and 66 percent in the soy group.

Soy isoflavone serum and prostate levels in the mice are similar to those found in Asian men who consume one to two servings of soy daily. In countries where soy is eaten regularly, prostate cancer occurs at significantly lower levels, the researchers said.

They said the results of the study suggest that three to four servings of tomato products per week and one to two servings of soy foods daily could protect against prostate cancer.

These findings also reinforce the recommendation that we should all eat a wide variety of whole fruits and vegetables, said the researchers.

"It's better to eat a whole tomato than to take a lycopene supplement. It's better to drink soy milk than to take soy isoflavones," Erdman said. "When you eat whole foods, you expose yourself to the entire array of cancer-fighting, bioactive components in these foods."