Several components of the humble tomato act together to help fight prostate cancer, say researchers.
Tomatoes are already known to have health benefits
It had been thought just one chemical, lycopene, had an anti-cancer effect.
But researchers at the Universities of Illinois and Ohio State found lycopene's effect is boosted by other chemicals in the fruit.
The finding, in the Journal of the National Cancer Institute, suggests lycopene-only dietary supplements have only a limited effect.
Lead researcher Professor John Erdman said: "It has been unclear whether lycopene itself is protective.
"This study suggests that lycopene is one factor involved in reducing the risk of prostate cancer.
"But it also suggests that taking lycopene as a dietary supplement is not as effective as eating whole tomatoes.
"We believe people should consume whole tomato products - in pastas, in salads, in tomato juice and even on pizza."
The researchers exposed laboratory rats to a chemical that causes prostate cancer, and then fed them on diets containing whole tomato powder, pure lycopene or no lycopene at all.
Rats who fed on tomato powder had a 26% lower risk of dying from their cancer than rats who ate no lycopene.
But the rats fed lycopene had a risk similar to control rats.
By the end of the study, prostate cancer had killed 80% of the control group, 72% of the lycopene-fed rats and 62% of the rats fed tomato powder.
The researchers also found that restricting the amount of food given to the rats cut their risk of developing prostate cancer - regardless of which diet they were on.
Separate research has shown that higher levels of lycopene in the blood is linked to a lower risk of prostate cancer.
Lycopene, the substance that makes tomatoes red, is effective at mopping up particles called free radicals which can damage the body's tissues.
But Dr Steven Clinton, who also worked on the study, said: "Our findings strongly suggest that risks of poor dietary habits cannot be reversed simply by taking a pill.
"We shouldn't expect easy solutions to complex problems. We must focus more on choosing a variety of healthy foods, exercising and watching our weight."