Eating tomatoes and broccoli together may offer better protection against cancer than eating either alone, say US researchers.
When combined with tomato, broccoli was even more powerful
Both are known to contain cancer-fighting chemicals, but the authors say when put together, they interact to maximise the effect.
Rats fed both foods together had less prostate cancer growth than rats fed either tomatoes or broccoli.
Dr John Erdman presented his findings at a US conference on food and cancer.
Dr Erdman, professor of Food Science and Human Nutrition at the University of Illinois, said: "We decided to look at these foods in combination because we believed it was a way to learn more about real diets eaten by real people.
"People don't eat nutrients, they eat food. And they don't eat one food, they eat many foods in combination."
His team had previously found rats fed lycopene - the chemical in tomatoes thought to fight cancer - were not afforded greater protection against prostate cancer.
But rats fed freeze-dried tomato powder, which contains the full range of vitamins and nutrients of normal tomatoes, were.
Similarly, the cancer-fighting compounds found in broccoli, called glucosinolates, have been shown to work better when they act with all of the other natural substances found in broccoli.
Dr Erdman's team examined how different food and nutrient combinations affected the growth of prostate tumours in rats.
Rats that ate a combination of broccoli and tomato powders had less growth of their tumours than rats who ate either of the two powders alone and rats given diets supplemented with lycopene.
A final group of rats was fed a normal diet and given a drug commonly prescribed to men with benign growth of the prostate which is also being tested for its potential to prevent prostate cancer.
All of the other diets were better at preventing tumour growth than the drug.
Dr Erdman said: "These preliminary results suggest that there is, in fact, an interactive protective effect between tomatoes and broccoli.
"Separately, these two foods appear to have enormous cancer-fighting potential. Together, they bring out the best in each other and maximise the cancer-fighting effect," he said.
He said other plant food combinations - fruits, vegetables, whole grains and beans - were likely to have a similar effect.
Dr Emma Knight, Science Information Officer for Cancer Research UK says: "Experts believe that changes to our diet could help prevent around one-third of cancers.
"But the link between diet and cancer is complex and it is difficult to identify the precise components of a food responsible for any protective effect.
"This research supports previous dietary studies showing that eating a diet rich in vegetables and fruits protects men from prostate cancer.
"It also backs current thinking that interactions between different substances in the diet are important and that supplements are not necessarily effective in preventing cancer.
"The best practical advice remains to increase your intake of a wide variety of fruits and vegetables, a message that is reinforced by this study's findings," she said.
Dr Erdman spoke at the International Research Conference on Food, Nutrition, and Cancer, set up by the American Institute for Cancer Research and the World Cancer Research Fund International.
His findings will be published in the Journal of Nutrition in December.