Both vegetables contain chemicals that have been shown to fight cancer - broccoli's glucosinolates and the lycopene found in tomatos have been hailed as powerful anti-cancer agents on their own.
But speaking at a diet and cancer conference in the US last week, an American researcher said that it was important to measure the complex interactions that take place in the overall diet. His study underlines a new but increasingly used approach to nutrition science.
"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," said John Erdman, professor of Food Science and Human Nutrition at the University of Illinois at Urbana. "People don't eat nutrients, they eat food. And they don't eat one food, they eat many foods in combination."
The study, scheduled for publication in the December 2004 issue of the Journal of Nutrition, showed that rats fed a combination of tomatoes and broccoli had markedly less prostate tumour growth than rats who ate diets containing either food alone - and also less tumour growth than rats who ate diets containing specific cancer-fighting substances isolated from tomatoes and broccoli.
The findings are not the first to demonstrate the benefits of food interactions. Last year the UK-based Institute of Food Research revealed that eating broccoli in combination with selenium-rich chicken could double the protection against cancer, making the anti-cancer components up to 13 times more powerful when put to work together.
Erdman's team compared the impact of lycopene alone to the tumour-suppressing activity of tomato powder in rats. In the same animal model, they tested diets containing broccoli powder, and in another group, a diet containing a combination of broccoli and tomato powder.
A final group of rats was fed a normal diet supplemented with finasteride, a drug commonly prescribed to men who suffer from benign prostatic hyperplasia.
Surprisingly, each type of diet was better able to suppress prostate tumour growth than the drug. But in the group that ate the combination tomato-broccoli diets, the average tumour weight was significantly lower than the control group and also lower than the broccoli, tomato and lycopene groups.
Jeff Prince, vice-president for education at the American Institute for Cancer Research (AICR) told reporters at the two-day International Research Conference on Food, Nutrition and Cance in Washington, DC thatErdman's study is one of several new papers mapping the interactivity of different substances and foods being presented at the conference.
"By addressing themselves to studying and measuring the complex interactions that take place in the overall diet, these studies represent a new approach that is gaining momentum in the scientific community - a movement that is no longer content to ascribe anti-cancer benefits to a single substance or pill,"he said.
Erdman is further exploring the synergy between tomato and broccoli powders in a new trial.
"The fact that some kind of food synergy exists is something most nutrition researchers have simply taken on faith," Erdman said. "This new experimental approach provides us with an opportunity to measure the synergy between foods."
He added that the interactivity is likely taking place in any diet high in a variety of plant foods.