"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," said lead researcher, John Erdman from the University of Illinois.
Writing in the January 15 issue of Cancer Research, Erdman and his co-workers report that lab rats with implanted prostate cancer cells fed a diet containing 10 per cent tomato powder and 10 per cent broccoli powder, the cancers had shrunk significantly more than the cancers in rats fed a control diet or a diet containing only broccoli or tomato powder, or supplemented with lycopene.
The researchers implanted Dunning R3327-H prostate tumours into 206 male Copenhagen rats and then assigned them to one of six dietary groups: 10 per cent broccoli powder plus 10 per cent tomato powder (10:10 combination); 5 per cent broccoli powder plus 5 per cent tomato powder (5:5 combination); only 10 per cent broccoli; only 10 per cent tomato; or lycopene supplementation (23 or 224 nanomoles per gram of diet - DSM lycopene beadlets).
After 22 weeks of feeding on the diets, the researchers reported that the lycopene supplements were associated with a seven and 18 per cent reduction in prostate size for the 23 or 224 nmol/g doses, respectively, while tomato only and broccoli only were associated with 34 and 42 per cent, respectively.
The 10:10 combination of broccoli and tomato was associated with a 52 per cent reduction in tumour weight, said the researchers.
"The combination of tomato and broccoli was more effective at slowing tumour growth that either tomato or broccoli alone and supports the public health recommendations to increase the intake of a variety of plant components," wrote the researchers.
The mechanism behind the apparent benefits, they said, is that the phytochemicals present in the vegetables, like lycopene in tomatoes and glucosinolates in broccoli, could induce apoptosis, or programmed cell death, in the cancer cells.
Further studies are needed to verify these findings, with a particular need for human studies: "These findings provide support for future human prevention trials based on dietary interventions," they said.
The researchers suggested that, based on the current results, a 55-year-old man concerned about prostate health could benefit by increasing their intake of the vegetables.
"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 half a 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 put broccoli on a pizza with half a cup of tomato paste," said lead author Kirstie Canene-Adams in a release.
Prof. Erdman said the study showed that eating whole foods is better than consuming their components: "It's better to eat tomatoes than to take a lycopene supplement. And cooked tomatoes may be better than raw tomatoes. Chopping and heating make the cancer-fighting constituents of tomatoes and broccoli more bioavailable."
"When tomatoes are cooked, for example, the water is removed and the healthful parts become more concentrated. That doesn't mean you should stay away from fresh produce. The lesson here, I think, is to eat a variety of fruits and vegetables prepared in a variety of ways," added Canene-Adams.
Commenting indepently on the research, Dr Julie Sharp, cancer information manager at British charity, Cancer Research UK, said: "While this work supports previous suggestions that both broccoli and tomatoes may contain chemicals with anticancer properties, their effects in humans are still unclear. This research has been done in the laboratory but studies of these vegetables in large numbers of people have produced conflicting results.
"However we do know that a healthy balanced diet can help to reduce the risk of cancer and should include plenty of fresh vegetables and fruit. Eating tomatoes and broccoli could help people get their five portions a day."
Over half a million news cases of prostate cancer are diagnosed every year world wide, and the cancer is the direct cause of over 200,000 deaths. More worryingly, the incidence of the disease is increasing with a rise of 1.7 per cent over 15 years.
Source: Cancer Research January 15, 2007, Volume 67, Pages 836-843 "Combinations of tomato and broccoli enhance antitumor activity in Dunning R3327-H prostate adenocarcinomas" Authors: K. Canene-Adams, B.L. Lindshield, E.H. Jeffery, and J.W. Erdman