Most of the studies examined the dietary differences between people with cancer and those without. In many cases, the primary difference was the consumption of tomatoes or tomato products, or the presence in the blood of high levels of lycopenes. Some studies merely measured ''high'' consumption versus none, while other researchers measured the actual tomatoes consumed. One study, for instance, showed a 50 percent reduction in oral cancer when comparing people who averaged one tomato a day with people who ate less than three a week. It's not clear how tomatoes lower the risk of cancer, Giovannucci said in the study, but he noted, ''Tomatoes are rich in several phytochemicals believed to have anticancer properties.'' The most prominent of these phytochemicals is a compound that leads to the formation of carotenoids, chemicals that can protect cells from the effects of oxidants.
Tomatoes are a particularly rich source of several carotenoids, including lycopene.
However, Giovannucci wrote that lycopene's benefits as a supplement, such as in a vitamin pill, have not been proven. Giovannucci said in the study.