Let your food be your medicine, and your medicine be your food. ...Hippocrates
Fruits and Vegetables
One of the most important messages of modern nutrition research is that a diet rich in fruits and vegetables protects against cancer. (The greatest message is that this same diet protects against almost all other diseases, too, including cardiovascular disease and diabetes.) There are many mechanisms by which fruits and vegetables are protective, and an enormous body of research supports the recommendation for people to eat more fruits and vegetables.
Block et al  reviewed about 200 studies of cancer and fruit and vegetable intake. A statistically significant protective effect of fruits and vegetables was found in 128 of 156 studies that gave relative risks. For most cancers, people in the lower quartile (1/4 of the population) who ate the least amount of fruits and vegetables had about twice the risk of cancer compared to those who in the upper quartile who ate the most fruits and vegetables. Even in lung cancer, after accounting for smoking, increasing fruits and vegetables reduces lung cancer; an additional 20 to 33 percent reduction in lung cancers is estimated .
Steinmetz and Potter reviewed the relationship between fruits, vegetables, and cancer in 206 human epidemiologic studies and 22 animal studies . They found "the evidence for a protective effect of greater vegetable and fruit consumption is consistent for cancers of the stomach, esophagus, lung, oral cavity and pharynx, endometrium, pancreas, and colon." Vegetables, and particularly raw vegetables, were found to be protective; 85% of the studies that queried raw vegetable consumption found a protective effect. Allium vegetables, carrots, green vegetables, cruciferous vegetables, and tomatoes also had a fairly consistent protective effect . Allium vegetables (garlic, onion, leeks, and scallions) are particularly potent and have separately been found to be protective for stomach and colorectal cancers [82,83] and prostate cancer .
There are many substances that are protective in fruits and vegetables, so that the entire effect is not very likely to be due to any single nutrient or phytochemical. Steinmetz and Potter list possible protective elements: dithiolthiones, isothiocyanates, indole-32-carbinol, allium compounds, isoflavones, protease inhibitors, saponins, phytosterols, inositol hexaphosphate, vitamin C, D-limonene, lutein, folic acid, beta carotene (and other carotenoids), lycopene, selenium, vitamin E, flavonoids, and dietary fiber .
A joint report by the World Cancer Research Fund and the American Institute for Cancer Research found convincing evidence that a high fruit and vegetable diet would reduce cancers of the mouth and pharynx, esophagus, lung, stomach, and colon and rectum; evidence of probable risk reduction was found for cancers of the larynx, pancreas, breast, and bladder .
Many of the recent reports from prospective population-based studies of diet and cancer have not found the same protective effects of fruits and vegetables that were reported earlier in the epidemiological and case-control studies [reviewed in ]. One explanation is that people's memory of what they ate in a case-cohort study may have been tainted by their disease state. Another problem might be that the food frequency questionnaires (FFQ) used to measure food intake might not be accurate enough to detect differences. Such a problem was noted in the EPIC study at the Norfolk, UK site. Using a food diary the researchers found a significant correlation between saturated fat intake and breast cancer, but using a FFQ there was no significant correlation . So, inaccurate measurement of fruit and vegetable intake might be part of the explanation as well.
It must be noted that upper intakes of fruits and vegetables in these studies are usually within the range of what people on an American omnivorous diet normally eat. In the Nurses Health Study the upper quintiles of fruit and vegetable intake were 4.5 and 6.2 servings/day, respectively . Similarly, the upper quintiles of fruit and vegetable intake in the Health Professionals Follow-up Study were 4.3 and 5.4 serving/day for fruits and vegetables, respectively . Intakes of fruits and vegetables on the Hallelujah Diet are much higher, with median reported intakes of six servings of fruits (646 g/day) and eleven servings of vegetables per day (971 g/day)  in addition to a green powder from the juice of barley leaves and alfalfa that is equivalent to approximately another 100 g/day of fresh dark greens. So, it is very possible that the range of intakes in the prospective population based studies do not have a wide enough intake on the upper end to detect the true possible impact of a very high intake of fruits and vegetables on cancer risk.