Cancer and the Vegetarian Diet - Fruits and Vegetables Reduce Risk of Cancer
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From William Harris, M.D.
December 21, 1999

Fruits and Vegetables Reduce Risk of Cancer

The World Cancer Research Fund (37) recommends a "predominantly plant-based diet" and lists fruits and vegetables as [convincing, probable, or possible] risk reducers for cancer of the bladder, breast, cervix, colon, endometrium, esophagus, kidney, larynx, liver, lung, mouth and pharynx, ovary, pancreas, prostate, rectum, stomach, and thyroid. This organization recommends five or more portions of vegetables and fruit daily, and "if eaten at all, red meat to provide no more than 10% of total energy" (Calories).

There are biochemical studies that suggest how plant foods protect against cancer. Since DNA damage is crucial to cancer, its cause and prevention should be reviewed. Important in current thinking is the effect of lipid peroxidation in the generation of free radicals, small molecular fragments of fat with incorporated oxygen. Lipid peroxidation is a branching chain reaction with devastating side effects due to the ability of the oxidized fat fragments to covalently bond with DNA, damaging its structure and function.

There is a large category of antioxidants, many of them man-made such as the food preservatives BHA and BHT. Naturally occurring antioxidants include vitamins C, E, the carotenoids (lycopene-[tomatoes], luteins and beta-carotene [leafy greens]) ellagic acid (4-carbon ring metabolic artifacts found in berries) (38), and saponins ( plant sterols attached to a short chain of sugars) (39). All of these substances help to quench the free radical chain reaction.


Not all of these antioxidants are listed in the USDA database, but of the ones that are, I sorted by nutrient/Calorie ratio to find the highest plant source and the highest animal source for -carotene, vitamin C, and vitamin E. Included were 232 foods including beans, dairy, eggs, fish, fruit, grains, meat, nuts, poultry, and vegetables. Sorting by nutrient/weight ratio produces roughly similar results.

Clearly animal source food is no anti-oxidant match for plant foods. It's likely that by the time animal source food reaches the table the animal's tissues have already utilized most of the anti-oxidants that were synthesized by the plants the animal ate. A diet high in plant food, particularly fruits and vegetables, will be high in these anti-oxidants, thus protective against cancer. A diet high in animal food will be low in these anti-oxidants, since the food itself is low and its presence in the diet displaces the fruits and vegetables that might otherwise be present.

It should be noted that of 20 flours, breads, grains, and grain products included in the 232 foods, all were well below 100% of the RDA/Calorie for these three antioxidants with the exception of wheat germ oil (vitamin E). This may bear slightly on a recent study showing no reduction in colon cancer by high fiber intake (40). Admittedly grains are high in fiber, but they are not high in cancer-protective anti-oxidants. The respondents with high fiber intake may have been consuming large amounts of cereals and grains as they had been advised to, but the cancer-preventive agents are mostly in fruits and vegetables. The same grain products added in 100 gram increments and averaged, also proved to have less than 100% of the RDA/Calorie for calcium, folate, and riboflavin. Ninety three vegetables treated in the same manner were well over 100% RDA/Cal for 18 common nutrients except for vitamin B12 and had 800% of the RDA/Cal for -carotene, 1250% for vitamin C, and 300% for Vitamin E.

Fiber, plentiful in grains, is not a nutrient since it is not absorbed. It acts, in the words of one medical editor, as "a sort of colonic broom" and while this may be advantageous, a repeat of the study, this time using fruits and vegetables, rather than fiber, as dietary intake markers might produce more favorable results.

Steinmetz and Potter (41) report that the cancer protective substances in fruits and vegetables include, in addition to antioxidants, the following: allium compounds (diallyl sulfide, allyl methyl trisulfide), coumarins, dietary fiber, dithiolthiones, flavonoids (quercetin, kaempferol), folic acid, indole-3-carbinol, inositol hexaphosphate, genistein, biochanin A, isothiocyanates, sulphorophane, d-limonene, phytosterols, protease inhibitors, and selenium.

The means by which these substances protect against cancer cell initiation include effects on cell differentiation, increased activity of enzymes that detoxify carcinogens, blocked formation of nitrosamines, altered estrogen metabolism, altered colonic milieu (including bacterial flora, bile acid composition, pH, fecal bulk), preserved integrity of intracellular matrixes, effects on DNA methylation, maintenance of normal DNA repair, increased apoptosis (programmed cell death) of cancer cells, and decreased cell proliferation.

Cancer cell metastasis may be blocked by a plant-based diet. German investigators have shown that vegetarian men have roughly twice the natural killer cell activity as age-matched omnivorous controls (42).

A recent study from Britain (43) concluded that: "Vegetables and fruit are almost invariably protective for the major cancers. The evidence is best for a protective effect of vegetables in the large bowel and for fruits and vegetables in stomach cancer.... High consumption of meat, especially red meat and processed meat, is linked with higher risk of bowel, breast, prostate, and pancreatic cancer. There is some evidence of an association with lung cancer, and of an association of barbecued meat and oesophageal cancer." This study also concluded that "up to 80% of bowel and breast cancer may be preventable by dietary change."

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