Potential role of flavonoids in the prevention of intestinal neoplasiaA review of their mode of action and their clinical perspectives

Intestinal neoplasia (adenomas and carcinomas) can possibly be prevented by a diet rich in vegetables and fruits, treatment with aspirin and other nonsteroidal antiinflammatory drugs, and early colonoscopic removal of adenomas. Ballast, fiber, and secondary plant products could play a major role in colon cancer prevention. Recently there has been much experimental work in vitro and in vivo about flavonoids as inducers of bioprevention. Flavonoids are secondary plant products with a wide variety of beneficial biological properties, and they possess anticarcinogenic, antimutagenic, and antioxidative modes of actions. Flavonoids are the main components of a healthy diet containing fruits and vegetables and are concentrated especially in tea, apples, and onions. We will focus this review on flavonoids which are derived from tea products such as proanthocyanidins (green tea) and flavons (camomille tea). Oral supplementation with bioflavonoids derived from tea could be used in humans to prevent growth of intestinal neoplasia such as adenomatous polyps of the colon. Flavonoids are a large group of natural compounds of which only a few have been used in animal models, cell cultures, and enzyme studies to inhibit mutagenic and carcinogenic events. Their clinical mode of action was evaluated by epidemiological studies, but no intervention studies in humans have been performed so far. In vitro flavonoids can bind electrophils, inactivate oxygen radicals, prevent lipid peroxidation, and inhibit DNA oxidation. In cell cultures they increase the rate of apoptosis, inhibit cell proliferation, and angiogenesis. In vivo they can induce the activities of protective enzymes (conjugating enzymes such as glutathione transferases and glucuronosyl transferases) of the intestine and the liver. In models of intestinal polyposis, flavonoids suppress polyp formation. Some epidemiological studies show a protective effect of flavonoids contained in fruits, vegetables, and tea.Flavonoid mixtures of tea origin supplied as nutritional supplements could be studied as a new way of bioprevention of intestinal neoplasia (colon adenomas and cancer). Therefore, a controlled, randomized clinical study should be performed to evaluate the efficacy of flavonoids.