Particular dietary practices have been associated with colorectal cancer in defined groups of subjects (cohort studies) or comparing affected cases with matched controls (case-control studies) (Trock et al, 1990). However, these studies demonstrate associations rather than specific causality. Diets high in vegetables (raw, green or cruciferous) are protective although the nature of the specific agent is unclear. High on the list is fibre (the complex carbohydrate constituent of plant cell walls), which could act by diluting or binding potential carcinogens in the lumen of the bowel. Fibre is poorly digested in the small intestine but is fermented by bacteria in the proximal colon to generate short-chain fatty acids, notably butyrate. Resistant starch (that is not digested in the small bowel) is another source of butyric acid. Butyrate serves as an essential respiratory nutrient for colonic epithelium and its lack may be a factor in the aetiology of colorectal cancer. Butyrate may also serve as a protective factor in its capacity as a differentiating agent. Vegetables are high in folate and absence of this vitamin may also serve as a risk factor.
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