The longterm goal of the proposed research is to elucidate the mechanisms of action of dietary fiber. Some are a consequence of its presence in the gut; others are undoubtedly a result of its metabolism, which occurs principally in the large bowel through the action of microflora. Microbial activity will, however, depend upon the substrates available. The effects of fiber on small bowel digestive enzyme activities, the viscosity of the lumenal contents, the rate of transit and mucin secretion all influence the substrate available for fermentation. The primary aim of this application is to study the nature and extent of metabolism of dietary fiber and other substrates in the gastrointestinal tract and begin to examine the distribution of the energy released from these substrates between bacteria and short chain fatty acids. Methods to characterize dietary fiber and to distinguish it from other polysaccharide sources in gut lumenal contents, i.e. bacteria, mucins and undigested food carbohydrate, are a major focus of our research. These methods will be used to determine the metabolism of dietary fiber by analyzing digesta collected from conscious minipigs with cannulas surgically implanted at different sites in the gastrointestinal tract. The proportion of substrate used for bacterial growth and metabolism will be evaluated by measuring bacterial mass and disappearance of digesta. Energy available for absorption will be assessed by measuring short chain fatty acids produced during in vitro incubation of digesta. We hypothesize that soluble fibers and mucin will be rapidly fermented in the proximal colon and that insoluble fiber will be metabolized principally in the distal colon. This study will address several significant human health issues, including the role of adequate dietary fiber in the treatment of obesity and diabetes mellitus and in the process of atherogenesis. This research also will test the hypothesis that fiber affects colon carcinogenesis by modifying microbial metabolism, in part, by altering the substrate available for fermentation.
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