In human colonic microflora, the majority of bacteria use nondigestible carbohydrates as the main if not exclusive feeding substrates. The metabolism of these compounds is quantitatively more important than amino acid fermentation, especially in the cecum and ascending segment where substrate availability is greatest. Carbohydrate fermentation is thus an important force driving the micro ecology and the physiology of the large intestine.8 Even though a wide variety of anaerobic bacteria are capable of fermenting many different types of carbohydrate, they use a small number of metabolic pathways to produce a limited range of end products.
5.3.2 Substrates of Colonic Carbohydrate Fermentation
The most important and most frequent nondigestible carbohydrates in human diet that serve as substrates for colonic fermentation are (Table 5.4):
• Nonstarch or plant cell wall polysaccharides including cellulose or D-d-(1""4) polymers of glucose (the major structural component in the cell wall of all plants); hemicelluloses or (□-d-(1""4) polymers of xylose, mannose and glucose, particularly arabinoxylans in cereals (rye and wheat); □-D-(1""3,4)-glucans in cereals (barley and oat) and xyloglucans
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