Fate of digestible starch
Most of the starch present in the diet is cooked and gelatinized. As such, most dietary starch is easily digested, accounting for approximately 95% of that consumed (Cassidy et al., 1994). Various diet, food processing and physiological factors are known to affect starch digestion, and these are listed in Table 8.3. Starch is digested/hydrolyzed enzymically and sequentially within the upper gastrointestinal tract (Gray, 1992, 2003; Levin, 1994). In the mouth some starch is digested to maltose via salivary amylase. In the small intestine, starch is initially digested in the lumen via pancreatic amylase to smaller compounds that include maltose, maltotriose and branched limit dextrins. Two more enzymes produced by the brush border (sucrase-isomaltase and glucoamylase) further hydrolyze the starch products to glucose which is actively absorbed through the enterocyte membrane. Most dietary starch is absorbed as glucose to participate in energy metabolism in the body.
Table 8.3 Food and physiological factors affecting the rate and extent of starch digestion
Food behavior Type of starchy food eaten
Amount of starchy food eaten Customs of food preparation and consumption Nature of the starch eaten Amylose content
Food processing Loss of cellular and native plant structure
Processing conditions Extent of gelatinization Particle size
Other food components - antinutrients, viscous fiber, fat
Physiology Health status
Individual physiological differences Extent of chewing Gastric emptying Viscosity
Gastrointestinal transit time Enzyme inhibition
RS is not digested, hydrolyzed or absorbed and so it does not contribute to plasma glucose levels. Instead RS passes into the large bowel where it contributes metabolic energy through bacterially fermented and absorbed SCFAs.
The large bowel is intensely populated with bacteria, with several hundred species present at about 1011-1012 CFU/g dry weight (Cummings and Macfarlane, 1991). These bacteria have a key role in salvaging undigested energy from food residues via metabolic pathways that generate SCFAs -such as acetate, propionate and butyrate. Some of the salvaged energy is utilized by the bacteria for growth but approximately 95% of the SCFAs are absorbed (Cummings and Macfarlane, 1991) and provide energy to the body. For example butyrate is the primary energy source for colonocytes and acetate is mainly used by muscle tissue (Salminen et al., 1998). SCFAs provide approximately 5-10% of our daily energy intake (Cummings, 1996).
Some of the RS is not fermented. The amount will depend on the type of RS, how much RS has been consumed, the composition of the colonic microflora, the digestive transit time and the health status of the individual. As much as 22% of RS can be excreted unfermented from the body (Phillips et al., 1995).
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