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Chapter: Introduction to Human Nutrition: Digestion and Metabolism of Carbohydrates

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Resistant starch

Resistant starch is starch that escapes digestion in the small intestine and enters the colon.

Resistant starch

 

Resistant starch is starch that escapes digestion in the small intestine and enters the colon. However, there is controversy over the amounts of resistant starch in foods because there is no universally accepted method for measuring it (different methods yield different results). The amount of resistant starch measured chemically is generally less than that observed to enter the colon (or leave the small intestine) in experiments in human volunteers.

 

In the 1970s and early 1980s it first became appar-ent that appreciable amounts of starch are not digested in the small bowel, from experiments showing that breath hydrogen increased after eating normal starchy foods. The only source of hydrogen gas in the human body is as a product of the anaerobic fermentation of carbohydrates by colonic bacteria . If a person consumed a load of an absorbable sugar such as glucose, breath hydrogen did not go up. In contrast, if lactulose (an unabsorbed disaccharide of fructose and galactose) was consumed, breath hydrogen increased rapidly, and the area under the breath hydrogen curve over an 8–12 hour period was directly proportional to the amount of lactulose consumed. If subjects ate common starchy foods such as white bread or potato, breath hydrogen levels increased to an extent that suggested that 5–10% of the starch was fermented in the colon. Subsequently, other ways of measuring carbohydrate entering the colon were developed. In one technique, subjects swallowed a tube that was passed through the stomach and along to the end of the small intestine so that the material leaving the small intestine and about to enter the colon could be sampled. Another method was to study people who have had their colons removed sur-gically and in whom the end of the ileum was sutured to a stoma in the body wall. In this way, the material leaving their small intestine could be collected quan-titatively in a bag attached to their abdomen. With these methods, the amount of carbohydrate leaving the small intestine can be measured directly. These methods confirmed that a substantial amount of starch enters the colon. 

The main forms of resistant starch (RS) are physi-cally enclosed starch, for example within intact cell structures (known at RS1); raw starch granules (RS2); and retrograded amylose (RS3). These kinds of starch can be identified chemically using methods developed by Englyst and colleagues (Englyst et al. 1996).


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