Metabolism
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Sugars, starches and the glycaemic index
Key points from this exercise:
Different carbohydrate foods have a different effect on blood glucose; the glycaemic index of a carbohydrate food is the extent to which it raises blood glucose compared with the equivalent amount of glucose or a reference carbohydrate that is rapidly digested and absorbed.
The insulin response to carbohydrate ingestion follows the glucose response.
Carbohydrates can be classified as sugars (mono- and disaccharides and sugar alcohols), oligosaccharides and polysaccharides.
The major monosaccharides are the hexoses glucose, galactose and fructose; the pentoses ribose and deoxyribose have important biochemical roles.
Glucose and galactose are absorbed by sodium-linked active transport, sharing the same carrier. Fructose, sugar alcohols and other monosaccharides are absorbed by passive carrier-mediated diffusion.
The major disaccharides are sucrose, lactose, maltose, isomaltose and trehalose. They are hydrolysed by enzymes in the brush border of intestinal mucosal cells, and the resulting monosaccharides are absorbed.
Starch is a large polymer of glucose. There are two main forms of starch: amylose is a linear polymer of glucose linked 1-4; amylopectin is a branched polymer, with branch points formed by 1-6 glycoside bonds.
Starch digestion is catalysed by amylase in saliva and pancreatic juice. Amylase attacks randomly, yielding a mixture of glucose, maltose, isomaltose and small oligosaccharides (dextrins).
Starch may be rapidly or slowly digested, or more or less completely resistant to digestion. This depends largely on whether or not the starch is gelatinised or crystalline. Starch enclosed in plant cell walls is inaccessible to amylase. Gelatinised starch is more accessible than crystalline.
Both resistant starch and non-starch polysaccharides provide a substrate for intestinal bacterial fermentation, and the resultant short-chain fatty acids provide a major fuel for colonic enterocytes. Butyrate may also have antiproliferative actions and provide protection against colorectal cancer.
Soluble non-starch polysaccharides increase the viscosity of the intestinal contents and slow the absorption of the products of digestion.
Insoluble non-starch polysaccharides provide bulk to the intestinal contents, so aiding peristalsis and preventing or treating diverticular disease of the colon. They also bind bile salts and potential carcinogens, preventing their absorption.