The strategy of digestion 311 Carbohydrate

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Dietary carbohydrate may take a number of forms. In most real meals (as opposed to the pure glucose loads studied in many experimental situations) there is a mixture of simple sugars, oligosaccharides and complex carbohydrates. Of the complex carbohydrates, some will be readily digestible starch, composed of the straight-chain amylose and the branching amylopectin, together with very small amounts of glycogen in animal tissues. Amylose consists of long chains of glucosyl units joined by a-1,4 links; amylopectin consists of chains of a-1,4-linked glucosyl units, with a-1,6-linked branches very like glycogen (see Fig. 1.8). There are other types of starch which are resistant to digestion in the small intestine, but fully digested in the large intestine; they are referred to as

Saliva

Oesoi

Gall t

Small

Liver

Saliva

Oesoi

Gall t

Small

Liver

Pancreas

Stomach

Fig. 3.1 Anatomy of the digestive tract and associated organs. Typical measurements are given in the text.

Large intestine

Pancreas

Stomach

Fig. 3.1 Anatomy of the digestive tract and associated organs. Typical measurements are given in the text.

resistant starch. Their chemical structure is identical to more easily digestible starch, but the polysaccharide chains are in a semicrystalline state that makes the bonds inaccessible to the usual enzymes of starch digestion. The remaining, less digestible, carbohydrate is referred to as non-starch polysaccharide or more generally as dietary fibre. Cellulose, one of the main components of the non-starch polysaccharide fraction, consists of long 0-1,4 linked chains of glucosyl units.

The digestible carbohydrates are for the most part absorbed from the small intestine in the form of monosaccharides. The strategy of the digestive process, then, is to have them in that form as they reach the small intestine. Digestion of dietary carbohydrate to monosaccharide units takes place in two stages: luminal digestion - digestion occurring in the intestinal lumen - and membrane digestion, the hydrolysis of certain small oligosaccharides by enzymes forming part of the microvillus membrane, the absorptive surface of the cells lining the small intestine.

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