Digestion and absorption of carbohydrates

Few animals are equipped with enzymes capable of attacking cellulose, although this polysaccharide plays a very large part in the nutrition of herbivorous animals. In these creatures the task of digesting cellulose is usually delegated to vast hordes of symbiotic micro­organisms, and the useful products of their activity consist in the main of short-chain fatty acids. The mechanisms of this degradation are complex, if only because many different kinds of microorganisms are involved. At the present time not a great deal is known about the details of the process, but they are being actively investigated.

Like cellulose, the so-called hemicelluloses and fructofuranosans are not digestible by the enzymes of most animals, although they can be handled by symbiotic microorganisms and probably yield products similar to those formed from cellulose.

The digestion of starch and glycogen is initiated by salivary amylase but, unless the eater chews each mouthful of food quite an unbelievable number of times, little digestion takes place in the mouth. The food, more or less intimately mixed with saliva, is swallowed and passes on into the stomach. Although the optimal pH for salivary digestion lies very near to neutrality, the secretion of the strongly acid gastric juice does not put a sudden end to salivary digestion, because it takes time for the acid to penetrate into the food bolus. The consistency of the food mass is therefore an important factor. Eventually, however, the free acid of the gastric contents reduces the pH to a value at which the salivary amylase is inactive and is actually destroyed, but in the meantime starch and glycogen alike have been at least partly broken down to yield maltose and, if digestion is not yet complete, some dextrins.

The gastric juice itself contains no carbohydrase, but a notable concentration of free hydrochloric acid is present and contributes something to the digestion of carbohydrates containing fructofuranose units. Fructofuranosides such as sucrose and inulin are hydro-lysed with great ease and rapidity by warm, dilute mineral acids, and it is believed, therefore, that substances such as these undergo at any rate a partial hydrolysis during their stay in the stomach. The hydrolytic activities of the hydrochloric acid are cut short when the chyme passes through the pyloric sphincter and into the duodenum, where it encounters the strongly alkaline pancreatic juice and bile. Here the pH rises nearly to neutrality, and under these conditions the amylase of the pancreatic juice has almost its optimal activity. This enzyme finishes the work begun by the salivary amylase, and the conversion of starch and glycogen into maltose is completed.

Maltose, however, is only a transitory product, for it is rapidly hydrolysed under the influence of a glucosidase, the so-called 'maltase' of the intestinal juice. This secretion also contains a powerful glucosaccharase which completes the hydrolysis of sucrose, and a p-galactosidase, 'lactase', that deals with lactose. Ultimately, therefore, the carbohydrates of the food are resolved into their constituent monosaccharides and in this form they are absorbed from the gut. It is improbable that appreciable quantities of di-or higher saccharides are absorbed because, as is known from injection experiments, disaccharides present in the blood stream are largely excreted unchanged, and it is only in exceptional and probably abnormal cases that disaccharides appear in the urine.

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