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
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.
A