Composition

Carbohydrates are composed of carbon, hydrogen, and oxygen. The most important are starch, sugar, and cellulose. Chemists recognize three main divisions: (i) Polysaccharides, or starches, cellulose, dextrin, and gums; (2) disaccharides, such as cane-sugar, lactose, and maltose; (3) monosaccharides, which include dextrose, or grape-sugar, and levulose, or fruit-sugar. Carbohydrates are eaten chiefly in the form of starch and cellulose, but must be transformed into dextrose, or grape-sugar, before they can enter the blood and contribute to the maintenance of bodily vigor.

Digestion

Starch occurs in the form of grains or small particles with a nondigestible envelope of cellulose about them. When, in cooking, it is subjected to heat and moisture, it swells, and the envelope ruptures, permitting the starch grains to escape. Therefore, in order that starch may be digested, cooking is essential. If starch, cereals, and vegetables are cooked imperfectly they are indigestible. When, after cooking, starch is taken into the mouth, the granules are, by mastication, more completely broken up and are incorporated with saliva so as to form a paste-like mixture. The thorough commingling of saliva with the starch is essential to good salivary digestion. Ptyalin, the digestive ferment of saliva, splits starch first into various dextrins and ultimately into maltose, which is the end-product of its digestion. These changes are only begun in the mouth, as food is rarely retained in it more than a few seconds, or one or two minutes at the longest. A comprehension of the process of salivary digestion makes evident the necessity of perfect and slow mastication. Although salivary digestion progresses best in a slightly alkaline or neutral medium, it can take place in a faintly acid mixture also. Therefore it is chiefly continued in the stomach before the contents of the latter become strongly acid. It is supposed to be checked by the end of the first half-hour or three-quarters of an hour of gastric digestion. As starches usually form a large part of our meals, it is impossible to convert much of them into maltose while they are in the stomach. By the end of the period of gastric digestion some starch is undigested in the stomach, much of it is converted into various dextrins, and a little into maltose. The churning movements of the stomach, as well as the process of salivary digestion, help to disintegrate and dissolve the starch clumps that are swallowed, so that they will be emptied into the duodenum in a state of fine division and suspension, if not of solution.

Such cane-sugar, milk-sugar, or fruit-sugar as is eaten is also partly digested in the mouth and stomach. When grape-sugar is eaten, it undergoes no digestive change.

As already explained, salivary digestion in the stomach is influenced by cooking and by mastication. The body-temperature especially promotes its progress. Ice, iced food, e.g., icecream, and iced water will hinder the formation of saliva and lessen the activity of ptyalin digestion. Very cold drinks at the beginning of a meal or with it are therefore not favorable for starch digestion in the stomach. Starch in cold foods, such as potato salads, cold oatmeal mush, and similar articles, is not readily transformed in the stomach and should be eaten only by those whose digestion is good. However, the chief changes that starch-containing foods undergo in the stomach are disintegration and comminution of the masses in which they are swallowed, and a very moderate degree of chemical alteration.

Salivary digestion is also delayed by eating, at the beginning of a meal, very acid fruit or food, which will rapidly acidify the contents of the stomach. Oranges and other acid fruits are therefore not so wholesome when eaten at the beginning of a meal as at the end. If fruit is eaten before breakfast, it should preferably be a sweet fruit or a compote.

When the digestion of starch in the stomach is imperfectly performed because of an excess of acid gastric juice, it may often be aided, if a glass of hot water is taken twenty or thirty minutes before eating, in order to lessen the secretory activity of the stomach and to dilute partly its juice. If only carbohydrates are eaten at one meal and only proteins at another, gastric digestion is often improved.

Carbohydrates are chiefly digested in the intestine. Amyl-opsin, a pancreatic ferment, is essential to this process. The changes that it effects in starch are similar to those wrought by ptyalin. Maltose is again its end-product. Several dex-trins are formed in the progress of the final transformation of starch. Dextrins, cane-sugar, milk-sugar, and grape-sugar also find their way into the intestine. The first of these is modified by amylopsin and converted into maltose.

Maltose is the final product of all carbohydrate digestion. It does not enter the blood, but is still further transformed into dextrose, either by the invertase of the intestinal juice, or possibly by the epithelial cells of the villi. Dextrose is the only carbohydrate found in the portal blood.