Having seen what each of the foodstuffs does in nourishing the body, we may now see how they are prepared for the use of the body in the digestive tract.

Digestion of carbohydrate.—The simplest carbohydrate is a sugar which cannot be broken up into other sugars. Such a simple sugar is called a monosaccharid. There are two common in foods, glucose and fructose; a third, galactose, is derived from more complex sugars. Two simple sugars united chemically make a double sugar or disaccharid; thus cane sugar or sucrose will yield glucose and fructose, while milk sugar or lactose will yield glucose and galactose, and maltose will yield two portions of glucose. These three disaccharids are the only common ones. Starches, dextrins, and cellulose or vegetable fiber are made of many simple glucose groups, and are hence called polysaccharids. All carbohydrates to be used by the body must be reduced to simple sugars. Glucose needs no digestion therefore, but the double sugars must be split by enzymes into two simple sugars in the intestinal juice, one for each kind, namely, sucrase (sucrose-splitting), maltase (maltose-splitting) and lactase (lactose-splitting). The digestion of starches and dextrins begins in the mouth, where amylase (starch-splitting) changes starch first to dextrin and finally to maltose, and maltase may change a little of the maltose so formed into glucose. In the stomach there are no enzymes acting on carbohydrates, but the digestion may continue under the influence of swallowed saliva for a time. In the pancreatic juice there is another amylase, which completes the splitting of starch to maltose, and then the intestinal maltase can reduce this to glucose, which will be absorbed. Cellulose cannot be digested and simply serves to add bulk to the diet.

Digestion of fat.—A fat is made up of two parts, one a fatty acid, the other glycerol. Fat cannot be absorbed by the body until it is split into these two parts. A fat splitting enzyme is called a lipase. There is none in the mouth; one in the stomach works only on fat in the state of emulsion; the most powerful is found in the pancreatic juice. Since fat cannot be digested in the mouth nor to any great extent in the stomach, it is bad to have food coated with it, for the protein and carbohydrates will have to wait till the fat is digested away, before they can be digested; that is, till the intestine is reached. This is one reason why pastries and fried foods are hard to digest.

Digestion of protein.—There are no enzymes in the mouth acting on protein. In the stomach, the hydrochloric acid helps to make it soften and swell, and then pepsin begins its digestion. Protein, like fat and carbohydrate, can be subdivided into smaller and smaller portions, finally being reduced to a form which the body can absorb, namely, amino acids, of which there may be 17 or 18 kinds from a single protein.

The digestion in the stomach produces chiefly large fragments of the original protein, called proteoses. In the pancreatic juice is a powerful enzyme called trypsin, which digests proteins, first to fragments, next smaller than proteoses, called peptones, and finally breaks these peptones into amino acids. In the intestinal juice is another enzyme called erepsin, which also forms amino acids from proteoses and peptones, thus finishing any digestion of protein left incomplete by the trypsin.

Carbohydrates, absorbed as glucose or other monosaccharids, are carried by the portal blood to the liver, and thence passed into the blood, to be burned in the muscles, if needed for fuel, or stored temporarily in the liver and muscles as glycogen (a polysaccharid yielding glucose) for future conversion to sugar when required as fuel.

Fats, passing through the intestinal wall as fatty acids and glycerol, enter the lymph largely as fat again, and finally pass to the blood to be burned in the muscles for fuel, or to be stored as fat until needed.

Proteins pass into the blood as amino acids. Those needed for building material are taken up by the cells (especially cells of the muscles) and those not required for this purpose are freed from their nitrogen (in the liver or muscles) and then burned for fuel.

For a fuller discussion of the fate of the absorbed foodstuffs see Chapter IV of Sherman’s “Chemistry of Food and Nutrition.”