A part of the digestion of the casein is performed by pepsin in the stomach and a part by the trypsin of the pancreatic juice.

The larger part of the digestion of the milk sugar or lactose, is performed by the pancreatic juice; although it is partly acted on by the saliva. Usually, however, the saliva is given little chance to become mixed with the milk, unless it is taken slowly and mixed with saliva by chewing movements. This is one reason why children should be given milk in which bread has been broken, rather than a piece of bread and a glass of milk. By swallowing the milk slowly, smaller curds are formed in the stomach and the milk is more thoroughly digested.

The salts of milk, to a large extent, the water, and perhaps a portion of the sugar are absorbed in the stomach.

When the fat (cream) is removed milk digests more readily, so that in cases in which the stomach is delicate, skimmed milk, clabbered milk, or buttermilk are often prescribed instead of sweet milk.

Boiled milk is also more readily digested by some; the lactalbumin is separated and rises to the top in a crinkly scum. The casein of boiled milk is also more readily digested, forming in small flakes in the stomach instead of in curds.

Sterilizing the milk by boiling will prevent the action of bacteria in producing fermentation and disordered digestion, and, if relished, milk can thus be treated. Pasteurized milk is more palatable than boiled milk.

Milk is often better assimilated if other food is not too suddenly cut off. When the diet is radically changed the digestive system is apt to show derangement. Therefore when for any cause an all-milk diet is desired, it is unwise to begin it at once, by feeding from eighteen to twenty glasses of milk a day. This amount may be approximated within a week’s time. The change in diet should be begun by cutting down all meats and legumes and gradually eliminating starches. In changing from a milk diet to a diet including more hearty foods, the transition should also be gradual.

If a milk diet is to be followed and the milk seems to disturb the stomach when taken in quantities, one may begin by taking it in very small quantities every fifteen minutes for the first hour. If one’s purpose is to gain in flesh the quantity may be increased to a glass, and time intervals be lengthened to every hour as the stomach becomes accustomed to caring for the milk. It should be sipped slowly and thoroughly mixed with saliva before being swallowed. The mouth should be carefully rinsed with equal parts of peroxide of hydrogen and water, or listerine and hot sterile water, each time milk is taken.

Milk, in whatever form it is taken, leaves a coating on the tongue and teeth. The heat of the mouth, especially if the patient is feverish, quickly causes changes which give a disagreeable taste and a chance for bacterial action. These bacterial products are carried into the stomach and excite digestive changes through which fermentation and gas formation appear and biliousness may result. This may be avoided if, after taking the milk, the mouth is carefully washed and, in feverish conditions, the tongue gently scraped or swabbed with absorbent cotton dipped in listerine or peroxide of hydrogen. Without such cleansing of the mouth milk may disagree.

When from two to three glasses of milk at a meal are taken, less solid food is needed, because the required nutriment is partially supplied by the milk. One reason why milk seems to disagree with many people, is because they lose sight of the fact that milk is an actual food, as well as a beverage, and they eat the usual quantity of food in addition to the milk. As one pint, or two glasses of milk, contains approximately the same amount of nutrition as one-third of a pound of beef, the amount of food to be taken in addition may be readily calculated.