The only substitute for breast milk that contains in any degree the same properties is the milk of some other mammal. No other fat or protein can take the place of that of milk. The milk of each kind of mammal is peculiarly adapted to its own young, and not to that of another kind. To overcome, as far as possible, this unfitness for the infant of cow's milk, which was intended for the calf, is the main problem of artificial feeding.

1. Cow's Milk And Human Milk Compared

Breast milk flows slowly, after proper effort, warm, living, non-acid and germ-free directly into the baby's mouth. Cow's milk is drawn into vessels, and stands for hours, even days, before it is given to the baby from a bottle. It becomes decidedly acid in reaction, teems with bacteria, is either kept cold, or else is heated so that its living properties, ferments, etc., are destroyed. It has a different chemical composition, a different physiological reaction with rennin, and has no antitoxins that are of value to the baby. Over and above all these differences there are, doubtless, others that are equally or even more important, that make up the difference between a food that is a specific and one that is a substitute.

Composition

The following table from Holt shows at a glance the comparative average composition of human and cow's milk:

The fat of cow's milk is apparently chemically much the same as that of the breast milk, but contains more volatile fatty acids and less lecithin.

The sugar is identical in both, but less abundant in cow's milk.

The proteins offer the most striking difference, cow's milk containing two or three times as much as human milk. The latter contains rather less casein than soluble proteins, lactalbumin, lac-toglobulin, etc., while cow's milk contains five or six times as much casein as lactalbumin. As a result of this difference in casein content, cow's milk, when mixed with rennin, or gastric juice, forms a jellylike, thick, semi-solid coagulum that contrasts strikingly with the fine, soft, flocculent precipitate of human milk treated in the same way. If similar conditions prevailed in the stomach, the baby fed on cow's milk would get a semi-solid food, that fed on human milk a liquid food. It is not difficult to see why casein has been considered the important disturber in the infant's digestion of cow's milk, especially when curds in the bowel movements are an almost unfailing accompaniment of milk indigestion.

The salts are relatively more abundant in cow's milk, especially the calcium and sodium salts. . Breast milk is richer in iron and potassium salts.

A comparison of the chemical composition of the two milks in our present state of knowledge gives us very little light on the great difference in their actions on the baby. The fat, in spite of great chemical similarity, is well borne in breast milk, and yet offers the most trouble of any food element in feeding cow's milk. The protein, on the other hand, with its decided chemical and physiological differences, seems to offer the least trouble of any food element, both in digestion and metabolism (see next chapter). The part played by the salts in this connection is imperfectly understood, but probably not important. It would be a simple matter to mix cream, milk, whey, water, and milk sugar so that the mixture would very closely approach the chemical composition of breast milk, and yet its action on the baby would be vitally different in most cases. Our aim, then, in adapting cow's milk to an infant, is not to make it like human milk, but to make it act like it as a food. While our food mixtures often bear little resemblance to breast milk, we nevertheless always keep in mind its composition as our standard and depart from it only so far as the difference in digestibility and assimilation of the different food elements of cow's milk makes it necessary to do so.

2. Adaptation Of Cow's Milk To The Healthy Infant

Whole Milk

The use of straight cow's milk has been enthusiastically advocated by some French and a few German writers. It is apparently well borne by some babies almost from the start, provided the amount is strictly limited and overfeeding avoided. Thus Budin recommends one tenth of the body weight daily of milk, and reports excellent results, "in infants after the fifth or sixth month, weighing thirteen to fifteen pounds." Nearly all of his babies are fed at the breast, however, for the first few months. Practically all other writers agree that the milk should be diluted and otherwise modified, at least during the first few months.

Modified Cow's Milk

The most primitive and most effective modification is dilution with water. A dilution of one part of milk with two parts of water is commonly well taken from the start. This reduces the protein to about the amount found in breast milk. It is the indispensable food element, as it alone contains nitrogen, and so should not be given for any length of time in less amount than in the natural food. To the twenty-four-hour food from one half to three quarters of an ounce of milk sugar is added to make up for the deficiency caused by dilution and to approximate the seven per cent found in breast milk. The proportion of milk is gradually increased, that of water lessened, and the amount of sugar increased, so that by the end of the first or second month the baby is getting equal parts of milk and water with the addition of three quarters to one ounce of milk sugar, giving a total of five to seven per cent of the latter. On this the child can be left for a long time. The same process is then continued until near the end of the first year the child is on whole milk. The sugar is gradually lessened, as more milk is added, and omitted when we reach straight milk. Cereal waters, or gruels, made up of barley, rice, wheat, or oatmeal flour, often seem of advantage from the start as diluents in place of plain water, but they should be used with caution till after the fifth or sixth month, when the ability to digest starch is well developed, and they should regularly form a part of the food. The additional food during the latter part of the year has been discussed under maternal nursing. The total amount of the twenty-four-hour food mixture is about a pint at the beginning, gradually increasing to a quart in the second half year. This amount need rarely be exceeded to any extent. Cane sugar is recommended by some (Jacobi) and malt sugar by others, in place of milk sugar. The latter is given preference here largely because it is the sugar of human milk.