That which principally characterizes milk as a food, and places it above other nutritive substances, is the fact that it contains all of the main nutrient groups, thus rendering it a complete food. This is proven by the fact that young animals, and young children, too, live upon it and are developed by means of it. It is, to be sure, necessary for the proper development of the young animals, and of nursing children, that the milk should be taken in the condition in which it occurs in the mammary glands of the corresponding species. Development is sometimes furthered by milk of a foreign nature, but the latter is not to be compared with mother’s milk. Bamberg has recently experimented, in the children’s clinic at the Charité in Berlin, with germ-free, raw, foreign milk, in comparison with cooked, foreign, germ-free milk. The children fed upon the raw milk thrived well, but the best results were obtained when they were nourished in the natural way. A series of other experiments have also proven that animals as well as man thrive and develop much better upon the maternal milk.
It is sometimes impossible to give the milk of the mother or of a wet-nurse, and it then becomes necessary to resort to the milk of animals. That most used, as is well known, is cows’ milk, which, while it contains very valuable nutritive substances and mineral salts, nevertheless differs greatly from human milk. That which most nearly approaches it is the milk of the ass, and the next is mares’ milk.
Cows milk contains on an average 35 to 40 grams of nitrogen substances per liter, together with 40 to 45 grams of milk-sugar and from 40 to 50 grams of fat. This will show what a valuable nutritive food milk is, for when only i liter of milk is taken in a day about 600 to 650 calories are taken, thus about one-fourth of the total amount of nourishment required per day. When 4 to 5 liters are taken in one day all the nourishment required will be furnishedfor a certain length of timeas may be seen in the various milk cures resorted to for diabetes, gout, fatty degeneration, or heart affections. It is to be remembered, however, that in order to do well on a milk diet it is necessary to take a rather larger quantity of milk than that corresponding to the required number of calories, because milk taken alone is not so well assimilated, owing to the fact that the flow of saliva and digestive fluids is not greatly stimulated by it. But little saliva is mixed with the milk, and consequently the carbohydrate content is poorly digested. Madinaveitia therefore recommends that when taking milk it should be kept in the mouth for a little while before swallowing it, and rolled about as much as possible, so that the saliva will be secreted, which will make the milk more digestible. While milk taken alone is poorly digested and assimilated, this can be remedied by taking some bread or cheese with it, as has been shown by Rubner.
He found in one case that 8.3 per cent. of the nitrogen, 6.4 per cent. of the fat, and 41.1 per cent. of the nutrient salts contained in the milk were not assimilated ; in another case 7 per cent. of the nitrogen, 7 per cent. of the fat content, and 24.1 per cent. of the salts were eliminated unused. When cheese was then taken with the milk, only 3.8 per cent. of the nitrogen, 7.1 per cent. of the fat, and 37.5 per cent. of the salts were lost. We see from the above how very poorly the nutrient salts in milk are assimilated. The digestibility and assimilation of milk depend to a certain degree upon whether it is taken raw or cooked. As a general thing, it may be said that raw milk is better digested and assimilated. Jessen showed that 600 c.c. of raw milk remained three and one-half hours in the stomach; skimmed milk, the same length of time. Sour milk was better digested, for it only remained three hours in the stomach. That most poorly digested was cooked milk, which remained the longest time (four hours) in the stomach. This goes to show that sour milk and buttermilk are best digested.
According to Listow, sterilized milk is more poorly assimilated than raw milk. He also found that milk is better digested when bread is taken with it.
The assimilation of milk is better accomplished in children than in adults. This is shown by the experiments of Rubner and Heuber.
According to Praunitz, milk is more poorly assimilated in the intestine in adults than other animal foods. At all events, we can certainly say that meat is much better assimilated in the intestine than milk. The following table shows the assimilability of the various kinds of milk and milk products, together with their composition, according to J. Konig.
We see from the above that of the various kinds of milk sheep’s milk is the most nourishing, and that cheese of all kinds has great nutritive value. When one drinks milk and takes bread and cheese at the same time, he will be well nourished, for in this combination we have an easily digested milk diet which will be sufficiently sustaining, as is shown in the case of shepherds, who often live for some time upon such food. This diet is rich in the most important nutritive elements ; in addition to large amounts of nitrogen, fat, and carbohydrates, it contains other indispensable substances. Milk contains much lecithin; woman’s milk contains more of it than cows’ milk. Burrow found in cows’ milk 0.049 to o.058 per cent. ; in woman’s milk, 0.057 to o.060 per cent. Nerking and Haensel found in woman’s milk lecithin to the amount of 0.0799 per cent., and in cows’ milk between 0.04 and 0.11 per cent. ; goats’ milk contained the same amount as woman’s milk.
Thus, it is plain that we absorb a considerable amount of lecithin when we take a quart of milk. Lecithin is said to favor the growth of young animals, and the digestion of fats is also improved by it.
In addition to its great nutritive value and lecithin con-tent, milk possesses other advantageous properties, containing as it does such valuable salts as phosphorus and lime in considerable quantities.
We may thus observe how much phosphorus and lime is contained in milk. It is most unfortunate that, as has already been stated, the assimilation of both the organic nutritive sub-stance and the salts is incomplete. The lime, in particular, is very imperfectly assimilated ; according to Forster, as much as 75 per cent. is lost in the child. In spite of all this, milk is a food from which our bodies absorb considerable lime. In iron, on the contrary, milk is poor; unskimmed milk contains 0.31 per cent., and cream 2.84 per cent. ; cream is likewise richer in phosphoric acid.
Very important substances also contained in milk, and which tend to make of it, as it were, a life-giving food, are certain ferments, which likewise help to render it more digestible. In boiled milk, these ferments are absent, thus making it a dead food in comparison with raw milk. Boiling and sterilization are unfortunately unavoidable when we are not certain of having pure, clean milk. As long as it remains in the udder of a healthy cow, the milk is certainly free from germs. Near the external orifices of the udder bacteria are found which have penetrated from the outside; therefore, that portion of the milk which is obtained first contains quite a number of these bacteria. The milking is, besides, often carried on in an uncleanly manner; more especially when the udder has not first been washed, the milk will contain a great many bacteria. These are not all of a harmless order; even the dangerous staphylococci and streptococci may be found among them. Some bacteria derived from the cow itself, as those of the mouth and hoof plague, can be transmitted to man. The transmission of tuberculosis from the cow to man is most improbable, as the milk of such cows is used with impunity. This has lately been shown by the numerous experiments made under the auspices of the Imperial Board of Health. In a large number of cases in which the milk of tuberculous cows had been taken for some time there were no injurious results; harm resulted only in a very few instances, always in individuals predisposed to tuberculosis.
The danger of contracting the disease through the agency of milk is not very great. The germs of other diseases, particularly those of typhoid fever and diphtheria, may be much more dangerousthey are often transmitted and cause actual epidemics, as I personally observed several winters ago during a stay in Copenhagen. The German Imperial Health Bureau proved infection by milk to have occurred in 51 out of 126 cases of typhoid fever during an epidemic. It is to be remembered that the typhoid germs thrive well in milk and increase rapidly in its lukewarm temperature. Heim found them living after thirty-five days, and the tubercle bacilli after fifteen days, even in sour milk.
In addition to typhoid-fever epidemics, diphtheria epidemics may also be caused, as has been stated by Power and Danger. According to Schlechtendal, at least 27 typhoid epidemics were caused from 1891 t0 1901 owing to negligently conducted dairies. The bacteria primarily find their way into the milk from dirt and unclean surroundings in general, or, again, through intentional watering of the milk. The surest way of preventing infection by milk is to have it sterilized. Even then a few germs remain, sometimes even of a dangerous character.
It is an interesting fact that, as was shown by Heim, the cholera bacilli soon die in raw milk, while in sterilized milk they remain active for ten days; the same results were reported by the Imperial Health Bureau; the diphtheria bacilli do not thrive as well in sterilized milk as in raw. Sterilization and the boiling of milk may, in general, be regarded as good pre-cautionary measures against a possible bacterial infection, but they have the disadvantage that not only the ferments, but also certain other important substances contained in the raw milk, may be impaired. Indeed, milk contains the same immunizing agents as are present in the blood, namely, alexin and opsonin, which, together with the internal secretions of various glands, enter into the milk from the blood. It would lead me too far to go into the details of this subject here, but I would call attention to the fact that the presence of the internal secretions in milk is shown in that in children having an inherited weakness of the thyroid gland no symptoms of this condition develop as long as they are taking the maternal milk. When the nursing has ceased, these symptoms, as a rule, very soon make their appearance. The various substances mentioned pass out from the mother into the milk. Fortunately, this is not the case with alcohol.