The milk of several animals, such as cows, goats, asses, mares, and camels, may be used for food, but in this country very little other than cows' milk is employed. The varieties of milk differ slightly in chemical composition, odour, and taste, but they all contain the elements which are necessary for the maintenance of life in fairly economical proportion, so that for infants they constitute a "complete " food which fully meets the requirements of the growing body, and in adults they will sustain life comfortably for many months.

For these reasons milk ranks among the most important of all foods, and it is necessary to determine to what extent it should be introduced into ordinary diet. A pint of milk may be said to represent approximately the nutrition contained in six ounces of beef or mutton. Although it furnishes so useful a food, milk is by no means essential to a diet designed for increasing bodily strength, and it is usually omitted from the menu of athletes in active training.

Milk taken alone constitutes an insufficient diet for adults in health, for to obtain the requisite 18.3 grammes of nitrogen 2,905 grammes of milk only are needed, against 4,652 which must be taken to obtain the necessary 328.9 grammes of carbon (Bauer). (See Force Production, page 20).

Adults who are able to eat any kind of food usually maintain their health in better condition by abstaining from milk except as used for cooking purposes, inasmuch as it makes many persons "bilious " to drink it, and produces constipation, particularly when taken in excess with other foods.

Sir Henry Thompson, in condemning the excessive drinking of milk by persons in goo'd health who are at the same time eating large meals of meat, says: "It is altogether superfluous and mostly mischievous as a drink for those who have reached adult age and can digest solid food." There are, however, many diseases in which a partial, or for a time an exclusive, milk diet is to be recommended.

Milk in considerable quantity - i. e., equal to one third or one half the total amount of food consumed - is often highly desirable in such diseases as tuberculosis, chronic Bright's disease, and anaemia.

Exclusive Milk Diet

An exclusive milk diet is useful in the following conditions and diseases:

1. In infancy for the first year, and sometimes for the first eighteen months.

2. All acute infectious diseases of young children.

3. Typhoid fever.

4. Acute Bright's disease, and at times in chronic nephritis.

5. Acute pyelitis.

6. Chronic gastric catarrh.

7. Gastric ulcer and carcinoma.

8. Neurasthenia.

A milk diet is so easy to prescribe, so cheap, and so easily procurable, that it is always the first resort of those who, from indifference or lack of knowledge of the first principles of dietetics, are unwilling or unable to take pains to study the peculiarities and needs of the individual case. It is an easily measured food, and it becomes a routine practice in hospitals - where almost always more attention is awarded to medication than to diet - to order milk diet for all patients who are not at the convalescent or full house table, and it is doubtless safe to err in this direction, but it is by no means always the best that can be done for the patients.

An exclusive milk diet in time becomes monotonous and wearisome to most adults, and may produce dyspepsia, constipation, and interfere with the functional activity of the liver. Aside from nausea, which the continued use of milk may excite, a positive loathing for the taste of it may be developed, unless the regimen is modified by occasional variation. This is a matter of considerable importance in feeding patients suffering from typhoid fever, chronic Bright's disease, chronic gastric catarrh, and other affections for which milk diet is often prescribed; for if other substances are substituted from time to time in small amounts, while milk is still retained as the chief food, it may be continued as such for a much longer time. On seeking the cause for the disagreement of milk, it is found in the fact that it contains too large a proportion of nitrogenous material as compared with the hydrocarbons, so that, in order to obtain sufficient of the latter, an excess of proteid is ingested, which interferes with normal digestion.

For a man in health a pure milk diet, as Ewald says, is "slow starvation," although it is an excellent food for a short time. If kept too long upon it, he develops a condition akin to scurvy. (See Diet in Scurvy.) This, however, does not invariably follow in disease, and J. K. Mitchell refers to a patient with diabetes and nephritis who lived upon milk alone for seven years, and kept in active business.

Infants and children tire of milk less easily than adults. Whereas the adult needs twenty-three ounces of water-free food per diem to maintain healthful equilibrium, he must consume nine pints of milk at a specific gravity of 1.030 to supply it. The excess of albumin, fat, and water which he would then obtain is wasteful for him, although it is good for the young. But in illness life is comfortably maintained upon a smaller quantity for a few weeks, and in typhoid fever four or five pints is an ample allowance, and it is often better to give less. (See Diet in Typhoid Fever.) In a case of chronic Bright's disease, if the patient is not confined to bed, it may be necessary to give six or seven pints.

In order to digest large quantities of milk it must be taken in measured doses at frequent intervals. In typhoid fever, for example, from three to five ounces may be given every two hours. In some diseases it is best to give two ounces every hour, in others six or eight ounces every three hours.

Milk leaves no coarse waste residue in the intestine like the indigestible fibre of meat or the cellulose of vegetables and fruits. Notwithstanding this fact, an exclusive milk diet yields considerable bulk of fecal matter, and a typhoid-fever patient living on milk alone often has daily evacuations of medium size. Rübner found that in health a diet of milk alone yields larger stools than either roast beef or egg alone. Yet by weight the absorption of milk is shown to be very complete, and four thousand grammes of milk ingested by the mouth, when perfectly digested, yield but one hundred grammes of feces (Rübner). There is a marked loss through the feces of the salts of lime which have been contained in the milk. These facts explain in part why milk is so constipating.

Prof. Charles E. Wait, in the course of nutrition investigations of a club of students at the University of Tennessee (U. S. Department of Agriculture, Bulletin No. 53, 1898), says:

"The average of ten experiments with an exclusive milk diet showed 92.1 per cent of the protein and 86.3 per cent of the carbohydrates to be digested. Five experiments made with an exclusive bread diet or with bread and sugar showed 82 per cent of the protein and 99 per cent of the carbohydrates to be digested. Five experiments with a diet of bread and milk showed 97.1 per cent of the protein and 98.7 per cent of the carbohydrates to be digested. In other words, the protein in milk alone or in bread alone seems to be much less completely digested than when the two are eaten together".