The chemical composition of breast milk, after the first week, does not appear to vary much during the normal period of lactation. Although the composition of breast milk has been given above, it must not be considered that an absolute standard has been fixed. As a matter of fact breast milk is constantly varying from day to day in the same woman, and even from hour to hour, under the same conditions of life and diet. Nature has fixed her own limits and the lesson to be drawn is that any artificial food should be within those limits also, but need not be of an absolutely uniform chemical composition. Even at a single nursing the milk at the beginning is often different from that at the close, so that an infant at the breast obtains not only a food but a meal which may be regarded as a series of courses. In artificial feeding therefore the same principle may be acted on and we may vary the composition of the food at the different feeding times, as long as the limits established by nature are not departed from. Too frequent suckling increases the solids in the milk and is thus a frequent cause of indigestion. The indigestion induces pain and crying, and the breast is given still more frequently, with disastrous results. The habits of life of the mother as regards exercise, diet, and alcohol have an important influence on the quality of the milk. It is generally agreed that the fatty element in the milk is the one chiefly affected by diet. The amount of fat can be increased by a full protein diet, composed of beef and mutton, fish, eggs, etc., while it can be diminished by a carbo-hydrate diet composed of vegetables, fruits, and farinaceous puddings. If a woman undergoes a course of stuffing with food and at the same time takes no exercise, the protein element in the milk tends to increase. The quantity of the milk will vary with the amount of fluid drunk, and the proportion of solid contents may be regulated to a certain extent by the fluid intake.
Hutchison quotes experiments of Baumm and Illner as to the effects of various diets on the milk of nursing mothers. They employed the following diets : (1) An abundant mixed diet, (2) a highly nitrogenous diet, (3) a diet rich in carbo-hydrates and fats, (4) a very fluid diet, (5) an ordinary diet with two to three pints of lager beer daily, and (6) a diet with much salt fish, pickles, and other salt foods. The result was that fat was the only ingredient of the milk on which the diet produced any appreciable effect. The amount of fat was sometimes increased 1 per cent on the first and second diets only. This may be regarded as the chemical side of the question, but one must also consider what may be called the biological aspect. Experience has shown that a healthy mother will probably rear an infant with much less trouble than one who is in a weak state of health. The diet and habits of the mother are therefore of the first importance in maintaining her health at the highest possible level, especially considering the extra strain which suckling imposes on her system. The mother's diet should be a mixed one, plain well cooked food being partaken of three times a day. It is not necessary to make any great change in her usual dietary supposing that to have been of an ordinary character. Fresh cows' milk and oatmeal porridge or gruel are always to be recommended as part of a nursing woman's diet. All highly spiced or seasoned dishes which would tend to impair her appetite or digestion should be avoided, but within these limits she should be allowed freedom to consult her own tastes. Her appetite will usually be increased during the nursing period and is the best guide as to the quantity she should take. No stuffing with food, which will probably lead to digestive disturbances, should be practised. As regards increasing the amount of her milk, should this be advisable, the addition of one or two pints of fresh cows' milk to her dietary will be the simplest and most efficient method.
It is popularly believed that the addition of alcohol to the diet, in the form of stout or beer or wine, is beneficial in improving the quality of the milk, as some say, and in strengthening the mother for her extra duties, as others think. This phase of the alcohol question is met by certain writers with the statement that as alcohol cannot be distilled from breast milk, even when considerable quantities of alcohol have been taken by the mother, the effect on the quality of the milk is nil. Holt as the result of some experiments found that the quality of the milk was altered and the quantity increased by giving nursing mothers an alcoholic extract of malt. The alteration in the milk consisted in an increase of the fat and to a less extent of the proteins. Probably exactly the same result would have followed the use of malt extract without the alcohol. Physicians generally have been led by experience to refrain from ordering alcohol during lactation either for the benefit of the mother or the improvement of her milk. If, however, the mother has been in the habit of taking alcohol in moderation, such as is represented by a glass of beer or stout or claret or port twice a day with meals, there seems no scientific justification for forbidding it. The practice by those not accustomed to alcohol of taking it freely during lactation will probably prove deleterious to both mother and child, and should be strongly condemned.
Variations in the quality of the milk during menstruation have not been determined chemically, and as a rule no bad effects can be traced clinically. In some cases it may be found that the infant suffers from convulsions or diarrhoea when the mother is menstruating, and only at those times. If any such disturbances occur one will naturally adopt some other method of feeding during menstruation. As a rule menstruation is not to be regarded as an indication for stopping nursing. The occurrence of pregnancy during lactation does not necessarily change the character of the milk or injuriously affect the child. On the other hand it is very questionable whether a woman can successfully perform the double function at the same time. In exceptional cases she may, but as a rule it is advisable to cease nursing if pregnancy occurs.
Worry, anxiety, and mental distress of any kind may so alter the milk as to render it unsuitable for an infant, as shown by digestive disturbances. Acute bodily illness in the mother will often necessitate the cessation of suckling. If such illness be brief and without after effects it may be possible to carry the infant through this period by artificial feeding and resume the breast-feeding later. During the acute illness the milk should be drawn off regularly by a breast pump. In the presence of constitutional disease on the part of the mother, such as tuberculosis or syphilis, it is not advisable to encourage breast-feeding. If on the other hand the infant is born with signs of active syphilis, there is no reason why the mother should not suckle it provided she is herself in a healthy state.
Wet nursing is not a popular institution owing to the many drawbacks attending its use. At the same time when the breast milk of the mother fails, and the best possible food seems necessary, there is no substitute equal to the milk of another woman. Sometimes, also, after a trial of artificial feeding without success or for an infant debilitated by acute illness the best method of treatment is by wet nursing. A wet nurse should be a healthy young woman of between twenty and thirty years of age, with an infant of about the same age as the one she is to nurse. The milk ought to be analysed in order to see that it conforms to an average standard. Her own infant, if alive, must be healthy and present no evidence of constitutional disease. The wet nurse must also be protected from possible infection through the infant she is to nurse. If syphilis be suspected or be definitely present in this infant wet nursing is not justifiable.