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Frequency of Feeding
Supplementary Feeding
Menstruation
Pregnancy
Smoking and Drugs
Social Pressures
The Effect of Diet on the Endocrinology and Metabolism of Lactation
Development of the Breast
Lactogenesis
Maintenance of Lactation
Oxytocin Release
References
4.1. It would give a totally erroneous impression to imply that maternal diet is the only, or indeed the major, variable influencing milk production. In recent years considerable interest has been focused on a number of sociological and physiological aspects including: economic, social and emotional pressures; the endocrine response; the operative condition of breast-feeding reflexes such as the suckling reflex; the frequency, duration, and intensity of the nursing stimulus; and so on. Many of these studies have been based in the metabolic ward or physiological laboratory, and there have unfortunately been few controlled community investigations on representative numbers of people living under different prevailing circumstances to clarify their wider public health significance.
4.2. When one compares communities in the Third World with those in the industrialized countries, frequency of breast-feeding provides one of the greatest differences. In industrialized countries, eight times, quickly falling to five times, per day is a common finding, while in Third World countries the rate may be 12-20 times. Even within an apparently homogeneous environment important differences can be detected, however, as recently shown by Delgado et al. (1). who studied poor people in Guatemala on a prospective longitudinal basis. They showed that, at three months old, babies who were larger in weight and length and who had been heavier at birth were breast-fed less frequently per 24 hr than their fellows. Their mothers were also heavier and their fathers were taller. The same tendencies remained even up to 12 months, particularly the association with father's height. Delgado has reasoned that father's height reflects long-term socio-economic status. Infant morbidity was also higher the greater the number of breast-feedings per day.
These results would seen to run counter to accepted dogma that frequent feeding is a good thing and to be encouraged as a way optimizing milk output. Such advice might be valid when the average rate of feeding is only five times per day, but a quite different interpretation suggests itself under the circumstances prevailing in poor Guatemalan communities. Delgado has reasoned that those mothers who are relatively better nourished during pregnancy and lactation, produce milk more easily that undernourished mothers, and thus readily satisfy the needs of their infants. As a result the infants of better nourished mothers obtain the quantity of milk needed in fewer breastfeedings, in less time, and with less effort. The higher morbidity rate found in frequent feeders indirectly reflects associated poverty and, presumably, the less than satisfactory hygienic circumstances that exist within such homes. These investigations need to be repeated in many more centres using the same protocol.
These conflicting observations on the importance of suckling frequency may be related to the behavioural responses of the infant. Suckling in babies is not only initiated in response to a need for nourishment but also in response to a need for security and comfort. In addition, suckling can be initiated by the mother in an attempt to comfort a distressed infant.
4.3. Another factor contributing to reduced milk outputs in many countries is that better nourished mothers are tempted to start supplementary feeding at too early a stage. Although true complementary feeding is to be encouraged between four and six months, it must be acknowledged that its ill-advised introduction, plus the apparent ease of bottle-feeding, frequently does result in the rapid abandonment of breast-feeding altogether.
The removal of some of the infant's drive for breast-milk also have an inhibitory effect on the mother's milk-producing capacity and she can be led into the mistaken belief that she is no longer physiologically capable of producing reasonable amounts of milk. On the other hand, if supplementary foods are introduced on a basis of infant choice (i.e. child-led introduction of supplementary foods) supplementation does not necessarily lead to the rapid abandonment of breast-feeding. The rational timing of the introduction of other foods is a complex issue. It is bound to vary with the specific physiological, sociological, and economic circumstances of each mother-baby pair. This topic is a subject of intense current debate and another United Nations report 12). It would be inappropriate for it to be pursued in greater detail here.
4.4. Although women do not ovulate during early lactation, the later stages of prolonged breast-feeding are frequently accompanied by menstruation. The factors influencing the onset of ovulation during breast-feeding form the main subject in the second part of this report. The reason why there is sometimes breast refusal during the menstrual cycle is unclear, but Hartmann and his colleagues (3) in Western Australia have found major, acute changes in milk composition occurring in the mid follicular and mid-luteal phases, though their exact signification awaits clarification.
4.5. There are widespread taboos inhibiting breast-feeding during pregnancy, not without good reason for marginally nourished communities, because of the double stress to the mother. Some tribes in Africa firmly believe that the child in the womb will poison the baby being breast-fed. Whatever the local reason, recognition of pregnancy usually results in an abrupt cessation of breast-feeding. This is the origin of the term "kwashiorkor": the illness produced by a child being displaced from the breast by a new pregnancy.
The renewed interest in breast-feeding and its duration for longer periods in the Western world has resulted in many women becoming pregnant while breast-feeding, and a few but increasing numbers of well-nourished and healthy women breast-feed throughout pregnancy. In Australia, Hartmann has reported tandem feeding, breastfeeding both the newborn and older infant. The consequences of such feeding practices both in developed and developing countries clearly merit further investigation.
4.6. Although the significance of therapeutic drug-taking for human lactation has been recognized, there is still an immense gap in this area. Also little is known of the effects of social drugs such as alcohol, caffeine, and nicotine. Hartmann has, however, observed some differences in milk composition at the initiation of lactation between smoking and non-smoking women that could interfere with the establishment of successful breast-feeding. Once this hurdle was overcome, however, there appeared to be little difference between the two groups of mothers.
4.7. Of all the factors influencing lactation, there can be little doubt about the overwhelming importance of social pressures. If the custom within a community is to breast-feed up to two years, then this is what the majority of women will do. If, however, the social attitude is that breast-feeding is rather distasteful, a not uncommon attitude in some communities in the industrialized world, there will be a low prevalence of breast-feeding. Such active discouragement compounds any real difficulties the mother may have, fuelling the worry that she may be physiologically inadequate and thus incapable of satisfying the dietary needs of her baby.
In many ways the mother in traditional societies is better off than her counterpart in the industrialized world. She at least has plenty opportunity to seek advice at a mother-to-mother level and is not dependent on the frequently uninformed views of health professionals. In this regard societies devoted to the promotion of breast-feeding, like the La Leche League, the Nursing Mothers Association of Australia and the National Childbirth Trust, in the United Kingdom, play an important role. It must be recognized, however, that such organizations can be just as much victims of misinformation as the health professionals. There is a widespread and urgent need for objective, broadly based studies on factors controlling lactation under both optimal and suboptimal breast-feeding situations in different socio-economic circumstances.
The Effect of Diet on the Endocrinology and Metabolism of Lactation
4.8. On a number of occasions in this report reference is made to possible alterations in the overall physiology of lactating women that could make them metabolically more efficient when forced to subsist on a diet of marginal adequacy. The same processes are also of significance to the duration of lactational amenorrhoea.
Unfortunately, the metabolic control of human lactation is a subject about which there are huge gaps in our knowledge. Most available information is from animal studies, and the relevance of this to humans is debatable(4). The problem is highlighted by the fact that it was not until 1970 that human prolactin was identified as a separate hormone, and radio-assay techniques for its estimation were not available until 1972.
The biological processes that finally lead to the delivery of milk to the baby can conveniently be considered under the following headings (41: (a) the development of secretory tissue within the breast; (b) the initiation of lactation, or lactogenesis; (c) the maintenance of lactation, or galactopoesis; and (d) milk ejection.
4.9. The enlargement of the gland at puberty is due to changes in the stroma, with deposition of periglandular adipose tissue. During the menstrual cycle there are changes in breast volume, probably accompanied by changes in the mammary epithelium, with increases in stromal oedema, luminal size, IgA, DNA and, perhaps, lactalbumin synthesis in the luteal phase of the cycle. These may be of great importance in preparing the breast for the major development of ductal and secretory tissue that takes place during pregnancy, but more research needs to be done on this subject.
The initial cycles after puberty are frequently anovular, as indeed they are during or immediately after lactation (see sections 7.2, 7.31. In the first trimester of pregnancy there is an extension and branching of the duct system; alveoli containing the secretory cells develop in the second trimester. The hormonal control of these changes is poorly understood, but studies in animals have suggested that insulin, cortisol, thyroid hormones, oestrogens and progesterone are required. Investigations in ruminants also indicate that placental lactogen (HPL) is necessary and levels of this hormone rise steadily throughout human pregnancy. Prolactin has also been implicated as being crucial, and this hormone exhibits massive increases in plasma concentration during pregnancy. Growth hormone, too, seems to play a specific role in some species, but a unique function is unlikely in humans, as placental lactogen and growth hormone have very similar actions.
It is quite clear that this is an area that should be investigated much more intensively, since inadequate breast development during pregnancy is known to be intimately implicated in poor lactational performance in animals. It is also reasonable to speculate that a poor diet during pregnancy may be just as significant to the preparation of the mammary glands for breast-feeding as is the development of the placenta for adequate intrauterine feeding.
4.10. Comparative studies in undernourished Gambian and well-nourished Cambridge women during pregnancy have indicated that a poor diet does lead to comparatively low circulation levels of insulin, T3 and oestradiol during each trimester IP.G. Lunn, personal communication). The concentrations of these hormones normally rise during pregnancy but this effect is by no means as great in the Gambia as it is in Cambridge. Unfortunately, the functional relationship between plasma concentrations and mammary-gland physiology is not fully understood, but insulin is the major anabolic hormone and the markedly elevated levels found in well-nourished pregnant women are presumably associated with placental, foetal, and breast growth as well as fat storage. Most of these are known to occur to a lesser extent in undernourished mothers, which would fit in with low plasma insulin levels.
If animals are made hypothyroid, mammary-gland development is reduced and eventual lactation is poor. Thus, the low T3 values found in the Gambia could also be of functional relevance. The comparatively low oestradiol concentrations are also probably tied up with the reduced fat deposition of the Gambian mothers. Cortisol concentrations were, however, very similar in the two groups, as were placental lactogen and progesterone. Prolactin concentrations were statistically significantly higher in the United Kingdom, but the overall difference was not great and the physiological significance, if any, must be small.
4.11. There is only limited information on the crucial metabolic and endocrine processes necessary for the initiation of lactation in humans. It seems, however, that as in the majority of animals so far studied, the main trigger for lactogenesis in the human is the drop in plasma progesterone level that occurs as a consequence of delivery and the loss of the placenta (4, 5). The period for which progesterone needs to be withdrawn is probably very short, since progestogens given for contraceptive purposes do not affect lactogenesis or lactation, provided they are given one or more days after delivery. It is generally postulated that the previously high concentrations of progesterone have had the effect of blocking the binding of prolactin to receptor sites in the mammary gland, thus preventing milk production during pregnancy. This is clearly not complete explanation, as women can produce a considerable amount of milk during pregnancy if they continue to nurse a previous baby. Furthermore, some secretory activity as well as the production of alfa-lactalbumin and lactose have been observed in non-nursing pregnant mothers before delivery.
The initiation of lactation is critically dependent on raised levels of prolactin around the time of delivery(4, 6). In this respect the malnourished mother appears not to be disadvantaged; certainly this is the case in the Gambia. In women who are unable to produce raised levels of prolactin, the initiation of lactation does not occur. Human growth hormone is not involved in human lactation, as it is in many other animals, since lactogenesis can occur in its absence.
Toaff (8) has hypothesized that the suckling reflex's initial impact overrides the inhibitory effects of gestogens (which are already present in the perinatal blood in massive amounts from placental origin). Suppression of the maintenance of lactation by oestrogens was, however, found in 25 reported studies after the post-partum period (8). The suppression is more substantial after several months of combined pills(9). There is, however, a need for further research, especially in developing countries to clarify the effect of oestrogens on lactation. There is also some argument about the size of women in relation to drug dosage.
4.12. There is much circumstantial evidence to support the view that prolactin continues to be of major importance for the maintenance of lactation. Del Pozo has demonstrated that if bromocryptine is administered, which totally inhibits prolactin secretion, milk production drops to zero within a few days even if mothers continue to use a breast pump (10). In well-nourished mothers it is known that basal prolactin concentrations fall rapidly after parturition, reaching normal values at two to three months(4, 11, 12). Thus, it would appear that only short bursts of prolactin secretion are necessary after each feed for the maintenance of lactation.
In undernourished mothers, however, high basal levels are maintained for periods up to two years(13, 14). Why this is so is not known, but it is reasonable to suppose that it is related to the need for frequent suckling, which appears to be the only way the baby can ensure for itself an adequate supply of milk. Dietary supplementation has definitely been shown to result in a significantly faster fall in basal plasma concentrations(14). The data in figure 15 (see FIG. 15. Plasma-Prolactin Concentrations in Lactating Mothers in the United Kingdom and the Gambia. Values given as Mean SEM. Number of Observations in Each Group was: UK 37; Gambia Wet Season 128, Dry Season 109, Dry Season with Supplement 37 (Source: ref. 14)) are from the Gambia. Even though total intake of milk by these children was not greater (section 3.26), and the frequency of feeding was apparently unaffected, it has been postulated that the child could satisfy his needs with much greater ease.
The role of insulin and corticosteroids should also receive attention, as these hormones are as important as prolactin in the initiation and maintenance of lactation (15).
4.13. Milk ejection is also an important variable in human lactation, especially under the particularly stressful social circumstances that can prevail in industrialized countries. Only small quantities of milk in the breast are available to the baby if no oxytocin is released during the initial period of suckling 14). A vicious downward spiral can result if a mother is upset and does not release oxytocin: the baby quite naturally becomes agitated because of a lack of milk, and equally naturally the mother becomes more and more upset, which can have the effect of reinforcing the inhibition of oxytocin release. Exactly why the oxytocin-releasing reflex is so sensitive in many women living in industrialized communities is not known. Some encouraging results have been obtained in mothers resorting to nasal oxytocin sprays. Quite obviously this is an area that merits considerably more research.
