NUTRITIONAL ASPECTS OF BREAST MILK
Human milk is not a constant substancebecausecolostrum differs from mature milk and the milk of the early puerperium differs from the milk of late lactation. Indeed, the content of milk varies at differing stages of the same feed. Nevertheless, the approximate concentrations of human milk and cow's milk show substantial differences (Table 10.6) with human milk having less protein but more fat and lactose. A number of specific components also differ between human milk and formulae, such as the long-chain polyunsaturated fatty acids, which have important neurodevelopmental consequences for the baby . There is no doubt that breast milk is the ideal nutrition for the human baby.
One of the most important secondary functions of breastfeeding is to protect the infant against infection. This is particularly important in developing countries where it has been estimated that in each year there are 500 million cases of diarrhoea in infants and children and about 20 million of these are fatal. The extent to which breastfeeding protects against infection in infants in developed countries, however, has been a matter of dispute. In a study from Dundee, Scotland, it was found that babies who had been breastfed for at least 3 months had greatly reduced incidences of vomiting and diarrhoea compared with babies who were either bottlefed from birth or completely weaned within a short time of delivery . This study also found that the protection against gastrointestinal illness in breastfed babies persisted beyond the period of breastfeeding itself and, in the developed country setting at least, was not undermined by the early introduction of at least some supplements. There was a smaller protection against respiratory tract infections but not against other illnesses.
Anumber of mechanisms contribute to the anti-infective properties of breast milk. Breast milk contains lactofer-rin which binds iron, and because E. coli requires iron for growth, the multiplication of this organism is inhibited. Breastfeeding also encourages colonization of the gut by non-pathogenic flora which will competitively inhibit pathogenic strains. In addition, there are bacteriocidal enzymes, such as lysozyme, present in breast milk, which will contribute to its protective effect.
The most specific anti-infective mechanism, however, is an immunological one. If a mother ingests a pathogen which she has previously encountered, the gut-associated lymphoid tissue situated in the Peyer's patches of the small intestine will respond by producing specific immunoglobulin A, which is transferred to the breast milk via the thoracic duct (Fig. 10.1). This immunoglob-ulin, which is present in large amounts in breast milk, is not absorbed from the infant's gastrointestinal tract but remains in the gut to attach to the specific offending pathogen against which it is directed. In this way the breastfed infant is given protection from the endemic infections in the environment against which the mother will already have immunity. Breast milk contains living cells, such as polymorphs, lymphocytes and plasma cells and although their functions are not yet fully understood they may also be active against invading pathogens.
A number of studies have shown positive associations between breastfeeding and improved childhood cognitive functions, such as increased intelligence quotient, which persist even after allowing for potential confounding variables. For example, one study found that, at 2 years of age, babies who had been breastfed for more than 4 months had a 9.1 point advantagein theBayley score. Other studies have shown similar but smaller benefits and preterm babies also have improved neurological development if exposed to breast milk [20,21].
The mechanism for the improved neurological development is not fully understood but the presence of long-chain rn-3 fatty acids in breast milk, particularly docosohexanoic acid, may beimportant; thecomposition of theinfant brain is sensitive to dietary intake but the relationship between the biochemical composition of brain lipid and cognitive function is not yet known. Nevertheless, the possible
Fig. 10.1 Pathways involved in the secretion of immunoglobulin A in breast milk by the enteromammary circulation. (Courtesy of Professor R.V. Short, Melbourne, Australia.)
beneficial effect of breastfeeding on cognitive function is a topic of great potential importance.
There are a number of reports that show lower incidences of atopic illness such as eczema and asthma in breastfed babies. This effect is particularly important when there is a family history of atopic illnesses . When the atopic illness is present, it is commonly associated with raised levels of immunoglobulin E, especially cow's milk protein.
Oddy et al.  suggest that, apart from a positive family history, the most important predisposing factor for atopic illness is the early introduction of weaning foods. The protective effect of breastfeeding against atopic illness, therefore, may be secondary, rather than primary, because breastfeeding mothers tend to introduce supplements at a later stage. Nevertheless, mothers with a family history of atopic illness should be informed of the advantages of breastfeeding and of the dangers of introducing supplements too quickly.
Breastfeeding may be associated with reduced juvenile-onset diabetes mellitus  and neoplastic disease in childhood . It is possible that some of these benefits are related to the avoidance of cow's milk during early life rather than to breastfeeding per se, for example, it is possible that early exposure to bovine serum albumin could trigger an autoimmune process leading to juvenile-onset diabetes. Breast milk is a particularly important ingredient in the diet of preterm infants as it appears to help in the prevention of necrotizing enterocolitis among these particularly vulnerable babies.
There is an epidemic of breast cancer among women of developed countries in the Western world. A number of recent studies have shown a reduced risk of breast cancer among women who have breastfed their babies . Because breastfeeding appears to have no effect on the incidence of postmenopausal breast cancer, its overall protective effect will be relatively small but the protection offered by lactation still represents an important advantage against a much feared and common disease.
The natural contraceptive effect of breastfeeding has received scant attention in the Western world because it is not a reliable method of family planning in all cases. Nevertheless, on a population basis, the antifertility effect of breastfeeding is large and of major importance in the developing world. It has to be remembered that the majority of women in the developing world do not use artificial contraception and rely on natural checks to their fertility. By far the most important of these natural checks is the inhibition of fertility by breastfeeding. In many developing countries mothers breastfeed for 2 years or more, with the effect that their babies are spaced at about 3-yearly intervals. In the developing world, more pregnancies are still prevented by breastfeeding than by all other methods of family planning combined. The current decline in breastfeeding in the developing world is a cause for great concern because, without a sharp rise in contraceptive usage, the loss of its antifertility effect will aggravate the population increase in these countries.
Artificially fed children have twice the risk of childhood obesity in comparison to breastfed children . Breastfed children have a significantly reduced blood pressure . These children have a significantly reduced chance of being obese as adults and dying prematurely from cardiovascular disease.
The mechanisms of lactational amenorrhoea are complex and incompletely understood. The key event is a suckling-induced change in the hypothalamic sensitivity to the feedback effects of ovarian steroids. During lactation, the hypothalamus becomes more sensitive to the negative feedback effects and less sensitive to the positive feedback effects of oestrogen. This means that if the pituitary secretes enough follicle-stimulating hormone and luteiniz-ing hormone to initiate the development of an ovarian follicle, the consequent oestrogen secretion will inhibit gonadotrophin production and the follicle will fail to mature. During lactation there is inhibition of the normal pulsatile release of luteinizing hormone from the anterior pituitary gland which is consistent with this explanation.
From a clinical standpoint, the major factor is the frequency and duration of the suckling stimulus although other factors such as maternal weight and diet may be important confounding factors. If supplementary food is introduced rapidly at an early stage, the suckling stimulus will fall and early ovulation and a return to fertility will be the consequence.
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