Title: Human Breast Milk
Key words: Breastfeeding, nutritional requirements, foetus, infant, neural development, calcium, Vitamin D
Date: May 1999
Category: 10. Reproduction
Author: Dr van Rhijn
Human Breast Milk
The Best Food For Human Babies For The First 4 6 Months Of Life
The rapidly growing and developing infant has high nutrient and energy requirements during the first five months of life. Breast milk is recognised as the best source of nourishment for babies during this phase and this is endorsed by a COMA report1. The evidence for this will be discussed, as currently there is an undesirably low rate of breastfeeding. Trends reveal that of the 63% of mothers who did actually start to breast-feed, 39% had already stopped after 6 weeks2.
The rate of growth is modulated by more than a dozen hormones, including insulin, insulin-like growth factor1, thyroxine, cortisol, pituitary and growth hormone. An adequate quantity of suitable milk is estimated at approximately 120 ml/kg body weight, providing the required average energy demand of 110 kcal/kg/day at one month and 85 kcal/kg/day at 6 months. The growing infant demands relatively more protein, vitamins (A, B2, Folate, C) and calcium than an adult.
Neural and vascular development in the foetus and neonate is actually dependent on an adequate supply of long chain polyunsaturated fatty acids (LC-PUFAs), especially arachidonic acid (20:4 n-6 AA) and docosahexaenoic acid (22:6 n-3 DHA) 3.
Preterm infants however have special nutritional needs due to their comparatively low body stores of fat, minerals and vitamins4. They grow better on an energy and micronutrient-enriched preterm formula5 and have higher IQ scores at 8 years of age 6, 7. Although mature human milk meets the needs of the growing infant during the first 4 months, the requirements exceed the supply eventually. Lactating mothers should receive supplementary calcium and Vit D to ensure adequate levels in their breast milk. Additional dietary sources of energy, protein, vitamins A & D, absorbable iron and zinc are usually needed by the age of six months.
Breast-feeding confers numerous advantages on the infant and also the mother. Human milk resembles living tissue such as blood and contains live cells and a wide range of biologically active components. Its composition is not uniform and changes during the course of lactation, even throughout a single feed as well as through diurnal variation. It is very dependent on the nutritional status of the mother8. Lactation, however, is a robust process and most lactating women produce relatively the same output of milk, typically 700 800 g/day9. The bioavailability of appropriate nutrients from breast milk is much more efficient than from formula milks. Dietary Reference Values (DRVs) for infants set by the Department of Health are based on estimates from data from formula-fed infants and have no value in practice for breast-fed infants10. Research concluded that human milk contains all the essential micro- and macronutrients, balanced in the proportions required for the healthy growth and well being of the infant. In fact, solids given with breast milk can actually reduce the absorption of nutrients in the milk and be microbiologically hazardous.
Breast Milk Composition
The principle carbohydrate is lactose, which provides approximately 39% (cows milk = 27%) of the energy required. Fat (4g/100ml 44% saturated) provides approximately 50% of the energy and is more readily digested than the fat in cows milk. Studies of doubly labelled water11 showed that breast milk provides 242kJ (58kcal - 65 kcal 12) per 100 ml 3 months postpartum at an energy cost of about 2.5MJ/day13. This is sufficient to cover the dietary needs of an infant growing along the 50th centile until at least 4 months14, and 6 months in developing countries. Some researchers, however, do question whether it is sufficient to meet the demands of a male infant beyond 3 months of age.
Human milk provides the biological requirement of proteins15 (0.8 0.9 g/100ml), consisting of whey protein (60% compared to 20% in cows milk) and casein (40% compared to 80% in cows milk). It also contains free amino acids, small peptides, amino sugars, creatine, creatinine, glycolipids and high concentrations of sulphurised taurine. The biochemical role of all these substances is not fully understood. Human milk is a rich source of cholesterol and fatty acids (39% monounsaturated & 12% polyunsaturated). The composition is influenced to a certain extent by the mothers diet, although the amount of LC-PUFAs, (AA and DHA) is relatively constant. There is evidence that infants are unable to synthesis DHA in sufficient quantities and this should be considered as an essential fatty acid for this delicate age group16.
Post mortem studies on brain phospholipids confirmed that breast-fed infants have higher concentrations of DHA than formula fed infants17. Deficiencies, found in non-fortified formula milks, may result in impaired neuro-development18, poor visual acuity19,20 and possible cot death21. Breast-feeding may confer prospective long-term benefits on neurological development22, visual function23 and cognitive behaviour24.
Human milk provides sufficient amounts of vitamin C (30 mg/day) and Vit A (300mcg/day), but may not be sufficient in Vit D (6mcg/day) as the aqueous fraction, which may be less biologically active than the fat-soluble form25. There is a wide safety margin for calcium to ensure adequate mineralisation of the skeleton, but it is noticeably low in iron (76mcg/100ml). This is compensated for by a 50 % better absorption rate (bound as lactoferrin) than from cows milk. Supplementation of iron26 and Vit D may be required as mentioned above. Haemorrhagic disease is more common in breastfed infants, which implies that human milk is an inadequate source of Vit K27.
The colostrum, produced during the first 10 days of breastfeeding, is unique in its content of proteins, 50% derived from immunoglobulins28, less fat but higher amounts of vitamins A and B12.
Three components, secretory IgA (1g/day), lactoferrin and lysosome, are considered to act synergistically to enhance the infants anti-infection defences against a wide range of bacterial, viral, fungal, parasitic and food antigens. Breast milk also provides microbial antigens, cytokines, lactoperoxidase, leucocytes, lymphocytes (T and B) and macrophages. There is clinical evidence that breast-feeding protects29 against gastro-intestinal30 and respiratory infections31, 32.
Further Advantages of Breast-feeding
There is evidence, albeit inconsistent, that breast milk may protect against the risk of IDDM33, 34 as well as allergies35. Breast milk is cheap, at the correct temperature and convenient, enabling infants to regulate their own food supply. Breast fed preterm infants may be more physiologically stable (oxygenation saturation) compared to bottle fed infants due to dissimilar patterns of sucking and breathing36, 37. It aids involution of the uterus, promotes loss of surplus weight from the mother and acts as a contraceptive. Very importantly, it also promotes bonding between mother and infant.
Caution and proper advice is warranted because of the risk of possible contaminants in breast milk, ranging from hormones, drugs, alcohol, nicotine and pesticides, especially if lipid soluble, being released during lactation. Atopic mothers should also exclude potent allergens (dairy products, eggs) to minimise sensitisation in the infant.
Breast milk provides the best nutrition for the developing infant but its nutritional value is dependent upon the diet and lifestyle of the mother, which should be optimal from the day of conception. In order to encourage women to breast-feed, adequate advice, support and encouragement need to be provided to mothers throughout pregnancy. Various schemes have been initiated to promote the importance of breast-feeding38.