3.4 Milk composition
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![]() Key Points - Case Study CJoanne is discharged early on day three and comes to see you for a follow-up on day five. The day following discharge her breasts became painful and hard and Rosie is again having difficulty latching on to the breast. Is this normal? What would you suggest Joanne do?
You see Joanne and Rosie again at 10 days. Joanne's breasts have settled and Rosie is feeding well. However, she has been feeding frequently at night and Joanne's husband, Tom, has suggested bringing the baby to bed with them. Joanne has heard that this increases the risk of SIDS. What would you tell her?
Joanne had been on the combined oral contraceptive pill before becoming pregnant. She intends to not use any contraception until she is six months postpartum. Is this a realistic option?
Joanne brings Rosie for immunization at four months. She is concerned about the composition of her milk. Although Rosie had been growing along the 50th percentile, her weight gain has started to slow.
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Joanne brings Rosie for immunization at four months. She is concerned about the composition of her milk. Although Rosie had been growing along the 50th percentile, her weight gain has started to slow.
Breastmilk is a complete food and drink for infants until six months of age. The composition of breastmilk includes carbohydrates, proteins, fats, vitamins and minerals in unique combinations and concentrations ideally suited to the developing human infants.
Breastmilk Composition
Carbohydrates
- Lactose
- Breastmilk contains the highest lactose concentration of any mammalian milk (approximately 68g/L), providing up to 40 percent of the infant's energy requirements1. Lactose is synthesised in the breast and is one of the most stable components of breastmilk, being independent of maternal diet. The osmotic effect of lactose in the alveoli is the main determinant of breastmilk volume2. As well as being broken down to galactose and glucose by lactase found in the brush borders of the small intestine, lactose also enhances calcium absorption. Galactose and glucose are absorbed and provide energy for the infant, especially the rapidly developing brain. Galactose is also used to produce galactolipids – important in the development of the central nervous system (3,2).
- Oligosaccharides, glycoproteins and glycolipids
- These molecules encourage the growth of non-pathogenic bacteria such as Lactobacillus bifidus in the infant's gut and provide the basis for important components of the central nervous system3. Oligosaccharides also appear to provide alternate binding sites for pathogenic bacteria and reduce the adherence of bacteria to epithelial cells. Some oligosaccharides are absorbed and excreted in the urine and may be the mechanism for the reduced incidence of urinary tract infections in breastfed infants1.
Protein and non-protein nitrogen
- Non-protein nitrogen
- This comprises up to 25 percent of the nitrogen found in breastmilk and consists of more than 200 nitrogenous compounds, including free amino acids, nucleotides, carnitine and urea – some of which appear to be essential for metabolism and growth. Nucleotides also stimulate the growth of Lactobacillus bifidus and inhibit the growth of pathogenic bacteria in the gut (1,4).
- Protein
-
Protein levels in breastmilk are approximately 9g/L, the lowest level among mammalian species, reflected in the human infant's slow growth. Casein contains a number of peptides that bind calcium, phosphorus and magnesium and are insoluble in the low pH of the stomach, forming a curd. Casein enables the delivery of calcium and phosphorus as well as other amino acids to the infant (3,4). In contrast, the whey portion of the milk contains the following soluble proteins:
- α-lactalbumin, the most common whey protein, appears to have a mainly nutritional role, providing amino acids necessary for growth and development of the infant, although it also has anti-infective properties. α-lactalbumin is also involved in lactose synthesis in the breast4.
- Immunoglobulins (especially IgA), lactoferrin and lysozyme are bacteriostatic and bactericidal to many bacteria, protozoa, viruses and fungi. Lactoferrin binds iron in milk and enhances iron absorption by the infant.
- Carrier proteins for vitamins and hormones – for example Vitamin B12 binding protein and corticosteroid-binding protein.
- Enzymes such as bile-salt stimulated lipase and amylase.
- Other active components such as epidermal growth factor, insulin, insulin-like-growth factor and relaxin5.
Fat
The fat content of human milk is approximately 40 g/L6 but varies (35 and 45 g/L) between women, between breastfeeds for the same woman and within a breastfeed1. Fats provide between 45 and 55 percent of the total energy needs of the infant as well as providing essential fatty acids and a mechanism for the delivery of fat-soluble vitamins1. The majority of fats (9899%) are in the form of triglycerides – 85 percent of which are medium and long chain fatty acids. These include essential fatty acids linoleic and alpha-linolenic acid as well as the omega-3 and omega-6 LCPUFAs arachindonic acid and docosahexaenoic acids, important for the development of the nervous system and membrane structure. Small amounts of phospholipids (integral for myelinization and development of the retina) and cholesterol are also present (1,6).
Medium chain fatty acids are synthesised in the breast while other fats are primarily derived from dietary sources. When the maternal diet is unable to provide the fats needed, long chain fatty acids are derived from the breakdown of maternal fat stores. Therefore the composition of fat in human milk, but not the concentration, will vary depending on maternal diet. Cholesterol levels are not affected by the maternal diet or blood levels6.
Fat concentration in breastmilk rises throughout the feed. However fat content is closely related to the degree of breast fullness with fat concentration being lowest when the breasts are full, and highest when the breasts are well drained. A milk sample with a fat content of more than 10 percent indicates that the breast is well drained7. For a woman with a large milk storage capacity whose infant does not drain the breast at each feed, the fat concentration at the beginning of one feed may be higher than the fat concentration at the end of another feed8. As a reflection of breast drainage, fat content is higher in the day and evening than in the morning and night8.

© J.Kent, Biochemistry & Molecular Biology, UWA
Breastmilk contains bile-salt stimulated lipase that assists lingula, gastric and pancreatic lipase to digest milk fats1.
Vitamins
Women with adequate vitamin intake and stores produce milk that is also has adequate vitamin concentrations. Increasing vitamin intake in these women does not necessarily increase milk vitamin concentrations. However, if the maternal vitamin status is low, milk vitamin concentrations also tend to be low and the infant may become deficient. Maternal supplementation in most of these situations rapidly returns milk concentrations to normal. Two vitamins require special mention.
- Vitamin D
- Vitamin D levels in breastmilk are not adequate to completely supply the needs of the infant, with sunlight being the natural source of Vitamin D for infants and their mothers. When direct sun exposure is restricted (e.g. climatic conditions, cultural clothing or to protect skin from solar damage) or skin colour reduces the effect of sun exposure, mothers and their infants may become Vitamin D deficient. Those most at risk of Vitamin D deficiency are dark-skinned women, women who are veiled and those who live in high latitudes where sun exposure, especially in winter, is limited. Some countries recommend supplementing all breastfed infants with Vitamin D9 while others recommend women at risk be monitored and their infants supplemented when necessary10.
- Vitamin B12
- Women with pernicious anaemia, on a strict vegan diet that is deficient in Vitamin B12 or with other causes of Vitamin B12 deficiency will produce milk that is also deficient in the vitamin. The infants may develop irreversible neurological sequelae if not diagnosed and supplemented11.
Minerals
In contrast to vitamins, mineral levels in milk are usually independent of maternal intake. Binding proteins and other transfer factors present in breastmilk enhance the absorption by the infant of minerals and other trace elements so that even apparent low concentrations of specific minerals have a high bioavailability. In contrast, infant formulas often have high mineral concentrations (up to 100 times that of breastmilk) to compensate for their low bioavailability12.
- Calcium
- Calcium levels in breastmilk are 2530 g/L and appear to be independent of maternal dietary calcium intake (12,13). Lactose and citrate in breastmilk aid calcium absorption. During the early months of lactation, there is a reduction in maternal bone mineralisation, particularly of the trabecular bones such as the spine and hip, although this is reversed later in lactation and after breastfeeding ceases. This demineralisation is not related to maternal calcium intake – calcium supplementation does not prevent or reduce the effect14. There is no evidence that breastfeeding has a detrimental effect on long-term bone mineralisation or is a risk factor for the development of osteoporosis. Women who become pregnant during or soon after lactation remineralise their bones in early pregnancy13.
- Iron
- Breastmilk appears to have very low iron levels (0.2-0.4mg/L), but its bioavailability is high so that term infants, who have laid down iron stores before birth, do not require additional sources of iron until around six months of age. Breastmilk concentrations appear to be independent of maternal iron status12. Lactoferrin binds iron in breastmilk and is thought to play a role in its absorption although other factors are also relevant. Vitamin C is known to increase iron absorption12. Phylates, present in cereals, interfere with the absorption of iron. As infant cereals (often supplemented with iron) are often the first weaning foods, this may be a reason why breastfed infants given solids before six months are more likely to be iron deficient at nine months than infants exclusively breastfed for at least six months12. Iron supplementation of infants who are not iron deficient appears to increase episodes of diarrhoea15.
- Iodine
- Iodine concentration in breastmilk varies depending on maternal iodine status. Although iodine is concentrated in breastmilk, in areas where iodine deficiency is prevalent and goitre formation common, iodine deficiency in breastfed infants can occur16. The use of iodised salt and other iodine supplements by the mother, where appropriate, ensures maternal iodine sufficiency and adequate iodine intake for the infant17.
Changes in milk composition
- Over time
- The concentration of various components of breastmilk changes over the course of lactation. Compared with mature milk (outlined previously), colostrum has higher concentrations of protein (especially immunoglobulins), sodium and chloride and lower concentrations of lactose, potassium, calcium and citrate7. Many of these differences are due to the closure of the tight junctions between the lactocytes with the onset of lactogenesis II. Lactose levels remain relatively stable over the course of lactation. However, fat content falls during the first four months of lactation, but rises again until 12 months while protein concentration decreases until six months and then remains stable18. While most micronutrient levels stay constant over time, there is a gradual fall in zinc, copper, Vitamin A and potassium levels. None of these variations reduces the overall nutritional value of breastmilk consumed by the infant – infant growth is related to milk volume consumed and not the concentration of particular macronutrients7. During weaning, breastmilk returns to a more colostral composition with higher concentrations of immunoglobulins, sodium and chloride19.
- Length of gestation
- The milk produced by mothers who have delivered prematurely differs from milk produced by mothers delivered at term. In particular, there are higher concentrations of protein, mainly IgA and lactoferrin, as well has increased levels of calcium, phosphorus and zinc3. While fat levels are similar, there is a higher proportion of medium chain fatty acids in pre-term milk6.
Milk yield
Milk production remains at a similar level from the end of the first month until the sixth month of lactation. Although the amount of milk needed for adequate growth varies considerably (4401220 ml/day) most women will produce between 710 and 803 mls/day8. Volume of milk produced at the end of the first week postpartum is predictive of milk production at six weeks20. From six months, with the addition of solid foods to the infant's diet, milk production gradually falls, although many women still produce over 500mls of breastmilk a day at 12 months21.
Notes
- # Picciano MF (2001) Nutrient Composition of Human Milk
- # Czank C et al. (2007) Human milk composition - carbohydrates
- # Lawrence RA et al. (2005) Breastfeeding: a guide for the medical profession
- # Czank C et al. Human milk composition - nitrogen and energy content
- # Walker M (1993) A fresh look at the risks of artificial infant feeding
- # Czank C et al. (2007) Human milk composition - fat
- # Kent JC (2007) How breastfeeding works
- # Kent JC et al. (2006) Volume and frequency of breastfeedings and fat content of breast milk throughout the day
- # AAP Policy Statement, Section on Breastfeeding (2005) Breastfeeding and the Use of Human Milk
- # Munns C et al. (2006) Prevention and treatment of infant and childhood vitamin D deficiency in Australia and New Zealand: a consensus statement
- # Rasmussen SA et al. (2001) Vitamin B12 deficiency in children and adolescents
- # Lonnerdal B (1997) Effects of milk and milk components on calcium, magnesium, and trace element absorption during infancy
- # Prentice A (2003) Micronutrients and the bone mineral content of the mother, fetus and newborn
- # Kovacs CS (2005) Calcium and bone metabolism during pregnancy and lactation
- # Allen LH (2005) Multiple micronutrients in pregnancy and lactation: an overview
- # Semba RD et al. (2001) Iodine in human milk: perspectives for infant health
- # Zimmermann MB (2007) The impact of iodised salt or iodine supplements on iodine status during pregnancy, lactation and infancy
- # Mitoulas LR et al. (2002) Variations in fat, lactose and protein in human milk over 24 h and throughout the first year of life
- # Hartmann PE et al. (1985) Variations in the yield and composition of human milk
- # Hill PD et al. (2005) Comparison of milk output between mothers of preterm and term infants: the first 6 weeks after birth
- # Kent JC et al. (1999) Breast volume and milk production during extended lactation in women