Where a mother is unable to breastfeed, or chooses not to, infant formula milks based on modified cows’ milk are the only recommended alternatives. The Codex Alimentarius Commission, the highest international body on food standards, recognises that infant formula is the only nutritionally complete substitute for breastmilk²_. While infant formula cannot reproduce all the nutritional qualities of breastmilk, it is a substantial improvement over traditional substitutes, such as cows’ milk. Cows’ milk is unsuitable as a main drink for infants under one year of age as it does not meet the infants’ nutritional requirements. The protein content in cows’ milk is too high and can put strain on an infant’s kidneys. Cows’ milk is also a poor source of vitamin D, needed for calcium absorption and bone development, and readily absorbable iron. In addition, cows’ milk provides insufficient amounts of calcium, magnesium, copper and vitamin C compared with breastmilk³. So for infants who are not being breastfed, infant formula is the sole source of nutrition during the critical periods of growth and development, and must meet very high quality and nutritional standards. However infant formula milks are evolving as the focus on particular functions, ingredients and components of human milk shift, and so this article aims to provide Healthcare Professionals with a reference guide to advise mothers about infant feeds. Breastmilk is a living, biological fluid with a unique composition, and scientists are continually working to identify and isolate the components of breastmilk and understand their specific functions. Breastmilk composition varies from mother to mother, from day to day, during the day and even during a feed (fore milk to hind milk). Differences are also noted between stages of lactation (colostrum, transitional and mature milk), tailored to the infants needs. Colostrum, produced from the first to the fifth day of lactation, is rich in many nutrients, particularly proteins. Transitional milk contains increasingly higher concentrations of fats and the milk sugar, lactose, until stabilising by about the fifteenth day as mature milk⁴. The breastmilk of mothers who deliver their infants prematurely contains higher concentrations of fats and protein and lower concentrations of lactose than the milk of women who deliver at term. Although concentrations of proteins, lactose, and other water-soluble components tend to remain similar throughout a single feeding, fat concentrations increase during an individual feeding⁵. The first is milk drawn at the beginning of a feed which is fore milk, a thin and watery milk which helps to quench the infant`s thirst. This is followed by the hind milk, which is much more creamy and helps to settle the infant.

The nutritional composition of breastmilk is determined in the laboratory, and the technique used depends on the component being studied⁶. Protein measurement can be determined by nitrogen, direct protein, or amino acid analysis. Fat content can be determined by creamatocrit, total lipid assay, or dry column method analysis. Carbohydrate analysis is usually done by 1 of 2 methods: total carbohydrate content or lactose assay. For mineral analysis, milk must be collected in a mineral-free container and be acid washed; then atomic absorption spectroscopy can be used. Vitamin analysis can be accomplished by 1 of 4 acceptable methods such as gas chromatography and High Performance Liquid Chromatography (HPLC). Either direct bomb calorimetry or indirect methods using conversion factors for major nutrients can be used to determine total energy content of milk. Although we do not know all the components of breastmilk, and possibly never will, scientists have been able to isolate over 80 specific components and determine their functions. Infant formula milks are continually being modified and improved as research improves our understanding of breastmilk composition.

The components of breastmilk
Breastmilk is about 87.5% water, with the remaining 12.5% containing carbohydrates, fats, proteins and mineral ash.

Carbohydrate
The carbohydrates in breastmilk are the sugar lactose and oligosaccharides. Lactose is important for the development of the central nervous system, as it is used in the tissues of the brain and spinal cord, and also provides energy for growth and development. Oligosaccharides are a complex mix of indigestible compounds and are important components of human milk. Some oligosaccharides have a prebiotic function and are referred to as “prebiotic oligosaccharides (OS)”. These play an important role in intestinal health. The intestine is one of the first lines of natural defence; in fact 2/3rds of the natural immune system is in the gut⁷. The colonisation of the infants gut with good bacteria promotes the development of the immune system. Good bacteria, such as bifidobacteria and lactobacilli, are important in preventing potentially harmful bacteria attaching to the gut lining and causing infections⁸. These friendly bacteria also stimulate the immune system to respond appropriately to any foreign bodies. Prebiotic OS stimulate the growth of friendly bacteria and increase their numbers in the infants gut. Breastmilk is a natural source of prebiotic OS, containing up to 1.5g/100ml⁹. Some infant formula milks now contain added prebiotic OS in concentrations similar to those found in breastmilk in order to increase the amount of friendly bacteria in formula fed infants. Research has found significant reductions in harmful bacteria¹⁰ and significant increases in bifidobacteria¹¹ in infants fed formula milks with added prebiotic OS, as well as a reduction in diarrhoeal disease^12,13. Formula-fed infants given a formula containing prebiotic OS had fewer infections requiring antibiotics¹⁴, and a reduction in antibodies to cows’ milk proteins¹⁵, which may have implications for the development of cows’ milk protein allergy.

Protein
Proteins are fundamental components of all living cells. Enzymes, hormones and antibodies are all proteins. The building blocks of protein are amino acids, which are used by the body to build new proteins for growth and development, immune system development, digestion and metabolism, and repair of body tissue¹⁶. Non-essential amino acids can be synthesised by the body, whereas essential amino acids must be obtained through dietary sources. The body needs a supply of all the amino acids to build the required proteins. Adults can eat a wide range of foods to ensure all amino acids are represented in the diet, however until infants are weaned onto solid foods, they rely on breastmilk or formula milk to meet their nutritional needs. The main proteins in breastmilk are whey and casein, however the ratio of whey to casein is higher in colostrum (80:20) and mature breastmilk (60:40) compared to cows’ milk (20:80). Formula milks based on modified cows’ milk had whey to casein ratios of 20:80 until the 1970s, when formula milk companies began to optimise protein profiles and developed milks that contain more whey (whey-dominant), and are therefore closer to the protein composition of breastmilk. Whey forms a soft curd in the stomach that is easily digested and reduces the gastric emptying time, whereas casein forms a firmer curd and takes longer to digest¹⁷, meaning the stomach feels fuller for longer. The whey component in breastmilk contains important proteins as such as lactoferrin, lysozyme, alpha-lactalbumin and immunoglobulins. Lactoferrin and lysozyme are important proteins in breastmilk. Many harmful bacteria require iron for growth, and lactoferrin inhibits bacteria by uptaking the iron. Lactoferrin has a direct antibiotic effect on bacteria such as staphylococci and E. coli. Lysozyme is an enzyme that inhibits bacteria by disrupting their cell walls. Both these proteins provide important protection against infections during infancy, while the immune system is developing. Alpha-lactalbumin promotes milk formation and helps provide the appropriate balance of essential amino acids in human milk. Immunoglobulins are antibodies, used by the immune system to identify and neutralise bacteria and viruses. The five main immunoglobulins are IgA, IgD IgE, IgG and IgM. Immunoglobulins are a major factor in the gastrointestinal protection system. The mother produces immunoglobulins when she comes in contact with a disease-causing agent, and passes antibodies onto the infant through breastmilk. Therefore the infant receives protection against the infectious agents that it is most likely to encounter in the first weeks of life while the immune system develops.

Fat
Infants’ energy needs are much greater than adults relative to their body weight, due to the high rate of growth in early life. Fats are a rich source of energy, and supply 40-50% of the total calories in human milk. The fat content of breastmilk changes constantly. Typically, the foremilk is lower in fat than the hind milk, and is thought to quench thirst. The rich hind milk that follows is thought to aid feelings of satiety in infants. Fatty acids are the building blocks of fats and over 200 fatty acids have been identified in human milk. Breastmilk contains equal amounts of saturated and unsaturated fatty acids. Unsaturated fatty acids can also be classified into two types, n-6 (omega 6) or n-3 (omega 3) fatty acids, based on their molecular structure. They include the essential fatty acids linoleic acid (LA) and α-linolenic acid (ALA), which are converted to Long Chain Polyunsaturated Fatty Acids (LCPs)¹⁸. The two main LCPs are Docosahexanoic acid (DHA) and Arachidonic acid (AA). These LCPs are major fatty acids in brain membranes, and are linked to brain development¹⁹. They are also thought to be responsible for improved vision, due to their high concentrations in the retina²⁰. Infants are unable to synthesise sufficient amounts of DHA and AA to meet their high requirements until after 16 weeks of age²¹, and so infants must rely on milk. Maternal diet is known to influence levels of DHA and AA in breastmilk. Fish-eating populations have higher breastmilk DHA concentrations than do populations that do not consume marine foods²². Many infant formula milks are enriched with LCPs, and studies have shown that infants fed LCP-enriched formulas have improved visual acuity²⁰ and cognitive development¹⁹ compared to those fed standard formulas.

Fat-soluble vitamins
The fat-soluble vitamins are vitamins A, D, E and K, and all are added to infant formula milks in appropriate quantities to meet the infant’s needs. Vitamin A is essential for normal growth and development, healthy skin and eyes, and also in immune function. Deficiency in infancy has been linked to poor growth and increased susceptibility to infection. Vitamin E is important in metabolism, and especially LCP function. Breastmilk is an excellent source of vitamin A and its precursor beta-carotene, also of vitamin E and, when consumed in recommended amounts, satisfies a full-term, healthy infant`s vitamin A and E requirements. Vitamin D is essential for calcium absorption, and deficiency can result in poor bone growth and rickets. The amount of vitamin D in breastmilk is usually low, and is influenced by mothers’ vitamin D status and intake. Exposure to sunlight increases vitamin D synthesis in the skin, however breastfeeding women and children under 4 are advised to take vitamin D supplements during periods when sunlight is limited²³.Vitamin K is important for maintaining the normal blood clotting function of the body. It is normally produced in sufficient quantities by bacteria in the intestines; newborns are susceptible to deficiency before bacterial colonisation of their intestines augments their low stores of vitamin K at birth. Newborns are routinely given vitamin K orally or intramuscularly in order to prevent haemorrhagic disease of the newborn. As a fat-soluble vitamin, Vitamin K is found mainly in colostrum and hind breastmilk.

Water-soluble vitamins
Water-soluble vitamins are not stored in the body, and so the diet must provide adequate amounts. The B vitamins are important in energy metabolism and expenditure. Vitamin B deficiency in infancy can lead to growth retardation, nerve damage and problems with skin, eyes and heart. Vitamin C is necessary for normal growth and development. It is needed to form collagen, a protein used to make skin, scar tissue, tendons, ligaments and blood vessels. Vitamin C is also important for the absorption of iron, and therefore the prevention of iron deficiency anaemia. The water-soluble vitamins are well provided in breastmilk, however their concentration can depend on the mother’s diet. Vitamin B6 is essential for early development of the central nervous system, and low breastmilk concentrations of this nutrient may not meet the infants required intake²⁴. Vegetarian and vegan mothers should be especially careful, as the major sources of vitamin B12 are animal foods, and following a strict vegetarian diet can lead to vitamin B12 deficiency. In breastfed infants, this has been shown to cause severe haematologic, metabolic, and neurologic abnormalities²⁵. Breastfeeding mothers should be advised to eat a balanced diet in order to maintain adequate levels of the water-soluble vitamins in their breastmilk. Formula milks have these important nutrients added at suitable levels to meet infant requirements.

Minerals
Calcium is a major nutrient of milk and contributes not only to bone growth but also to muscle contraction, nerve transmission, and blood clotting. Although the calcium content of breastmilk is much lower than that of cows’ milk, much more is absorbed by the breastfed infant. Phosphorus is also essential for bone growth. Breastmilk has a much lower phosphorus content compared to cows’ milk, which is important firstly because the infant’s immature kidneys would be unable to deal with high levels, and secondly, because the 2:1 ratio of calcium to phosphorus enhances calcium absorption. The ratio of calcium to phosphorus in cows’ milk is 1.2:1, which means less calcium is absorbed²⁶. Breastmilk, therefore, provides the right balance of these nutrients in appropriate quantities.

Iron is low in both breastmilk and cows’ milk, however the iron in cows’ milk is bound in a casein complex and is more difficult to absorb compared to the iron in breastmilk. Infant formula milks contain higher levels of iron compared to breastmilk, to compensate for its poorer bioavailability. About 20-50% of iron in breastmilk is absorbed, compared to 4-7% of iron in formula milk²⁷. Iron deficiency in infancy has been linked to poorer cognitive and motor development²⁸, and dietary iron is important in infancy, especially after 4-6 months of age when the infants own iron stores deplete²⁹.

Selenium is an essential micronutrient that functions mainly in association with proteins or as a constituent of them. Selenium is an important antioxidant, providing protection against free radicals. Breastmilk contains higher levels of selenium than formula milks, and human milk is fundamental for the infant’s optimum selenium status. Reports consistently show higher selenium concentrations in breastfed infants and in infants fed formula milk supplements with selenium, compared with infants fed formulas with no added selenium³⁰.

Zinc, found in every cell of the body, is an important constituent of many enzymes, and is essential for growth, development and immune function. The concentration of zinc in breastmilk declines rapidly over the first 3 months of lactation. As with iron, human milk contains less zinc than cows’ milk, but much more is absorbed (higher bioavailability): 60% for breastmilk, 43 to 50% for cows’ milk, and only just 27 to 32% for infant formulas³¹. This lower absorption ratio is offset by higher concentrations in infant formulae.

Nucleotides
Nucleotides are the chemical compounds that form the basis of DNA, and are therefore found in every cell in the body. They play significant roles in many biological processes. There is wide variation in reports of the amount of nucleotides found in breastmilk, depending on methods of analysis and the individual nucleotides measured. Colostrum provides the newborn infant with the highest concentration of nucleotides and during the first four weeks of lactation, the nucleotide concentration falls by about a half. Nucleotide content is lower in cows’ milk, and its composition different to that of breastmilk. Nucleotides are not considered to be essential as they can be produced by the body from simpler components. However at times of rapid growth and development, such as infancy, they are deemed conditionally essential; normal synthesis in the body may not meet requirements, and the body must rely on dietary sources. Nucleotides have therefore been added to some infant formulas in several countries since 1965³², and this has been shown to enhance the gastrointestinal and immune system in formula fed infants³³.

Enzymes
Many enzymes are supplied in breastmilk in far greater amounts than in cows’ milk. Amylase activity, which aids starch digestion, is 60 times greater in colostrum and 40 times greater in mature breastmilk than in cows’ milk. Breastmilk also provides the infant with lipase, the enzyme responsible for breaking fats down for digestion and absorption. Digestion in breast fed infants is also enhanced by many other enzymes that are present in higher levels in breastmilk than in cows’ milk, including transaminase catalase, cholinesterase, lactate dehydrogenase and proteases.

Hormones
Recently, hormones which have a function in appetite regulation and energy metabolism have been identified in breastmilk. Leptin is a hormone which regulates food intake and energy balance, and it is thought to be a factor in the prevention of obesity³⁴. Leptin is present in breastmilk but not in formula milks³⁵, and breastfed infants have higher leptin levels compared to formula-fed infants. The presence of leptin in breastmilk may have a positive effect on satiety and regulation of energy intake, however the long-term consequences of leptin and the role of leptin in promoting later obesity are unknown.

Prostaglandins
Prostaglandins are derived from the essential fatty acids, linoleic and linolenic acids. They affect many physiologic functions, including gastrointestinal secretion and absorption, smooth muscle contraction and local circulation. Breastmilk has been found to contain small amounts of prostaglandins E2 and F2, which are not present in cows’ milk or formula milk³⁶. These prostaglandins may play nutritional, immunologic, or hormonal roles in the newborn infant.

Growth factors
Breastmilk contains several proteins called growth factors which may be important for growth and maturation of the gastrointestinal tract in the neonate³⁷.

Issues and debates
Infant formula is today perhaps the world’s most regulated food. Current UK legislation specifies minimum and maximum levels for the energy, protein, fat, carbohydrate, vitamin and mineral content of formulas. It also specifies which nutritional substances and food additives are approved for infants. The modification of cows’ milk in the manufacture of formula milks began in the 1890s, as an answer to the high rate of infant mortality. Early infant formulas consisted of cows’ milk, sugar, and water, which were dangerously high in protein, provide a high renal solute load, are poorly absorbed, and deficient in many essential nutrients. This resulted in deficiency diseases such as scurvy and iron deficiency anaemia. The understanding of the differences between breast and cows’ milk was well advanced by the beginning of the twentieth century. With increasing nutritional knowledge and improved processing techniques, formula milks that resembled breastmilk in overall composition (fat, carbohydrate and protein) began to be mass-produced and marketed, dedicated to the cause of reducing infant mortality and childhood malnutrition. Use of formula milks rose from the mid-1900s, until the 1970s when the natural childbirth movement and breastfeeding researchers began to highlight the importance of breastfeeding. Manufacturers of formula milk shifted their focus, concentrating less on reproducing the major breastmilk constituents in formula milks, and more towards subtle aspects of its composition, with the aim of reproducing in formula fed infants the functional effects seen in breastfed infants. Various themes in this progress have emerged and many stand to be further refined in the future. Major advances in the past were the addition of nucleotides and LCPs to formula milks, and more recent advances are the health outcomes associated with prebiotic OS and alterations in protein profiles.

A future development in infant formula may be the addition of leptin. Newspaper reports in April 2007 told of how British scientists are working on a leptin-enriched infant formula which would chemically restructure the metabolic system of children to prevent excess weight gain and could halt the rise in childhood obesity³⁸. The researchers say this formula milk is less than 10 years away, however its development has caused controversy and has raised many medical and ethical concerns. Other scientists have stated that obesity is a social problem, requiring lifestyle changes, not an artificial ‘quick fix’, while others have questioned the ability of leptin to be ingested through formula milks, as it is easily destroyed by stomach acids. However the scientists responsible for the research state that supplementing formulas with leptin is simply adding something that is present in breastmilk and absent in formula.

Summary and conclusions
Breastmilk is undoubtedly the best form of nutrition for the first six months of life, and should be encouraged and promoted widely. However many women, especially in the UK, either cannot breastfeed, choose not to breastfeed, breastfeed for short periods of time, or to combine breast and formula milk feeding. Infant formula milks are the only recommended alternative to breastmilk in infants under one year of age, and parents often turn to healthcare professionals for advice on use of formula milks. As our knowledge on the specific components of breastmilk increases, the composition of formula milks change to be closer in composition to breastmilk. Recent changes include the addition of prebiotic oligosaccharides and the modification of formula milk protein composition. These changes are not universal and formulae are not all the same, therefore it is crucial that Healthcare Professionals are informed about advances in formula milks, so that if mothers decide not to breastfeed, they can make informed decisions about replacement milks.

Click here for a handy download that summarises the composition of infant milk

References
1. Koletzko B. et al. Short and long term effects of breast feeding on child health. New York: Springer. 2000.
2. Codex Alimentarius. Codex standard 72 on infant formula. 1987.
3. Food Standards Agency. McCance and Widdowson’s The Composition of Foods. Cambridge: Royal Society of Chemistry; 2002.
4. Blanc B. World Rev Nutr Diet 1981; 36: 1-89.
5. Hall B. Am J Clin Nutr 1979; 32: 304-12.
6. Lönnerdal B. et al. JPGN 1984; 3: 290-295
7. Hanson LA. Monatsschr Kinderheilkd. 1998; 146 (suppl 1): S2-S6.
8. Gibson GR. et al. J Nutr 1995; 125: 1401-12.
9. Dai D. et al. JPGN 2000; 30: S23-S33.
10. Boehm G. et al. J Clin Gastroenterol 2004; 38(suppl 2): S76-S79.
11. Moro G. et al. J Pediatr Gastroenterol Nutr 2002; 34: 291-295.
12. Morrow AL. et al. J Pediatr 2004; 145: 297-303.
13. Bruzzese E et al. Clin Nutr 2009; 28: 156-161.
14. Arslanoglu S et al. J Nutr 2008; 138: 1091-1095.
15. van Hoffen E et al. Allergy 2009; 64: 484-487.
16. American Academy of Pediatrics, Committee on Nutrition Handbook. Elk Grove Village, IL: American Academy of Pediatrics; 2004.
17. Billeaud C. et al. Eur J Clin Nutr 1990; 44: 577-583.
18. Innis SM. Adv Exp Med Biol 2004; 554: 27-43.
19. Williatts P. et al. Lancet 1998; 352: 688-691.
20. SanGiovanni JP, et al. Pediatrics 2000; 15: 1292-8.
21. Lauritzen L. et al. Progr Lipid Res 2001; 40; 1-94.
22. Olafsdottir AS. et al. Ann Nutr Metab 2006; 50: 270-6.
23. Department of Health (2007) Press release (accessed 28/12/2007): http://www.gnn.gov.uk/Content/Detail.asp?ReleaseID=341224&NewsAreaID=2.
24. Andon MB. Am J Clin Nutr 1989; 50: 1050-8.
25. Kuhne T. et al. Eur J Pediatr, 1991; 150: 205-8.
26. Greer FR. J Nutr 1989; 119: 1846-51.
27. Griffin IJ. et al. Pediatr Clin North Am 2001; 48: 401-13.
28. Lozoff B. et al. Pediatrics 2000; 105: e51.
29. Booth IW. et al. Arch Dis Child 1997; 76: 549-554.
30. Lönnerdal B. Acta Paediatr 1994; 83: 367-73.
31. Bosscher D. et al. Int J Food Sci Nutr 2001; 52: 173-82.
32. Cosgrove M, Nutrition 1998; 14: 748-751.
33. Carver JD. Acta Paediatr Suppl, 1999; 88: 83-8.
34. Von Kries R. et al. BMJ 1999; 319: 147-50.
35. Savino F. et al. Acta Paediatr 2002; 91: 897-902.
36. Reid B. et al. Pediatrics 1980; 66: 870-872.
37. Hirai C. et al. JPGN 2002; 34: 524-8.
38. http://www.timesonline.co.uk/tol/news/uk/health/article1690434.ece (accessed 22nd Dec 07)