The composition of cow’s milk
Cow’s milk composition can vary widely between different breeds and during different stages of lactation. In the first few days after birth, a special type of milk called colostrum is excreted which is rich in fats and protein. Colostrum also contains important infection-fighting antibodies which strengthen the immune system of the young mammal. The transition from colostrum to true milk occurs within a few days following birth.
All milk produced by animals contains carbohydrate, protein, fat, minerals and vitamins but the major component is water. Water dilutes the milk allowing its secretion from the body; without water it would be impossible to express milk. Additionally, the water in milk is essential to the newborn for hydration. Cow’s milk contains a similar amount of water to human milk – around 87 per cent.
The major carbohydrate in mammalian milk is a disaccharide (or sugar) called lactose. For lactose to be digested, it must be broken down in the intestine by the enzyme lactase to its component monosaccharides glucose and galactose. Glucose can then supply energy to the young animal. Many people are unable to consume cow’s milk and dairy products because they are unable to digest lactose after weaning. Most infants possess the enzyme lactase and can therefore digest lactose, but this ability is lost in many people after weaning (commonly after the age of two). In global terms lactose intolerance is very common, occurring in around 90-100 per cent of Asians, 65-70 per cent of Africans, but just 10 per cent of Caucasians(Robbins, 2001). Therefore most of the world’s population are unable to digest milk after weaning.
Protein provides energy and is required for the growth and repair of tissue such as skin and muscle. Caseins are the primary group of proteins in cow’s milk, making up around 80 per cent of the total protein content. The remaining portion is made up from whey proteins. There are four types of casein (alpha-, beta-, gamma- and kappa-casein) that combine to make up a structure known as a casein micelle. The micellar structure of casein is important in the production of cheese; it also plays a significant role in cow’s milk allergies (see Allergies).
The principal fat in milk is a complex combination of lipids called triglycerols (esters of three fatty acids with one molecule of glycerol). There are more than 400 fatty acids in cow’s milk ranging in carbon atom chain length from four carbon atoms to 26 (National Dairy Council, US, 2005). Fatty acids are described as saturated or unsaturated depending on the amount of hydrogen in the carbon chain of the molecule; milk contains both saturated and unsaturated fatty acids. Unsaturated fatty acids may be further classified as monounsaturated or polyunsaturated (depending on the number of double bonds in the carbon chain of the fatty acid molecule). Again, milk contains fatty acids from both groups but most of the fat in whole cow’s milk (around 65 per cent) is the saturated type.
Polyunsaturated fats include fatty acids called the omega-6 and omega-3 fatty acids (these names refer to the position of the double bond in the carbon chain of the fatty acid molecule). Milk contains the omega-6 essential fatty acid linoleic acid and the omega-3 fatty acid linolenic acid. These are called essential fatty acids because they are essential to health but cannot be made within the body and so must be obtained from the diet. While milk does contain linoleic acid and linolenic acid (both with chains of 18 carbon atoms) it does so at relatively low levels.
There has been much excitement recently about the so-called conjugated linoleic acids (CLAs) in cow’s milk. The term ‘conjugated’ refers to the molecular arrangement of the molecule. CLAs are described as positional and geometric isomers of linoleic acid; this means that CLAs are made up of exactly the same components as normal linoleic acid, just in a different arrangement. CLA in one particular configuration (cis-9, trans-11 CLA) is believed to possess a range of potential health benefits for humans (McGuire and McGuire, 2000). However, the majority of studies on weight loss, cancer, cardiovascular disease, insulin sensitivity and diabetes and immune function have been conducted on animals and it has been acknowledged that variations exist between different animals’ responses to CLAs. A recent review of 17 studies on humans concluded that CLA does not affect body weight or body composition and has a limited effect on immune function (Tricon et al., 2005). Furthermore some detrimental effects of CLA have been observed in mice and some reports suggest that CLAs can elicit pro-carcinogenic effects (Wahle et al., 2004). Despite warnings from researchers that until we know more, CLA supplementation in humans should be considered with caution, the dairy industry sees this molecule as a new marketing opportunity and research into producing CLA-enriched milk by manipulating the diet of dairy cows has already begun (Lock and Garnsworthy, 2002).
In addition to the fatty acids discussed there are small amounts of phospholipids and other fats present in milk including fat soluble vitamins.
Minerals found in cow’s milk include sodium, potassium, calcium, magnesium, phosphorus and chloride, zinc, iron (although at extremely low levels), selenium, iodine and trace amounts of copper and manganese (FSA, 2002). Vitamins in cow’s milk include retinol, carotene, vitamin E, thiamin, riboflavin, niacin, vitamin B6, vitamin B12, folate, pantothenate, biotin, vitamin C and trace amounts of vitamin D (FSA, 2002). In the US, milk is fortified with additional vitamin D; this has important implications as we shall see later (seeOsteoporosis).
Although cow’s milk contains all these nutrients it is important to note that these vitamins are contained at very low levels. Furthermore, the mineral content is so out of balance with human biochemistry that it is difficult for us to absorb the optimum amounts required for health.
Milk contains no dietary fibre.
Whole milk, cheese, butter and many other dairy products contain high levels of saturated fat, cholesterol and animal protein all of which are not required in the diet and have been linked to a wide range of illnesses and diseases. For example, excess saturated fat and cholesterol in the diet is associated with an increased risk of heart disease and stroke. Cross cultural studies show that as the consumption of saturated fat, cholesterol and animal protein increases from country to country, so does the incidence of the so-called diseases of affluence such as obesity, heart disease, diabetes, osteoporosis and certain cancers. It has been suggested that this is because of genetic differences between different races. However, when people migrate from an area of low incidence of the so-called affluent diseases to an area of high incidence, they soon acquire the same high incidence shared by the population into which they have moved. This correlation must then be attributed, at least in part, to environmental factors such as diet and lifestyle. So if you can increase the risk of disease by changing your diet and lifestyle, it stands to reason that you can reduce the risk of disease by changing your diet and lifestyle. The WHO state that there are major health benefits in eating more fruit and vegetables, as well as nuts and whole grains and moving from saturated animal fats to unsaturated vegetable oil-based fats (WHO, 2006c).
In addition to saturated fat, cholesterol and animal protein, a wide range of undesirable components occur in cow’s milk and dairy products. The modern dairy cow is prone to both stress and disease. In the UK, cows suffer from a range of infectious diseases including brucellosis, bovine tuberculosis, foot and mouth disease, viral pneumonia and Johne’s disease. As a result of an infectious disease a wide range of contaminants can occur in milk. Mastitis (inflammation of the mammary gland) is a widespread condition affecting cattle in the UK in which all or part of the udder suffers from an infection caused by bacteria entering through the teat (MDC, 2004). Mastitis may be referred to as subclinical (no symptoms) or clinical whereby symptoms include swelling, pain, hardness, milk clots or discoloured milk. The cow responds to the infection by generating white blood cells (somatic cells) which migrate to the affected area in an effort to combat the infection. These cells, along with cellular debris and necrotic (dead) tissue, are a component of pus and are excreted into the milk.
The number of somatic cells in the milk (the somatic cell count) provides an indication of the level of infection present. The somatic cell count usually forms part of a payment structure to farmers with defined thresholds of concentration determining the qualification for bonus payments or penalty charges (Berry et al., 2003). In the European Union the somatic cell limit is a maximum of 400,000 cells per ml in bulk milk (Dairy Products (Hygiene) Regulations, 1995). This means that milk containing 400 million pus cells per litre can be sold legally for human consumption. So one teaspoonful of milk could contain up to two million pus cells! It could be even worse, as concerns have been raised about the efficiency of cell counting techniques (Berry et al., 2003).
Mastitis effects the quality of milk in many ways; the total protein content is decreased, the amounts of calcium, phosphorus and potassium content are decreased, the taste deteriorates (becomes bitter), and the levels of undesirable components rise. These include enzymes such as plasmin and lipase, immunoglobulins (Blowey and Edmondson, 2000) and microbes. Mastitis is treated with antibiotics delivered directly into the udder. These drugs can also end up in the milk, so milk from treated cows must not be marketed until the recommended withholding period has elapsed (MDC, 2004). Mastitis occurs in around 50 per cent of cows in the UK (Blowey and Edmondson, 2000).
Milk contains many biologically active molecules including enzymes, hormones and growth factors. In 1992, Pennsylvania State University endocrinologist Clark Grosvenor published an extensive review of some of the known bioactive hormones and growth factors found in a typical glass of milk in the US. The list included seven pituitary (an endocrine gland in the brain) hormones, seven steroid hormones, seven hypothalamic (another brain endocrine gland) hormones, eight gastrointestinal peptides (chains of two or more amino acids), six thyroid and parathyroid hormones, 11 growth factors, and nine other biologically active compounds (Grosvenor et al., 1992). Other biologically important proteins and peptides in milk include immunoglobulins, allergens, enzymes, casomorphins (casein peptide fragments) and cyclic nucleotides (signalling molecules). The concern here is that these signalling molecules that have evolved to direct the rapid growth of a calf into a cow may initiate inappropriate signalling pathways in the human body that may lead to illnesses and diseases such as cancer.
All milk produced by mammals is a medium for transporting hundreds of different chemical messengers. It has been suggested that milk actively communicates between the maternal mammary epithelia and the infant’s gastrointestinal system directing and educating the immune, metabolic and microflora systems within the infant (German et al., 1992). Indeed, research indicates that many of these molecules survive the environment of the infant’s gut and are absorbed into the circulation where they may exert an influence on the infant’s immune system, gastrointestinal tract, neuroendocrine system, or take some other effect. This has evolved as a useful mechanism between mothers and infants of the same species, but the effects of bioactive substances in milk taken from one species and consumed by another are largely unknown. The concern is that the bioactive molecules in cow’s milk may direct undesirable regulation, growth and differentiation of various tissues in the human infant. Of particular concern for example is the insulin-like growth factor 1 (IGF-1) which occurs naturally in milk and has been linked to several cancers in humans (seeIGF-1).
The WHO states that as a global public health recommendation, infants should be exclusively breast fed for the first six months of life to achieve optimal growth, development and health (WHO, 2001). They conclude that in general this is the healthiest start to life for a baby. It is interesting to note that when given the choice between human breast milk and cow’s milk infant formula, newborn babies demonstrate a preference (by turning their head and mouthing) for human milk regardless of their individual postnatal feeding experience (Marlier and Schaal, 2005).
Breast feeding is important for many reasons. Babies receive an important boost to their immune system in the first few days of breast feeding as important antibodies are passed from the mother to the infant in the colostrum (the fluid expressed before the so-called true milk). These antibodies protect the baby from infection. Breast fed babies are less likely to suffer many serious illnesses including gastroenteritis, respiratory and ear infections, eczema and asthma as children. Adults who were breast fed as babies are less likely to have risk factors for heart disease such as obesity, high blood pressure and high cholesterol levels (UNICEF, 2005). This was confirmed recently in a study of over 2,000 children from Estonia and Denmark. It was found that that children who were breast fed as infants had lower blood pressure than those who were not; the longer the child was breast fed, the greater the difference (Lawlor et al., 2005). The implications are that breast feeding plays a role in reducing heart disease in adults.
Furthermore, breast feeding is free! You do not need to wash and sterilise an endless number of bottles. You will not be up in the night mixing and testing the milk to see if it is cool enough; breast milk comes ready mixed at the perfect temperature. The act of breast feeding is also important for bonding the mother and baby relationship. Yet British breast feeding rates are amongst the lowest in Europe. At birth, only 69 per cent of UK babies are breast fed and this figure falls rapidly to 55 per cent at one week (Hamlyn et al., 2002).
The use of formula milk while in hospital is a strong indicator for a mother giving up breast feeding after leaving hospital; 40 per cent of breast feeding mothers whose babies had been given formula milk in hospital stopped breast feeding within two weeks compared to only 13 per cent of breast feeding mothers whose babies had not been given formula milk (Hamlyn et al., 2002). Regrettably, at six months of age, just one in five babies in the UK are still receiving breast milk, despite the fact that the WHO, UNICEF and the UK Government all recommend that babies should be fed only breast milk for their first six months of life.
Some mothers are unable to, or choose not to, breast feed and in these circumstances infant formula milk is used. Formula milk is designed to meet the nutritional requirements of the infant and must comply with strict UK and EC legislation which specifies the nutritional composition of the feeds. Soya-based infant formulas provide a safe feeding option for most infants that meet all the nutritional requirements of the infant with none of the detrimental effects associated with the consumption of cow’s milk formulas. Under no circumstances should a child under 12 months be given ‘normal’ cow’s, goat’s, soya or any other milk that is not specifically formulated for an infant (for a review on the safety of soya see Appendix I).
In 1924, local education authorities (LEAs) in the UK were permitted to provide children with free milk. This was the start of the movement to introduce milk to school-aged children that would continue to this day. In a recent paper published in the Economic History Review, Dr Peter Atkins of Durham University reviewed the motivations behind the introduction of cow’s milk in schools during the first half of the twentieth century (Atkins, 2005). Atkins stated that the nutritional benefits of school milk were debatable, possibly even negative in those areas where it replaced other foods, but noted that the dairy industry did well, creating new markets at a time of depression (Atkins, 2005).
In 1946, the School Milk Act provided free milk to all school children. A third of a pint of milk was provided to all children under the age of 18 years until 1968 when Harold Wilson’s Government withdrew free milk from secondary schools. This policy was extended in 1971 when Margaret Thatcher (then secretary of state for education) withdrew free school milk from children over seven. This was an economic decision, not one based on a nutritional assessment of the value of milk, and for this she earned the nickname ‘Thatcher, Thatcher, milk snatcher’ – although many children were delighted at not having to drink the warm sickly odorous milk at school anymore!
The school milk scheme was introduced in 1977 by the European Union (EU) to encourage the consumption of milk in schools. The scheme requires member states to make subsidised milk available to primary and nursery schools wishing to take part, but participation is entirely a matter for the school or LEA. The European Commission had originally indicated that it wished to abolish the subsidy because the scheme was not providing value for money. The UK did not accept these conclusions and fought hard to retain the scheme. A compromise was secured whereby in 2001 the subsidy rate was reduced from 95 to 75 per cent. The UK Government tops up the subsidy to its original level in England, up to a maximum total expenditure of £1.5 million each year. In the academic year 2003 to 2004, around one million school children in England drank 34.9 million litres of subsidised milk at a cost of around £7 million (Defra, 2005a).
The move to increase milk consumption in schools is gathering momentum; the School Milk Project (TSMP), set up in 1998 by the Women’s Food and Farming Union, aims to increase the uptake of milk in primary schools. It receives funding from the Milk Development Council (MDC) which was established following the re-organisation of the milk industry in 1994. The MDC is funded by a statutory levy on all milk sold off farms in Great Britain; the annual income from the levy is over £7 million (MDC, 2005). Primarily the MDC funds research and development into milk production methods, it also funds TSMP which employs ‘facilitators’ to promote the uptake of school milk through direct contact with LEAs, schools and dairy suppliers.
The charity Milk For Schools (MFS) was founded in 1994. Set up to educate the public in the field of school based nutrition, MFS is a registered member of the United Nations Food and Agriculture Organisation (UNFAO) School Milk Network, which initiated the first World School Milk Day on 27th September 2000. In October 2004 Dairy UK was established as a cross-industry body representing processors and distributors of liquid milk and dairy products, as well as milk producer co-operatives. In 2005 the EU and Dairy UK joined forces with the MDC to promote milk consumption in primary schools (Dairy UK, 2005). Schools were targeted with ‘Teacher’s Guides to Health and Fitness’ and School Milk Week commenced on 10th October 2005. Previous school milk weeks have generated over 6,000 new school milk drinkers or as Dairy UK put it “over one million new serving opportunities per annum” (Dairy UK, 2005).
There is undoubtedly some very clever marketing going on here, in fact the 30-year decline in milk consumption may be coming to an end. Liz Broadbent, director of market development at MDC, points out that this growth (worth £4 million to Britain’s dairy farmers), is the first credible and seemingly sustainable rise in the past three decades. Research indicates the extra milk is being in used in porridge, tea and coffee. Evidence suggests this rise is due to successful promotion and marketing of specific products. This explains the industry’s recent move to abandon generic promotions (just telling everyone to drink more milk) instead choosing to focus on specific products for specific groups, hence the MDC’s latest campaign specifically targeting teenage girls. The research has also discovered a growing number of low milk consumers among the more affluent members of the population including single professionals and young parents who did not receive free milk themselves at school.
This group, that are not passing on a milk-drinking habit to the next generation the MDC notes, account for around half of the population but consume only a quarter of the volume. The MDC targets particular groups in an attempt to generate new consumers, who will, in turn, make new consumers of their children. Broadbent states that convenience, innovation and habit are the key, and while cost is not an issue for this group, providing milk in a form they like is. The other route Broadbent suggests is through the school milk programmes which are redeveloping the milk drinking habit at an early age.
MDC’s school milk project and match-funded school milk bar initiative have generated half a million new milk drinkers and accounts for 20 million litres of milk. But its real value to the dairy industry is the reinstatement of milk as a ‘normal’ commodity for regular family consumption now and in the future. The policy of introducing school milk begs the question, are the dairy industry nurturing our children? Or simply nurturing a future loyal adult consumer base?