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 (alphaS1, alphaS2, beta 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. 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). Most of the fat in whole cow’s milk (around 65 per cent) is the saturated type. Around 30 per cent is monounsaturated and just five per cent polyunsaturated. Saturated fatty acids are associated with high blood cholesterol and heart disease.
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 over the last ten years about 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. It has been suggested that some forms of CLA may confer a range of potential human health benefits (McGuire and McGuire, 2000, Whigham et al., 2007). However, the majority of studies on weight loss, cancer, cardiovascular disease, insulin sensitivity and diabetes and immune function have been conducted on animals (or in vitro) and it has been acknowledged that variations exist between different animals’ responses to CLAs. Human studies have produced mixed results; a 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). One form of CLA is suspected of having pro-diabetic effects in individuals who are already at risk of developing diabetes (McCrorie et al., 2011).
In summary, there is no substantive evidence of a consistent benefit of CLA on any human health conditions and evidence regarding effectiveness of CLA in humans is inconclusive (Silveira et al., 2007). 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, began over a decade ago (Lock and Garnsworthy, 2002). As stated above, cow’s milk and dairy products are significant source of saturated fat in the human diet. Altering the fatty acid composition of dairy foods would mean that people could comply with government health recommendations on fat intake without having to change their eating habits (Shingfield et al., 2013). This brave new world approach to how we eat is called nutrigenomics. The simpler alternative would be to just eat healthier food!
In addition to the fatty acids discussed there are small amounts of phospholipids and other fats present in milk including fat soluble vitamins.
Minerals and 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 (see Osteoporosis).
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.