Lactose – What is it?
Lactose, probably most frequently associated with the term ‘lactose intolerance’, is a simple carbohydrate occurring naturally in the milk of most mammals. A disaccharide, it is made up of two molecules, glucose and galactose – see the post Mono, Di and Poly Carb. The lactose content of human milk is around 7%1
Lactose intolerance
Milk is the main source of nutrition for human infants, so to thrive babies need the ability to digest lactose.
Produced in the proximal part of the small intestine, the enzyme lactase hydrolyses or breaks down lactose into its component parts of glucose and galactose. These single sugars can then be absorbed through the wall of the small intestine and into the bloodstream.
Production of lactase begins to diminish after weaning and during childhood, although the mechanism for this is only partially understood. This is known as primary lactase deficiency. In 65-70% of the world population lactase activity does not persist into adulthood resulting in the potential for an intolerance to lactose. Non-persistence of the lactase enzyme is genetically determined, whilst distinct genetic polymorphisms (more than one variant of a DNA sequence) convey the persistence of lactase activity in some adults. These polymorphisms appear to have prevalence in certain cultures and ethnicities; for example, in Scandinavia only around 10% of the population are lactase deficient, compared with more than 90% of the population in Southeast Asia.2
Lactase deficiency however, does not automatically indicate an intolerance to lactose. There is evidence to suggest that one cup of milk containing 12 to 15 g of lactose is tolerated by most adults known to be ‘lactose-malabsorbers’3.
If undigested in the small intestine, lactose will pass into the colon where microbes which possess lactase enzymes, e.g. Lactobacillus, can break down and ferment the lactose, producing lactate, short chain fatty acids and gases. One study suggests that colonic microbes in adults who are intolerant to lactose can ferment the lactose more quickly than the microbes in adults who have a greater tolerance for lactose4. It is the faster fermentation rate which leads to the symptoms of intolerance, notably abdominal distension and pain, rumbling sounds in the stomach, flatus and diarrhoea.
Where adults are known to have persistence of lactase activity, secondary lactase deficiency can occur because of health issues such as gastrointestinal infection, inflammatory bowel disease and abdominal surgery.
Lactose intolerance can sometimes be misinterpreted as an allergy to milk, but these are not the same thing. An allergy is an over-reaction of the immune system to a specific food protein and can cause a range of symptoms from a mild rash to life threatening breathing difficulties. Milk is one of 14 allergens regulated by UK food law. Food businesses are required to advise a consumer if any of these allergens are used as an ingredient in their products – see the post Setting Standards.
Milk – a complex food
Milk contains a wide range of naturally occurring nutrients including carbohydrate, protein, fat, vitamins, minerals, immunoglobulins and cytokines which convey passive immunity to the infant, hormones, growth factors and enzymes. Table 1 below illustrates some of the main components of cow’s milk.
Table 1: Composition of bovine milk
| Milk component | Concentration in 1 litre of whole milk |
|---|---|
| Total fatty acids | 33g/L |
| Saturated fatty acids | 19g/L |
| Oleic acid | 8g/L |
| Linoleic acid | 1.2g/L |
| Alpha linoleic acid | 0.75g/L |
| Protein (casein and whey) | 32g/L |
| Lactose | 53g/L |
| Calcium | 1.1g/L |
| Magnesium | 100mg/L |
| Zinc | 4mg/L |
| Selenium | 37µg/L |
| Vitamin E | 0.6mg/L |
| Vitamin A | 280µg/L |
| Folate | 50µg/L |
| Riboflavin | 1.83mg/L |
| Vitamin B12 | 4.4µg/L |
Source: Bovine milk in human nutrition5
Some countries fortify milk with Vitamin D – this does not happen in the UK
Dairy, or foods made from, or containing milk, are a large food group. Dairy can be liquid or solid, fermented or non-fermented, making it difficult to generalise in terms of nutritional value. The following sections are a quick resume of some of the current topics of interest surrounding the marketing of milk and, aside from lactose, milk-related health concerns.
Milk sales
Sales of whole cow’s milk have certainly declined over the last few decades. It is true that consumers now have more options when it comes to buying milk, compared to the doorstep deliveries in glass bottles from the ‘milkman’ of the 1960’s and 1970’s. There has been a recent surge in the availability and popularity of plant-based milks, probably because they are viewed as a more environmentally friendly option and less likely to cause lactose intolerance. A word of caution here though; since they are formulated using industrial substances not normally found in a domestic kitchen, most plant-based milks can be classified as ultra-processed foods6.
Milk and saturated fat
Because of its saturated fat content, the World Health Organisation and the UK Government both recommend lower fat milk – see the post Plating It Up. Maybe, however, the tide is turning. See this article from the British Heart Foundation which suggests that ‘despite their saturated fat content, dairy foods like milk, cheese and yoghurt have a neutral or even positive effect’ [on heart and circulatory health].
Milk and calcium
Is dairy good for your bones? Whilst there are many lifestyle factors which contribute to bone health, such as weight bearing exercise, and sun exposure for vitamin D production, it is also important to have a diet rich in calcium. Dairy foods are one of these sources, but there are others, for example, green leafy vegetables, nuts, sardines and foods fortified with calcium. However, current research seems to indicate that a ‘greater intake of milk and dairy products [is] not associated with a lower risk of osteoporosis and hip fracture’7.
Fat and sugar
Particularly appealing to the palate, many manufactured and ultra-processed foods contain high measures of fat and sugar – think pastries, cakes and ice cream. However, very few naturally occurring foods contain significant amounts of fat and sugar together. Milk is one of these exceptions!
References
- Schaafsma, G. Lactose and lactose derivatives as bioactive ingredients in human nutrition. Int. Dairy J. 2008;18:458–465. Available from: https://doi.org/10.1016/j.idairyj.2007.11.013
- Bayless, T.M., Brown, E., Paige, D.M. Lactase non-persistence and lactose intolerance. Curr Gastroenterol Rep.2017;19:23.
- Shaukat, A., Levitt, M. D., Taylor, B. C., MacDonald, R., Shamliyan, T. A., Kane, R. L., Wilt, T. J. Systematic review: effective management strategies for lactose intolerance. Ann Intern Med. 2010;152:797–803. Available from: https://doi.org/10.7326/0003-4819-152-12-201006150-00241
- He, T., Priebe, M. G., Harmsen, H. J. M., Stellaard, F., Sun, X., Welling, G. W., Vonk, R. J. Colonic fermentation may play a role in lactose intolerance in humans. J Nutr. 2006;136:58–63. Available from: https://doi.org/10.1093/jn/136.1.58
- Haug, A., Høstmark, A.T. & Harstad, O.M. Bovine milk in human nutrition – a review.Lipids Health Dis6, 25 (2007). Available from: https://doi.org/10.1186/1476-511X-6-25
- Drewnowski, A. Perspective: Identifying Ultra-Processed Plant-Based Milk Alternatives in the USDA Branded Food Products Database. Advances in Nutrition, Volume 12, Issue 6, 2021, Pages 2068-2075, ISSN 2161-8313. Available from: https://doi.org/10.1093/advances/nmab089
- Malmir, H., Larijani, B., & Esmaillzadeh, A. (2019). Consumption of milk and dairy products and risk of osteoporosis and hip fracture: a systematic review and Meta-analysis.Critical Reviews in Food Science and Nutrition,60(10), 1722–1737. Available from: https://doi.org/10.1080/10408398.2019.1590800
