A staggering 7 billion litres of milk are consumed in the UK every year. But did you know that adult humans would not be able to digest dairy products without a genetic mutation? Milk contains lactose — a disaccharide. As infants, humans feed exclusively on milk, and they use an enzyme, lactase, to hydrolyse the lactose. Following weaning, and as we get older and consume less milk, the production of lactase decreases and humans become more and more intolerant to lactose. When ancient humans began to farm animals during the agricultural revolution and started to consume dairy products beyond infancy, a genetic mutation allowed some of us to continue to produce lactase. This mutation spread through the population. Today, most adult humans in the Western world can digest milk, whereas many Asian people, whose traditional diet does not include dairy products, are lactose intolerant.
Lactase is produced in cells lining the small intestine, and uses water to hydrolyse lactose into glucose and galactose (see Figure 1). Both these monosaccharides enter the bloodstream as nutrients. If lactase is absent, undigested lactose reaches the colon where bacteria ferment it, producing methane, carbon dioxide and hydrogen — leading to bloating, flatulence and abdominal pain. Unabsorbed lactose can also have an osmotic effect, drawing water into the colon to cause diarrhoea. Symptoms can be so severe that some intolerant individuals have to cut dairy products out of their diet completely.
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