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A sugar, found in milk, and forming 5% of cow"s milk. It"s commercially prepared, from the whey left over during cheese making. Unlike sucrose, it"s tasteless.

Lactose makes up between 4 and 7 per cent of the milk of mammals; but to absorb lactose, newborns have to secrete the enzyme lactase which breaks down lactose into its components, glucose and galactose, which are absorbable.

However, after weaning and growing to adulthood, many people lose their ability to secrete lactase and cannot therefore digest milk or dairy products.

This condition is known as 'lactose intolerance' and affects some 70-90 per cent of oriental, black, native American and Mediterranean adults, whereas the rate among northern and western Europeans is a low as 15 per cent.

The symptoms of intolerance include abdominal discomfort, bloating and diarrhoea in response to drinking even small amounts of milk.

For people with lactose intolerance who wish to drink milk, the enzyme lactase is obtainable from some health food shops as a supplement. For more information on nutrition see 'Nutrients A to Z' by Dr. Michael Sharon (Prion Books)

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