Lactose, the sugar in milk, is broken down in the intestines by the enzyme lactase. Most people are able to digest lactose properly during infancy and early childhood, but as they grow older this ability declines. Approximately 75% of adults worldwide are lactose intolerant and those with the highest intolerance are Native Americans and Asians and only slightly lower than these are the blacks, Jews, Hispanics, and southern Europeans. Lactose intolerance is lowest among northern Europeans and their descendants. In the US some 25% of Caucasians, 51% of Hispanics and 75% of all African Americans, Jews, and Native Americans have insufficient levels of lactase to digest dairy products and 90% of Asian Americans are lactose intolerant.20,21 It has been found that 90% of African people are lactase deficient, and in the case of the rural Zulu of South Africa it was found that they showed no change in blood glucose concentrations after ingesting 50g of lactose.22 When milk and dairy products are digested, lactose is broken down by the enzyme lactase into glucose and galactose (fig. 4.8).
The presence of lactose is a feature of mammalian milk, but the concentration of this sugar is normally geared to the needs of the species, as are the concentrations of all the other components of milk. In the case of humans, mother’s milk does not only contain the essential nutrients that are required for growth and development, but also contains the bacterium bacillus bifidus that assists in the digestion of lactose. In cow’s milk, however, the bacterial composition differs from that of human milk, and if cow’s milk is fed to infants, this can interfere with the digestion of lactose. Moreover, human milk has a higher carbohydrate concentration (7%-7,5%) than cow’s milk (4,5%-5%) and contains some 200m mol/litre of lactose which makes it sweeter than cow’s milk. The lower protein and higher carbohydrate content of human milk is also more suited to the needs of infants, because their growth rates are considerably lower than those of calves.
After the conversion of lactose to glucose and galactose, the available galactose is not utilized as such, but is converted to glucose in the liver by a series of steps requiring the initial presence of the enzyme galactokinase. The production of both the enzymes lactase and galactokinase declines with age, and the capacity to digest and utilize the products of lactose in adult life is thus curtailed. A deficiency in the enzyme lactase will result in fermentation of lactose by intestinal bacteria, which can result in abdominal distress such as the development of excessive gas, cramping, bloating, borborygmi (stomach rumbling), altered bowel habits and diarrhoea.21 The severity of the symptoms depends on the quantity of lactose consumed and the level of intolerance.