Lactose Intolerance an inability to digest lactose (milk sugar), so that drinking milk causes a digestive upset. Among white Europeans and a few other groups it is an unusual disability. But in the rest of the world, especially Africa and Asia, it is usual for the ability to digest milk to be lost as a person grows up. It seems likely that human beings originally had the capability to produce lactase in infancy and for as long as they needed mother’s milk, but not thereafter, when the capability would have been superfluous. However, at some time after human beings in C. and N. Europe (but not S. Europe where even today some 70 per cent of Sicilians are to a certain degree lactose intolerant) had begun to use the milk of domesticated animals and were in a position to benefit considerably from a prolongation of their ability to produce lactase, a new gene which provided for such prolongation would have been favoured by natural selection, so that this portion of the human race became able to digest lactose in adulthood and even into old age.