Up to 75% of people worldwide begin to lose the ability to digest lactose as they grow into adults. This loss of ability ranges from 5% in northern Europe, to over 71% in Sicily and to over 90% in parts of Africa and Asia. There is no cure for lactose intolerance. People need to change their diet to only include food with very little lactose. There are many milk substitutes.
Milk is not a fermented product. If is not digested, then it may ferment in the small intestine which can cause a problem called pseudoallergy. Amino acids are changed to other substances, which may act in much the same way as histamine in a true allergy.
Lactase evolution in humans
The ability to digest lactose into adulthood ('lactase persistence') was useful to humans after the invention of animal husbandry. People kept animals which could provide milk.
A mutation on chromosome 2 stops the shutdown in lactase production. This makes it possible for those with the mutation to continue drinking fresh milk (and eating other dairy products) throughout their lives.
This appears to be a recent adaptation to dairy products. It occurred in both northern Europe and east Africa in people with a historically pastoral lifestyle. Lactase persistence, allowing lactose digestion to continue into adulthood, is a dominant allele, making lactose intolerance a recessive trait.
Genetic studies suggest that the oldest mutations associated with lactase persistence have only become common in human populations in the last 10,000 years. Because of this, lactase persistence is often given as an example of recent human evolution. As lactase persistence is genetic, but animal husbandry a cultural trait, this is gene–culture coevolution.
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