Why some nations can not drink milk?

Why some people and even whole nations do not tolerate dairy products?

"In medical language, milk intolerance has its name: lactase deficiency. The fact is that in dairy products in large quantities contains carbohydrate lactose, for the assimilation of which the enzyme of lactase is responsible in the body. Evolutionously stipulate that the activity of this enzyme can and should change during life. So, the peak of activity falls on the children’s age, when the need for milk is highest. With age, the generation of the enzyme decreases, which affects lovers of dairy products: the unusted lactose accumulates in the intestine and causes a lot of unpleasant consequences (discomfort, bloating, meteorism, diarrhea).

The formation of lactase in the body corresponds to a certain gene, and, therefore, any features of digestion of dairy products have a genetic basis. People who received from parents an active lactase gene, with pleasure drink milk all life. Those who got not the most active (but the most correct from the point of view of nature) gene, forced with age to limit themselves in milk."

"Among the Dutch and Danes, only 2% of adults are not able to absorb whole milk, while among the Chinese and the Japanese – more than 90%. ours from a third to half adults do not absorb milk. The ability to preserve the synthesis of lactase in adulthood arose millennium back in people as a result of mutations that spread from peoples engaged in milk animal husbandry. At the same time in different regions of the world, mutations appeared independent. The frequency map of lactase insufficiency in different regions of the world is presented in the figure above.

The red regions in which more than 90% of the adult population are able to absorb the milk, and the blue is 10%."