lactose intolerance

Some what I consider to be interesting facts about lactose intolerance.

• Mammals digest sugars in mother’s milk until weaning.
• The intestinal enzyme for digestion is lactase. Lactase production in humans usually drops by 90% after the age of four. A mutation on chromosome 2 results in keeping lactase production active in some humans.
• It is not clear When this adaptation occurred, and whether it occurred only once and then was inherited or whether evolution “came up with it” several times. However, there seems to be quite some research on it.
• There is large cultural variation when it comes to lactose intolerance. In Northern Europe, about 2% of the population is intolerant. In Africa the range is between 20 and 80%. China 93%, Thailand 98%, American Indians 100%. The Navajos used the milk powder lavishly bestowed upon them by the government to … color their adobes.
• Cats in Europa are not lactose intolerant (although some individuals very well might be), cats in Asia are. My explanation for this would be co-evolution: Europeans fed their cats milk, thinking they were doing something good. That introduced a high selection pressure (the same selection pressure that was imposed on humans when the mutation occurred and was adaptive), so cats with the mutation did better. In Asia, people wouldn’t even come up with the idea of feeding grown-up mammals milk.

Source: lecture about evolution & public health by Gilbert S. Omenn