The normal mammalian condition is for the young to lose the ability to effectively digest milk sugar (lactose) at the end of the weaning period (a species-specific length of time usually equal to roughly 3% of lifespan). It has been established that certain human populations have undergone a mutation on chromosome 2 which results in a bypass of the normal shutdown in lactase production, allowing members of these groups to continue consumption of fresh milk and fresh milk products throughout their lives.
There is some debate on exactly where and when the mutation(s) occurred, some arguing for separate mutation events in Sweden and the Arabian Peninsula near 4000 BC which converged as they spread, while others argue for a single event in the Middle East at about 4500 BC which radiated from there. Whatever the precise origin, most western Eurasians and people of western Eurasian ancestry show the effects of this mutation while most eastern Eurasians, sub-saharan Africans and native peoples of the Americas and Pacific Islands do not .
Without lactase, the lactose in milk remains uncleaved and unabsorbed, and instead gut bacteria metabolise it, producing copious amounts of gas by fermentation. This causes a range of unpleasant abdominal symptoms, including stomach cramps, flatulence and diarrhea. Like other unabsorbed sugars, e.g. mannitol, the lactose raises the osmotic pressure of the colon contents, preventing the colon from resorbing water and hence causing a laxative effect to add to the excessive gas production.
One solution to this problem (other than avoiding milk) is lactose-free milk, which is produced by passing milk over lactase enzyme bound to an inert carrier: once the molecule is cleaved, there are no lactose ill-effects, whatever the milk drinker's ancestry. The milk sold for pet cats is another example of lactose reduced milk. Cats have a very short generation time compared to humans, and have been around people since animal husbandry began, so it would not be surprising if at least some cats have made a similar adaptation to dietary lactose, but not every cat has European ancestry - some of the oriental breeds are particularly sensitive to lactose.
In recent years (1990-2000), more and more lactose-reduced and lactose-free dairy products have become available. Some of these products are cottage cheese, American cheese and ice cream. These products are made using milk-substitutes such as soy milk, almond milk, or rice milk. Another recent solution has been a pill which artificially provides the missing enzyme, allowing a person to tolerate milk products for a period of a few hours after taking the pill.
See also: Gastroenterology