Symptoms and signs
Without lactase, the lactose in milk remains uncleaved and unabsorbed, and instead gut bacteria metabolize 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) there has been an increase in the number of lactose-reduced and lactose-free dairy products. 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.