The enzyme that breaks down lactose is called lactase. When the intestine does not contain lactase, then lactose intolerance can occur. It is a troublesome and annoying problem, but it is never a serious one. As commonly expected, infants and small children have the enzyme lactase so they can digest mothers' milk. However, during childhood, lactase begins to disappear in many people.
Some ethnic groups are more likely to develop lactose intolerance.
So the condition is very common. Lactic acid and other acidic chemicals result. It is these products that create the symptoms of lactose intolerance. They usually occur 30 minutes to two hours after ingesting lactose-containing foods. The severity of symptoms usually depends on the amount of lactose ingested and how much of the enzyme, lactase, remains in the intestinal tract.