People who are lactose intolerant cannot digest and absorb the milk sugar lactose, because they produce only low levels of the lactase enzyme. This causes digestive symptoms like gas, bloating and diarrhea, when dairy products are consumed.
Lactase persistence is the opposite of lactose intolerance. People who are lactase persistent are able to produce the lactase enzyme for their entire lives, meaning they can continue to consume dairy products right through adulthood.