Lactose
From Cookipedia
Lactose is a sugar that is found most notably in milk. Lactose makes up around 2–8% of milk (by weight), although the amount varies among species and individuals. It is extracted from sweet or sour whey.
Lactose intolerance
People who are lactose intolerant may suffer uncomfortable or socially unacceptable symptoms of too much lactose consumption. In these people, lactose is not broken down and provides food for gas-producing gut flora. This can lead to bloating, flatulence, and other gastrointestinal symptoms.