A lactose-free diet is one in which the milk sugar lactose is avoided. Most typically, someone assumes a lactose-free diet when they are diagnosed as lactose intolerant. In these instances, someone is usually lacking the enzyme lactase, wherein they are unable to properly able to digest lactose in their small intestine. When this occurs, the undigested lactose makes its way to the large intestine where it is digested by the bacteria that reside there, causing symptoms such as gas, bloating, abdominal pain, cramps, diarrhea and vomiting. Read More.