Lactose intolerance is the inability to digest and absorb the milk sugar lactose. Normally the upper small intestinal lining produces the enzyme lactase that splits lactose into 2 readily absorbable forms of sugars. When lactose is undigested, bacteria within the intestine ferment the sugar. This produces gas and lactic acid, which causes abdominal bloating, flatulence and diarrhea. Lactose enzyme supplements are readily available over-the-counter and can solve the problem in most people.