symptoms lactose intolerance
1 Lactose intolerance is caused by a deficiency of the intestinal enzyme lactase that splits lactose into two smaller sugars, glucose and galactose, and allows lactose to be absorbed from the intestine.  The primary symptoms of lactose intolerance are diarrhea, flatulence (passing gas), and abdominal pain. Abdominal bloating, abdominal distention, and nausea also may occur.