Normal people absorb 90 over percent of milk's lactose (milk sugar) while those with lactose intolerance absorb only 25 to 60% of the milk sugar. The rest lingers in the colon and ferments, causing abdominal pain, bloating, excess gas and diarrhea.
What to do if you are lactose intolerant? For one, you can avoid milk. Another is to take yoghurt or kefir. The lactose in them have been predigested by the friendly bacteria. One more trick is to drink chocolate milk. Dr. Chong M. Lee, Ph. D., a professor of food science and nutrition at the University of Rhode Island tested 35 subjects and found that in more than half of the cases, the abdominal cramping, bloating and other signs of lactose intolerance were blocked. In test tube studies, cocoa increased lactase activity by 500 to 600%, Dr. Lee speculate that most likely, the cocoa stimulate enzyme activity. (source: Food the Miracle Medicine by Jean Carper)
One more alternative is to drink soymilk. Now with the availability of automatic soymilk maker, it is a breeze to make your own soymilk at home.