For two days do not eat any foods containing lactose. Drink two glasses of skim milk on an empty stomach. If the symptoms develop soon afterward, this indicates lactose intolerance. Knowledge of the causes and carefully monitoring one's intake of lactose can reduce discomfort. Some people can tolerate small amounts of lactose without too much of a problem by eating it with a meal.
Whole milk is tolerated better than skim milk.
Plain unflavored yogurt with active bacterial cultures may be a good substitute, however, frozen yogurt usually has been pasteurized and the bacteria eliminated. Read labels and be aware of its presence. Several dairy products have been developed to address the problem of lactose intolerance. It is possible to buy lactose-reduced products that have had lactase enzyme added. Persons who still have symptoms after using such products can buy a tablet or liquid to add to milk and rernove more of the lactose.