Intolerance to lactose
Lactose is the principal carbohydrate of milk. It's a pretty big compound (named disaccharide) formed by two smaller components: glucose and galactose. Such a big compound as lactose cannot get through the intestinal wall and into the bloodstream, so we need "something" to break it into smaller pieces. This "something" is an enzyme named lactase. The more milk and milk products we consume, the more lactase we need.
Normally there's plenty of lactase in the digestive systems of infants and children, but as we grow older most of us lose the ability to produce lactase in large quantities, usually too little to handle more than a glass or two of milk at a time. When this drop in lactase production falls below certain minimums the intolerance to lactose appears.
Without enough lactase in the digestive fluids, the lactose of milk and milk products isn't broken (hydrolyzed) effectively, so lactose passes along the intestinal path to a region where it undergoes fermentation to gases such as carbon dioxide and hydrogen and to acid lactic, a bowel irritant. The combination easily produces gastric distress and diarrhea.
No known way exists to increase the amount of lactase enzyme the body can make, but symptoms can be controlled through diet. For those who react to very small amounts of lactose or have trouble limiting their intake of foods that contain lactose, lactase additives are available from drug stores without a prescription. At somewhat higher cost, shoppers can buy lactose-reduced milk at most supermarkets. The milk contains all of the other nutrients found in regular milk and remains fresh for about the same length of time.