Because Amy has a form of lactose intolerance, she cannot digest milk unless it is in the form of yogurt, hard cheese, or cottage cheese. Drinking any milk products that contain lactose, such as the chocolate shake, will result in discomfort.
Lactose, the carbohydrate component of milk, must be broken into its two monosaccharides, glucose and galactose, to be absorbed in the small intestine, most often in the jejunum. As babies, most people produce lactase, the enzyme that breaks up lactose, which is useful since human breast milk has the highest amounts of lactose of all mammals. Many people, however, lose the ability to produce lactase as they become adults and can no longer tolerate milk or milk products unless they have been partially broken down, as in yogurt. In the United States, about 75% of African Americans, 50% of Hispanics, and 20% of Caucasians are lactose intolerant.
Amy makes some lactase, so some of the sugar from the shake will be absorbed in the intestine. The severity of the symptoms will depend on how much lactase she makes, because total absence of the enzyme is extremely rare. A short time after drinking the shake, she will experience diarrhea, gas, bloating, and abdominal cramps. The sugar is retained in the small and large intestines and causes more water to be retained in the digestive tube than normal. This accounts for the bloating feeling in the abdomen. The lactose cannot be used by her body, but the bacteria in her intestine can use it. These bacteria ferment the sugar and produce the gases hydrogen and carbon dioxide. The bacteria also produce acid, which affects abdominal sensory nerves and causes the abdominal cramping.
Anyone with lactose intolerance should avoid milk, milk solids, whey (the liquid from milk), and casein, which is milk protein. Lactose is also found in breads, cereals, instant soups, instant potatoes, salad dressings, and nondairy powdered creamers. Drinking acidophilus milk or taking a pill containing lactase can also help avoid the digestive problems. In addition, about 20% of prescription drugs and 5% of over-the-counter drugs contain lactose. People with lactose intolerance need to be careful not to become deficient in calcium or riboflavin, a B vitamin.
Biologists suggest that the persistence of the production of lactase into adulthood by some people may have occurred because of the development of dairy farming thousands of years ago. People whose ancestors depended on dairy farming tend to continue to be able use milk all their lives.
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