Lactose intolerance refers to the inability of the body to digest lactose.
Lactose is the predominant form of sugar present in milk. The enzyme lactase, which is normally produced by cells lining the small intestine, breaks down lactose into substances that can be absorbed into the bloodstream. When dairy products are ingested, the lactose reaches the digestive system and is broken down by lactase into the simpler sugars glucose and galactose, which can then be absorbed into the bloodstream. Lactose intolerance occurs when, due to a deficiency of lactase, lactose is not completely broken down and consequently blood sugar levels do not rise. While not usually a dangerous condition, lactose intolerance can cause severe discomfort.
Lactose intolerance is also referred to as lactase deficiency, milk intolerance, dairy product intolerance, or disaccharidase deficiency.
From 30 to 50 million Americans suffer from the symptoms of lactose intolerance by the age of 20. People from cultures in which adult consumption of milk and milk products occurred earliest are less likely to be lactose intolerant than people from areas where dairy farming began more recently. The prevalence of deficiency in production of the lactase enzyme, therefore, varies among different ethnic groups. Among Asian populations it is almost 100 percent, with symptoms occurring around the age of five; among Native Americans it is 80 percent; among blacks it is 70 percent, with symptoms appearing by the age of 10; and among American Caucasians, the prevalence of lactose intolerance is only 20 percent. However, individuals who are mildly or moderately deficient in the production of the lactase enzyme may not exhibit symptoms of lactose intolerance.
Causes and symptoms
Lactose intolerance can be caused by some diseases of the digestive system (for example, celiac sprue and gastroenteritis) and by injuries to the small intestine that result in a decreased production of lactase. While rare, some children are also born unable to produce the enzyme. For most people, however, lactase deficiency develops naturally because, after about two years of age, the body produces less lactase. Before humans became dairy farmers, they usually did not continue to drink milk, so their bodies did not produce lactase after early childhood.
Symptoms of lactose intolerance include nausea, cramps, diarrhea, floating and foul-smelling stools, bloating, and intestinal gas. The symptoms usually occur between 30 minutes to two hours after eating or drinking lactose-containing foods. A child may also exhibit weight loss, slow growth, and malnutrition.
When to call the doctor
To diagnosis lactose intolerance, usually healthcare professionals measure the absorption of lactose in the digestive system by using the lactose tolerance test, the hydrogen breath test, or the stool acidity test. Each of these can be performed as an outpatient in a hospital, clinic, or doctor's office.
Children who are to take the lactose tolerance test must fast before being tested. They then drink a lactose-containing liquid for the test; medical personnel take blood samples during the next two hours to measure the children's blood glucose level. The blood glucose level, or blood sugar level, indicates how well the body is digesting the lactose. A diagnosis of lactose intolerance is confirmed when blood glucose level does not rise. This test is not administered to infants and very young children because of the risk of dehydration from drinking the lactose-containing liquid, which can cause diarrhea in those who are lactose intolerant, resulting in dehydration.
Hydrogen is usually detected only in small amounts in the breath. However, when undigested lactose found in the colon is fermented by bacteria, hydrogen in the breath is produced in greater quantities. The hydrogen is exhaled after being absorbed from the intestines and carried through the bloodstream to the lungs. The hydrogen breath test involves having the child drink a lactose-containing beverage. Healthcare professionals monitor the breath at regular intervals to see if the hydrogen levels rise, which indicates improper lactose digestion. Children taking the test who have had certain foods, medications, or cigarettes before the test may get inaccurate results. While the test is useful for children and adults, infants and young children should not take it because of the risk of dehydration from diarrhea in those who are lactose intolerant.
The stool acidity test measures the amount of acid in the stool. This is a safe test for newborns and young children. The test detects lactic acid and other short-chain fatty acids from undigested lactose fermented by bacteria in the colon. Glucose may also be found in the stool sample, resulting from unabsorbed lactose in the colon.
Some parents may try to self-diagnose lactose intolerance in their child by using an elimination diet, a diet that eliminates obvious milk and milk products. However, because there are so many food products that may contain hidden sources of milk, such a diet should be supervised by a dietician or developed by following a guide to a lactose-eliminating diet. A simpler way to self-diagnose lactose intolerance is by a milk challenge. The child fasts overnight, drinks a glass of milk in the morning, and then fasts for the next three to five hours. If the child is lactose intolerant, the child should experience symptoms within several hours. If symptoms do occur, the child should be evaluated by a healthcare professional to rule out the possibility of a milk allergy. However, milk allergies are rare and usually only occurs in infants and young children.
Most children affected by lactose intolerance do well if they limit their intake of lactose-containing food and drinks. Individuals differ in the amounts they can handle before experiencing symptoms. Many children may only need to eliminate major milk-containing products from their diet, while others who are intolerant to even small amounts of lactose may be required to follow severe dietary restrictions.
Foods that contain lactose include milk, low-fat milk, skim milk, chocolate milk, buttermilk, sweetened condensed milk, dried whole milk, instant nonfat dry milk, low-fat yogurts, frozen yogurt, ice cream, ice milk, sherbet, cheese, cottage cheese, low-fat cottage cheese, cream, and butter. Other foods that may contain hidden lactose are: nondairy creamers, powdered artificial sweeteners, foods containing milk power or nonfat milk solids, bread, cake, margarine, creamed soups, pancakes, waffles, processed breakfast cereals, salad dressings, lunch meats, puddings, custards, confections, and some meat products. Lactose is also used as the base for more than 20 percent of prescription drugs and 6 percent of over-the-counter drugs.
For infants younger than two years of age, soy formulas are adequate substitutes for milk. Toddlers may drink rice or soymilk, while older children who are sensitive to lactose can take lactase enzymes, which are available without a prescription. Using the liquid form of lactase enzymes, children can add a few drops in their milk, put the milk in the refrigerator and drink it after 24 hours, when the lactase enzymes have reduced the lactose content by 70 percent. If the milk is heated first and double the amount of lactase liquid enzymes is added, the milk will be 90 percent lactose-free. Supermarkets also carry lactose-reduced milk and other products, which contain nutrients found in the regular products but without the lactose.
In the early 2000s, researchers have developed a chewable lactase enzyme tablet. Taking three to six tablets just before eating helps some children digest lactose-containing solid foods.
Eliminating milk from the diet can result in deficiencies of calcium, vitamin D, riboflavin, and protein. Milk substitutes for children are a necessity, as other sources of calcium are required. Fermented milk products such as yogurt are often tolerated. Buttermilk and cheeses have less lactose than milk. Goat's milk can sometimes be tolerated but should be consumed with meals.
Lactose intolerance is easy to manage and is not considered dangerous. People of all ages, but especially children, have to replace the calcium that is lost by cutting back on milk products; this can be accomplished by taking supplements and eating calcium-rich foods, such as broccoli, kale, canned salmon with bones, calcium-fortified foods, and tofu. They may also add lactase enzymes to dairy products to reduce lactose content as
Often lactose intolerance is a natural occurrence that cannot be avoided. However, people can prevent symptoms by managing the condition with diet and lactase supplements.
Parents must guard the health of a child who is lactose intolerant by carefully managing the child's diet to avoid foods that will result in symptoms while providing foods that contain necessary nutrients for the child's health and growth.
Glucose—A simple sugar that serves as the body's main source of energy.
Lactase—The enzyme produced by cells that line the small intestine that allows the body to break down lactose.
Lactose—A sugar found in milk and milk products.
Dobler, Merri Lou. Lactose Intolerance Nutrition Guide. Chicago, IL: American Dietetic Association, 2004.
The Official Patient's Sourcebook on Lactose Intolerance: A Revised and Updated Directory for the Internet Age. San Diego, CA: Icon Health Publications, 2002.
American Dietetic Association. 120 South Riverside Plaza, Suite 2000 Chicago, IL 60606–6995. Web site: <www.eatright.org/Public/>.
Judith Sims Lisette Hilton