Lactose intolerance is the inability to digest significant amounts of lactose, the predominant sugar of milk. This inability results from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine. Lactase breaks down milk sugar into simpler forms that can then be absorbed into the bloodstream.
When there is not enough lactase to digest the amount of lactose consumed, the results, although not usually dangerous, may be very distressing. While not all persons deficient in lactase have symptoms, those who do are considered to be lactose intolerant.
Common symptoms of lactose intolerance include:
These symptoms begin about 30 minutes to two hours after eating or drinking foods containing lactose. The severity of symptoms varies depending on the amount of lactose each individual can tolerate.
Some causes of lactose intolerance are well known. For most people, lactase deficiency is a condition that develops naturally over time. After age 2, the body begins to produce less lactase. However, many people may not experience symptoms until they are much older.
Other, less common causes include certain digestive diseases and injuries to the small intestine, which can reduce the amount of enzymes produced. In rare cases, children are born without the ability to produce lactase.
Between 30 and 50 million Americans are lactose intolerant. Certain ethnic and racial populations are more widely affected than others. As many as 75 percent of all African Americans and American Indians and 90 percent of Asian Americans are lactose intolerant. The condition is least common among persons of northern European descent.
Researchers have identified a genetic variation associated with lactose intolerance; this discovery may be useful in developing a diagnostic test to identify people with this condition.
What are the most common tests used to measure the absorption of lactose in the digestive system?
- Lactose tolerance test
- Hydrogen breath test
- Stool acidity test
These tests are performed on an outpatient basis at a hospital, clinic, or doctors office.
The lactose tolerance test begins with the individual fasting (not eating) before the test and then drinking a liquid that contains lactose. Several blood samples are taken over a two-hour period to measure the person's blood glucose (blood sugar) level, which indicates how well the body is able to digest lactose.
Normally, when lactose reaches the digestive system, the lactase enzyme breaks it down into glucose and galactose. The liver then changes the galactose into glucose, which enters the bloodstream and raises the person's blood glucose level. If lactose is incompletely broken down, the blood glucose level does not rise and a diagnosis of lactose intolerance is confirmed.
The hydrogen breath test is available for children and adults and measures the amount of hydrogen in a person's breath. Normally, very little hydrogen is detectable. However, undigested lactose in the colon is fermented by bacteria, and various gases, including hydrogen, are produced.
The hydrogen is absorbed from the intestines, carried through the bloodstream to the lungs and exhaled. The patient drinks a lactose-loaded beverage and the breath is analyzed at regular intervals. Raised levels of hydrogen in the breath indicate improper digestion of lactose. Certain foods, medications and cigarettes can affect the accuracy of the test and should be avoided before taking it.
If necessary, a stool acidity test, which measures the amount of acid in the stool, is preferred for infants and young children. The other tests may cause diarrhea -- and consequently, dehydration -- in these young patients. Undigested lactose fermented by bacteria in the colon creates lactic acid and other short-chain fatty acids that can be detected in a stool sample. In addition, glucose may be present in the sample as a result of unabsorbed lactose in the colon.
Fortunately, lactose intolerance is relatively easy to treat. No treatment can improve the body's ability to produce lactase, but symptoms can be controlled through diet.
How much lactose is too much? Young children with lactase deficiency should not eat any foods containing lactose. Most older children and adults need not avoid lactose completely. However, people differ in the amounts and types of foods they can handle.
For example, one person may have symptoms after drinking a small glass of milk, while another can drink one glass but not two. Others may be able to manage ice cream and aged cheeses, such as cheddar and Swiss, but not other dairy products. Dietary control of lactose intolerance depends on people learning through trial and error how much lactose they can handle.
For those who react to very small amounts of lactose or have trouble limiting their intake of foods that contain it, lactase enzymes are available without a prescription to help people digest foods that contain lactose:
- Tablets can be taken with the first bite of dairy food.
- Lactase enzyme is also available as a liquid. Adding a few drops of the enzyme will convert the lactose in milk or cream, making it more digestible for people with lactose intolerance.
Lactose-reduced milk and other products are available at most supermarkets. The milk contains all of the nutrients found in regular milk and remains fresh for about the same length of time, or longer if it is super-pasteurized.
Diet and Nutrition
Milk and other dairy products are a major source of nutrients in the American diet. The most important of these nutrients is calcium. Calcium is essential for the growth and repair of bones throughout life. In the middle and later years, a shortage of calcium may lead to thin, fragile bones that break easily, a condition called osteoporosis. A concern, then, for both children and adults with lactose intolerance, is getting enough calcium in a diet that includes little or no milk.
In planning meals, making sure that each day's diet includes enough calcium is important, even if the diet does not contain dairy products.
Many nondairy foods are high in calcium, including:
- Green vegetables, such as broccoli and kale
- Fish with soft, edible bones, such as salmon and sardines
- Fortified orange juice
However, factors other than calcium and lactose content should be kept in mind when planning a diet. Calcium is absorbed and used only when there is enough vitamin D in the body. A balanced diet should provide an adequate supply of vitamin D. Sources of vitamin D include eggs and liver. However, sunlight helps the body naturally absorb or synthesize vitamin D, and with enough exposure to the sun, food sources may not be necessary.
Some people with lactose intolerance may think they are not getting enough calcium and vitamin D in their diet. Consultation with a doctor or dietitian may be helpful in deciding whether any dietary supplements are needed. Taking vitamins or minerals of the wrong kind or in the wrong amounts can be harmful. A dietitian can help in planning meals that will provide the most nutrients with the least chance of causing discomfort.
Although milk and foods made from milk are the only natural sources, lactose is often added to prepared foods. People with very low tolerance for lactose should know about the many food products that may contain even small amounts of lactose, such as:
- Bread and other baked goods
- Processed breakfast cereals
- Instant potatoes, soups, and breakfast drinks
- Lunch meats (other than kosher)
- Salad dressings
- Candies and other snacks
- Mixes for pancakes, biscuits, and cookies
- Powdered meal-replacement supplements
Some products labeled nondairy, such as powdered coffee creamer and whipped toppings, may also include ingredients that are derived from milk and therefore contain lactose. Smart shoppers learn to read food labels with care.
If any of these are listed on a label, the product contains lactose:
- Milk by-products
- Dry milk solids
- Nonfat dry milk powder
In addition, lactose is used as the base for more than 20 percent of prescription drugs and about 6 percent of over-the-counter medicines. Many types of birth control pills, for example, contain lactose, as do some tablets for stomach acid and gas. However, these products typically affect only people with severe lactose intolerance.