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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. (see figure 1). 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, though 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 include nausea, cramps, bloating, gas, and diarreah, which begin about thirty 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 instance, certain digestive diseases and injuries to the small intestine can reduce the amount of enzymes produced. In rare cases, children are born without the ability to produce lactase.
For most people, though, lactase deficiency is a condition that developes naturally over time. After about the age of two years, the body begins to produce less lactase. However, many people may not experience symptoms until they are much older.
Between thirty and fifty million americans are lactose intolerant. Certain ethnic and racial populations are more widly affected than others. As many as seventy five percent of all African-Americans and Native Americans and ninety percent of Asian Americans are lactose intolerant. The condition is least common among persons of northern European descent. Lactose intolerance is not the same as a milk allergy.
How is lactose intolerance diagnosed?
The most common tests used to measure the absorption of lactose in the digestive system are the lactose tolerance test, the hydrogen breath test, and the stool acidity test. These tests are performed on an outpatient basis at a hospital, clinic, or doctor's 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 down lactose into glucose and galactose. The liver then changes the galactose into glucose, which enters the blood stream 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 measures the amount of hydrogen in the breath. Normally, very little hydrogen is detectable in the breath. However, undigested lactose in the colon is fermented by bacteria, and various gasses, including hydrogen, are produced. The hydrogen is absorbed from the intestines and carried through the bloodstream to the lungs, and exhaled. In the test, 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 tests accuracy and should be avoided before taking the test. This test is available for children and adults.
The lactose intolerance and hydrogen breath tests are not given to infants and very young children who are suspected of being lactose intolerant. A large lactose load may be dangerous for very young individuals because they are more prone to dehydration that can result from diarrhea caused by the lactose. If a baby or young child is experiencing the symptoms of lactose intolerance, many pediatricians simply recommend changing from cow's milk to soy formula and waiting for symptoms to abate.
If necessary, a stool acidity test, which measures the amount of acid in the stool, may be given to infants and young children. 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.
How is lactose intolerance treated?
Fortunately, lactose intolerance is relatively easy to treat. No treatment exists to improve the body's ability to produce lactase, but symptoms can be controlled through diet.
Young children with lactase deficiency should not eat any foods containing lactose. Most older children and adults need not avoid lactose completely, but individuals differ in the amounts of lactose they can handle. For example, one person may suffer 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 each person's learning through trial and error how much lactose he or she can handle.
For those who react to very small amounts of lactose or have trouble limiting their intake of foods containing lactose, lactase enzymes are available without a perscription. One form is liquid for use with milk. A few drops are added to a quart of milk, and after twenty four hours in the refrigerator, the lactose content is reduced by seventy percent. The process works faster if the milk is heated first, and adding a double amount of lactase liquid produces milk that is ninety percent lactose free. A more recent development is a chewable lactase enzyme tablet that helps people digest solid foods that contain lactose. Three to six tablets are taken just before a meal or snack.
Lactose-reduced milk and other products are available at many 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.
some of the above information used with permission from NIDDK
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