What is lactose intolerance?

Lactose intolerance is the inability to digest significant amounts of lactose, the predominant sugar found in 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 people deficient in lactase have symptoms, those who do are said to be lactose intolerant.

Common symptoms include nausea, cramps, bloating, wind, and diarrhoea, which begin about 30 minutes to two hours after eating or drinking foods containing lactose. The severity of symptoms varies according to 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, lactase deficiency or lactose maldigestion is a condition that develops 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.

Certain ethnic and racial populations are most likely to be affected, such as people of Hispanic, south Indian, African-Caribbean, and Asian ethnicity. The condition is least common among people of northern European descent. Individuals who are lactose intolerant do not tolerate any lactose, whereas the majority of people suffer from lactose maldigestion and can tolerate small amounts of lactose.

How is lactose intolerance diagnosed?

The most common tests for measuring 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 at hospital outpatient departments or by a GP.

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 lactase 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 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 gases, including hydrogen, are produced. The hydrogen is absorbed from the intestines, carried through the bloodstream to the lungs, and exhaled. In the test, the patient drinks a lactose-loaded beverage, and the breath is analysed at regular intervals. Raised levels of hydrogen in the breath indicate improper digestion of lactose. Certain foods, medications, and cigarettes can affect the test's accuracy and should be avoided before taking the test. This test is available for children and adults.