Overview

Lactose intolerance is a digestive disorder in which the small intestine does not produce enough of the enzyme lactase to split lactose, the sugar found in milk and many dairy products, into absorbable simple sugars. When lactose is not digested in the small intestine it passes into the colon, where bacteria ferment it and produce gas and other byproducts. Symptoms typically include bloating, abdominal pain, wind, and loose stools that occur after consuming lactose-containing foods or drinks.

How it works and main characteristics

Lactose is a disaccharide composed of glucose and galactose. Lactase, produced by cells lining the small intestine, cleaves lactose into these monosaccharides so they can be absorbed. Reduced lactase levels mean lactose remains intact and reaches the colon. Bacterial fermentation then generates hydrogen, methane and short-chain fatty acids; the gases cause discomfort while the acids can draw water into the bowel, producing diarrhea.

Causes and types

Lactose intolerance arises in several ways. The most common form is primary lactase nonpersistence, a gradual decline in lactase after childhood that affects a large portion of the global population. Secondary lactose intolerance results from intestinal injury or illness (for example gastroenteritis, celiac disease or inflammatory bowel disease) and may be temporary. A rare congenital form exists when infants are born with little or no lactase due to genetic causes and present with severe symptoms from the first feedings.

Symptoms and diagnosis

Typical symptoms appear within a few hours of lactose ingestion and include cramping, bloating, flatulence and watery stools. Diagnosis can be suggested by symptom patterns and confirmed with tests such as the hydrogen breath test, lactose tolerance blood test, stool acidity test in young children, or genetic tests for lactase persistence variants. A supervised elimination and reintroduction of lactose under medical guidance is also commonly used to identify intolerance.

Management and dietary approaches

There is no cure for lactase nonpersistence, but symptoms are usually manageable. Strategies include reducing or avoiding lactose-containing foods, choosing lactose-free or low-lactose dairy products, consuming fermented dairy (yogurt and many aged cheeses) that contain less lactose, or taking over-the-counter lactase enzyme preparations with meals. People following restrictive diets should ensure adequate calcium and vitamin D intake through fortified foods or supplements and consider dietetic advice to maintain balanced nutrition.

Epidemiology, history and notable facts

The prevalence of lactase nonpersistence varies widely by population: only a small minority of people in some northern European groups retain high lactase activity into adulthood, while many adults in parts of Africa, Asia and southern Europe have reduced lactase and symptomatic lactose intolerance. The capacity to digest lactose as an adult (lactase persistence) has evolved in certain populations alongside the cultural practice of dairying. This variation underlies differences in diet and public health approaches to dairy consumption around the world.

Distinction from milk allergy

Lactose intolerance is not the same as a milk allergy. Lactose intolerance is an enzyme deficiency and causes digestive symptoms; milk allergy is an immune response to milk proteins and can cause hives, respiratory symptoms, vomiting and, in severe cases, anaphylaxis. Because the mechanisms differ, the diagnostic tests and treatments for each condition are also different.

Further resources

If symptoms are persistent or severe, consult a healthcare professional for testing and individualized dietary advice.