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Lactose intolerance

A common digestive condition caused by insufficient lactase enzyme, reducing the ability to digest lactose in milk and many dairy products; covers causes, symptoms, diagnosis and management.

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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.

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Pig lactation
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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.

Questions and answers

Q: What is lactose intolerance?

A: Lactose intolerance is when a person cannot digest milk or milk products, such as cheese or yogurt. This is because they do not have the enzyme called lactase which breaks down the double sugar molecule of lactose into two simple sugars, glucose and galactose.

Q: How common is it?

A: It varies from 5% in northern Europe to over 71% in Sicily and over 90% in parts of Africa and Asia.

Q: Is there a cure for lactose intolerance?

A: No, there is no cure for lactose intolerance. Those people need to change their diet to eat and drink substances with very little lactose. There are many milk substitutes available.

Q: Is milk allergy the same as lactose intolerance?

A: No, they are not the same thing. Milk allergy is an immune reaction to some of the proteins in milk whereas lactose intolerance occurs when someone does not have enough of the enzyme needed to break down the double sugar molecule of lactose into two simple sugars, glucose and galactose.

Q: What happens if someone with lactose intolerance drinks milk?

A: If someone with lactose intolerance drinks milk that has not been digested properly then it may ferment in their small intestine which can cause a problem called pseudoallergy where amino acids are changed to other substances which may act similarly to histamine in a true allergy.

Q: Are all mammals born drinking mother's milk?

A: Yes, all mammals start off on mother's milk but almost all change to a non-milk diet at some point - this process we call weaning.

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