Lactose Intolerance

Lactose intolerance is the constellation of gastrointestinal symptoms — bloating, abdominal pain, flatulence, and diarrhoea — that follows lactose ingestion in people whose small intestine produces too little of the brush-border enzyme lactase. It is the most common form of disaccharidase deficiency and a prototype of a single-enzyme malabsorption. This topic explains why undigested lactose causes symptoms and how lactase activity varies across populations.

Definition

Lactose intolerance is the occurrence of gastrointestinal symptoms attributable to lactose malabsorption, which results from reduced activity of the brush-border enzyme lactase (lactase-phlorizin hydrolase); when symptoms are absent, the underlying state is termed lactose malabsorption rather than intolerance.

Scope

The entry covers the digestion of lactose by intestinal lactase, the genetics of lactase persistence and non-persistence, the difference between lactose malabsorption and symptomatic lactose intolerance, and the secondary causes of lactase deficiency. It also frames lactose intolerance within the broader category of disaccharidase deficiencies. The content is reference material, not personal dietary or medical advice.

Core questions

• How is lactose normally digested and absorbed in the small intestine?
• Why does lactase activity decline after weaning in most of humanity?
• What distinguishes lactose malabsorption from symptomatic lactose intolerance?
• What primary and secondary causes reduce lactase activity?

Key concepts

• Lactase (lactase-phlorizin hydrolase) and brush-border hydrolysis
• Lactase persistence versus non-persistence
• Adult-type hypolactasia (primary lactase deficiency)
• Lactose malabsorption versus lactose intolerance
• Secondary lactase deficiency from mucosal injury
• Hydrogen breath testing

Mechanisms

Dietary lactose is a disaccharide that must be split by the brush-border enzyme lactase into glucose and galactose before its components can be absorbed (Kiela & Ghishan, 2016). When lactase activity is low, unhydrolysed lactose passes into the colon, where it exerts an osmotic effect and is fermented by gut bacteria, producing gas and short-chain fatty acids that cause the bloating, pain, and diarrhoea of lactose intolerance (Misselwitz, Butter, Verbeke, & Fox, 2019). In most of the world's people, lactase expression declines after weaning (lactase non-persistence), whereas a regulatory genetic variant upstream of the lactase gene confers continued, lifelong lactase production (lactase persistence) (Enattah et al., 2002). Lactase deficiency can also be secondary, arising when mucosal injury from other diseases damages the enzyme-bearing brush border.

Clinical relevance

Lactose intolerance is a frequent cause of recurrent abdominal symptoms and is often distinguished from milk allergy and other functional bowel conditions; it can also signal underlying mucosal disease when it appears as a secondary deficiency. This entry describes the condition for reference; it does not prescribe specific diets or tests, which should be individualised by a clinician.

Epidemiology

Lactase non-persistence is the ancestral human condition and affects a large majority of the global population, with marked geographic and ethnic variation: lactase persistence is common in northern European and some pastoralist populations and much less common across much of Asia, Africa, and the Americas. The proportion of people with lactose malabsorption who experience symptoms depends on the dose ingested and other factors (Misselwitz et al., 2019).

Evidence & guidelines

The pathophysiology and diagnostic approach are summarised in major reviews such as Misselwitz et al. (2019), and the genetic basis of adult-type hypolactasia was established by Enattah et al. (2002). These sources, together with breath-test methodology described in the literature, constitute the reference evidence for this topic.

History

Lactose intolerance was clarified in the mid-twentieth century when reduced intestinal lactase activity was linked to the symptoms following milk ingestion, and population studies revealed striking ethnic differences in adult lactase levels. The genetic explanation followed in 2002, when a regulatory variant near the lactase gene was associated with adult-type hypolactasia, recasting lactase non-persistence as the normal human default and persistence as a derived, geographically clustered trait shaped by the history of dairying.

Key figures

• Nabil Sabri Enattah
• Mark R. Fox

Seminal works

• misselwitz-2019
• enattah-2002

Frequently asked questions

What is the difference between lactose malabsorption and lactose intolerance?

Lactose malabsorption is the failure to fully digest lactose because of low lactase activity, whereas lactose intolerance is the presence of symptoms (such as bloating and diarrhoea) caused by that malabsorption; a person can malabsorb lactose without having symptoms.

Is lactose intolerance the same as a milk allergy?

No. Lactose intolerance is an enzyme-deficiency problem with digesting milk sugar, while a milk allergy is an immune reaction to milk proteins; they have different mechanisms and are evaluated differently.