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Back to Lactose Intolerance and Milk Allergy

Lactose Intolerance: Definition and Diagnosis

Lactose intolerance is a condition that can affect some individuals who have a deficiency in the enzyme lactase resulting in an inability to digest lactose properly. To diagnose lactose intolerance, an objective test such as the hydrogen breath test is needed.


  • Lactose intolerance is a clinical syndrome characterized by the occurrence of gastrointestinal symptoms among individuals with lactose malabsorption;
  • If lactose intolerance is suspected, its confirmation via clinical diagnosis is important to avoid unnecessary dietary restrictions.

Definition of lactose intolerance

Lactose is a disaccharide found naturally in mammalian milk, including cow’s milk. During normal digestion, it is broken down into glucose and galactose in the small intestine by the enzyme lactase. However, some people are unable to properly digest and absorb lactose.

Lactose intolerance is a clinical syndrome and is defined as the onset of gastrointestinal symptoms following the ingestion of lactose by an individual with lactose malabsorption.1,2 In a blinded, single-dose challenge of ingested lactose, the same symptoms are not observed when the person ingests an indistinguishable placebo.1

The symptoms of lactose intolerance result from bacterial fermentation of undigested lactose in the colon.1 They are directly related to the quantity of lactose ingested and independent of the cause of lactose malabsorption.2 Common gastrointestinal symptoms that may occur after lactose ingestion in individuals with lactose intolerance include abdominal pain, diarrhea, flatulence and bloating.

Lactose malabsorption
Lactose malabsorption is the physiologic condition underlying lactose intolerance. Lactose malabsorption occurs due to a decreased ability to digest lactose caused by an insufficient amount of lactase in the small intestine.1,2

Lactase deficiency
The decline or absence of intestinal lactase is referred to as lactase deficiency. There are several types of lactase deficiency:2,3

  1. Lactase nonpersistence, also known as primary lactase deficiency, is the progressive physiological decline in intestinal lactase due to a decrease in the genetic expression of lactase in early childhood.
  2. Secondary lactase deficiency is a temporary condition caused by certain factors that injure the small intestinal mucosa, e.g. acute gastroenteritis, inflammatory diseases such as Crohn’s disease, celiac disease, and some medications. It can occur at any age and is reversible upon correction of the causative factor.
  3. Congenital lactase deficiency, also known as congenital alactasia, is an extremely rare autosomal recessive genetic disorder that has been reported in only a few infants. In these cases, newborns do not produce lactase and cannot digest lactose. The condition is evident as it results in severe diarrhea on first exposure to breast milk.

Diagnosis of lactose intolerance

If lactose intolerance is suspected, standardized and objective testing should be done to confirm diagnosis, as it is important to distinguish lactose intolerance from other causes of gastrointestinal symptoms. Self-diagnosis of lactose intolerance is often incorrect and can lead to unnecessary dietary restrictions.2

The hydrogen breath test is the most objective test to diagnose lactose malabsorption.1,2 The hydrogen breath test requires that patients consume a standard dose of lactose (usually 50 g of lactose, which is equivalent to the amount in 1 L of milk) after fasting. Hydrogen levels in the breath are then measured over a 3-hour period.2,4 The fermentation of undigested lactose by intestinal flora produces hydrogen, carbon dioxide and methane, which are eliminated by the lungs through the breath. These gases also cause bloating, flatulence, abdominal pain and diarrhea. Lactose malabsorption is diagnosed if hydrogen levels are elevated. In the case of lactose intolerance, gastrointestinal symptoms will also occur. The test is widely used, but its reliability depends on the activity of bacterial flora.4 False-negative tests can occur due to several conditions that may affect the intestinal flora, e.g. recent use of antibiotics or antimicrobial agents.

Some other less common tests used to diagnose lactose malabsorption are:2,4

  1. Intestinal biopsy
    Lactase activity from an intestinal sample is measured through direct biochemical assay. However, because of the invasiveness of intestinal biopsies, lactose tolerance tests have been developed.
  2. Lactose tolerance test
    This older test has mostly been replaced by the hydrogen breath test due to its high rate of false-negative and false-positive results. The test consists of taking a series of blood glucose measurements over 3 hours, after the ingestion of a lactose load. A diagnosis of lactose malabsorption is consistent with a rise in blood glucose of about <20 mg/dL. False-positive results are often obtained because of a lack of increase in blood glucose, which is attributable to normal insulin response.
  3. Genetic testing
    Genetic testing can be conducted for the common polymorphisms that are linked to lactase nonpersistence. However, other polymorphic variants can affect diagnostic accuracy.


Lactose malabsorption is most objectively diagnosed by the hydrogen breath test. However, the majority of people with lactose malabsorption do not have clinical lactose intolerance and do not experience gastrointestinal symptoms following lactose ingestion.

For information, see:

  1. Prevalence of Lactose Intolerance
  2. Lactose Intolerance: Health Authorities’ Recommendations


  1. Suchy FJ et al. NIH Consensus Development Conference Statement: lactose intolerance and health. NIH Consens State Sci Statements 2010;27:1-27.
  2. Heyman MB. Lactose intolerance in infants, children, and adolescents. Pediatrics 2006;118:1279-1286.
  3. Miller GD et al. Handbook of Dairy Foods and Nutrition, 3rd ed. Chapter 8: Lactose digestion. Boca Raton, FL: CRC Press. 2007.
  4. Mattar R et al. Lactose intolerance: diagnosis, genetic, and clinical factors. Clin Exp Gastroenterol 2012;5:113-121.

Keywords: lactose intolerance , lactose malabsorption , national institutes of health , prevalence

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