• Also Known As:
  • Lactase Deficiency
  • Carbohydrate Intolerance
  • Disaccharidase Deficiency
  • Congenital Alactasia
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What is lactose intolerance?

Lactose intolerance is the decreased ability to digest lactose, a type of sugar found in the milk of mammals such as cows and goats as well as in human breast milk. It is also an ingredient in other dairy products such as cheese, cottage cheese, yoghurt, ice cream, and butter. People who are lactose intolerant can develop abdominal symptoms within 30 minutes to 2 hours of consuming dairy products. The severity of symptoms depends on the type and amount of dairy product consumed and often varies from person to person, changing as a person ages.

Lactose is a sugar with a complex structure (a disaccharide). Before it can be absorbed and used by the body, lactose must be broken down into the simpler sugars of glucose and galactose (monosaccharides). This digestion step is initiated by lactase, an enzyme produced by cells lining the small intestine. If an individual does not produce enough lactase, then the undigested lactose passes through the small intestine to the large intestine, where bacteria break down the lactose, producing excess hydrogen gas and lactic acid, which inhibits salt and water absorption. This results in diarrhea and abdominal cramps.

Production of the enzyme lactase begins in a developing baby during pregnancy and peaks near birth. Almost all babies can digest milk, although premature infants may initially have some degree of intolerance. Normal lactase production mirrors an infant’s consumption of milk as its primary source of nutrition. The lactase level decreases after the first couple of years and continues to decline as an individual ages.

However, about 35% of adults in the world continue to produce lactase throughout adulthood and are thus capable of digesting lactose with no or minimal symptoms. This is known as lactase persistence (LP) and is associated with an evolutionary development among specific races and ethnicities. Northern European groups have the highest rate of lactase persistence, thus the lowest incidence of lactose intolerance, while Asians and Native Americans have the highest incidence of lactose intolerance in adults:

Ethnic Group or Race % of Adults with Lactose Intolerance
Southeast Asians 98%
Asian Americans 90%
Indian adults 50%
Descendants of N. Europeans 5%
African American adults 79%
Mexican American males 55%


About Lactose Intolerance


In addition to the natural decline of lactase production with age, a variety of diseases and conditions can cause a secondary form of lactose intolerance. Damage to the small intestines and/or causes of general malabsorption may lead to the inability to absorb lactose properly. Celiac disease, radiation therapy, chemotherapy, inflammatory bowel disease (IBS), and infections caused by parasites or an overgrowth of bacteria can lead to decreased lactose digestion. In some individuals, stomach contents move more rapidly than usual through the small intestine. They too may experience lactose intolerance due to insufficient time for lactose digestion to occur.

Lactase deficiency can be divided into three clinical syndromes: congenital lactase deficiency (alactasia), primary adult lactase deficiency (hypolactasia), and secondary lactase deficiency.

Congenital Lactase Deficiency
Two genes are associated with the production of the lactase enzyme: the LCT gene controls lactase production and the MCM6 gene controls the expression of the LCT gene. Mutations of the LCT gene result in congenital lactase deficiency and are an autosomal recessive inheritance, thus requiring one mutated gene from each parent. This is a rare form of lactose intolerance that begins at birth and severely inhibits a baby’s ability to digest milk or milk-based formulas.

Primary Adult Lactase Deficiency
This is the most common form of lactose intolerance and is associated with the decreased production of lactase in adults. The decreased production of this enzyme renders an individual less likely to properly digest lactose; the intensity varies based on racial and ethnic distribution. The MCM6 gene regulates the LCT gene, resulting in a decrease in the production of lactose over time (nonpersistance) and is considered a normal “wild type” gene. Some people, primarily northern Europeans, have an inherited MCM6 gene mutation that leads to persistence in the ability to digest lactose into adulthood. People without this mutation lose the ability to produce lactase as they age, resulting in lactose intolerance.

Secondary Lactose Intolerance
Many of the symptoms of lactose intolerance can also be seen with other gastrointestinal conditions generally caused by disease or damage to the small intestine that result in a loss of lactase. Testing is important to differentiate among the various conditions that have similar signs and symptoms, but have different prognoses and treatments. Treatment of the primary condition may reverse the lactose intolerance. Other conditions that may include similar symptoms are:

  • Celiac sprue
  • Crohn disease
  • Bacterial growth syndrome
  • Whipple disease
  • Tropical sprue
  • Short bowel syndrome
  • AIDS
  • Cystic fibrosis
  • Giardia infections
  • Zollenger Ellison syndrome
  • Chemo or radiation therapy

Signs and Symptoms

Signs and symptoms associated with lactose intolerance are due primarily to undigested lactose reaching the large intestine. Extra fluids are drawn into the large intestine, where bacteria break down the lactose, producing hydrogen gas and lactic acid. This can cause:

  • Abdominal pain and cramps
  • Abdominal bloating
  • Excessive gas, flatulence
  • Nausea
  • Diarrhea

The type and severity of symptoms vary by individual and by the amount and kind of dairy products consumed. Many people with lactose intolerance can consume small amounts of dairy products with few to no symptoms.

Lactose intolerance should not be confused with an allergy to cow’s milk. A milk allergy involves a reaction by the immune system and is directed against a protein in the milk, not a sugar. Although some of the symptoms may be similar, a person with a milk allergy can have a mild to severe or even life-threatening reaction within minutes to hours after drinking milk. In addition to digestive symptoms, those with a milk allergy may experience hives and wheezing. Lactose intolerance is more common in adults than in children. Milk allergies are common in young children but are also frequently outgrown.


Laboratory Tests
The goals with testing are to diagnose lactose intolerance and distinguish it from other conditions with similar symptoms. Testing may include:

  • Lactose tolerance tests, including:
    • Hydrogen breath test – currently, this is the most reliable test used to diagnose lactose intolerance. Breath samples are taken before and at timed intervals after a patient drinks a fluid containing a standard amount of lactose. With lactose intolerance, undigested lactose is broken down by bacteria in the large intestine, producing excess hydrogen gas. The hydrogen moves to the circulation and is eventually exhaled by the lungs. Increasing levels of hydrogen in the breath samples over time strongly indicates lactose intolerance.
    • Lactose tolerance blood test – sometimes performed to help diagnose lactose intolerance; blood samples are collected and tested for glucose at timed intervals after a standard oral dose of lactose is given. If glucose levels do not rise, then lactose intolerance may be present. This test is rarely done in clinical practice.
  • Stool acidity (pH) test – occasionally performed, usually on infants or children who cannot undergo other types of testing; stool can be acidic with lactose intolerance.
  • Potential testing that is rarely ordered clinically includes:
    • Lactase activity performed on a biopsy from the small intestine
    • Genetic testing of the LCT (lactase) gene

Non-Laboratory Tests
A healthcare practitioner may have a patient abstain from all dairy products for a couple of weeks to see if symptoms resolve (dietary elimination), then re-introduce dairy to see if symptoms re-emerge. This is not diagnostic of lactose intolerance but is consistent with it.


Lactose intolerance cannot be prevented or “cured” but can be managed. Most people who have lactose intolerance can tolerate small amounts of dairy products. Affected individuals should discuss with their healthcare provider actions that can be taken to avoid symptoms. Examples include:

  • Eat smaller amounts of dairy products spread throughout the day.
  • Eat dairy products with other foods to slow their passage through the intestines.
  • Eat dairy products that have lower lactose levels, such as yogurts and hard cheeses.
  • Drink and eat lactose-reduced milk and other dairy products.
  • Pills and liquids that contain the lactase enzyme are available and may be taken before or with meals to help digest any lactose present and avoid symptoms.
  • Substitute cow’s milk with soy or rice milk, which do not contain lactose.
  • Obtain calcium from other sources besides dairy products, such as spinach, broccoli, salmon, sardines, and dried beans; consider talking to your healthcare provider about the need for calcium supplements.

Those who cannot tolerate even small amounts of lactose should be aware that it is present in many forms in processed foods and may appear as additives such as whey, dry milk solids, and milk by-products.

For individuals with underlying conditions that cause lactose intolerance, treatment and/or resolution of the underlying cause may improve or reverse the intolerance.

Babies who have been diagnosed with rare congenital lactose intolerance may need to be given special formulas that do not contain milk.

View Sources

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