What is lactose intolerance?
Lactose intolerance is a condition that occurs when there is decreased ability of the body to digest a type of sugar called lactose. Lactose is 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. It is a disaccharide, which means it is composed of two simple sugars joined together. Before it can be absorbed and used by the body, lactose must be broken down into its simpler sugars of glucose and galactose monosaccharides). This digestion step is initiated by lactase, an enzyme produced by cells lining the small intestine.
Production of the enzyme lactase begins in a developing baby during pregnancy and peaks near birth. Normal lactase production mirrors an infant's need for the enzyme when milk is the primary source of nutrition. Almost all babies can digest milk, although premature infants may initially have some degree of intolerance. Lactase levels naturally decrease after the first couple of years i.e. when the diet is less reliant on milk and dairy products. Levels continue to decline as an individual ages.
If an individual does not produce enough lactase (they are lactase deficient), then the undigested lactose passes through the small intestine to the large intestine. In the large intestine, bacteria break down the lactose producing excess hydrogen gas and lactic acid and drawing extra fluid into the gut. Combined, these factors result in the typical symptoms of lactose intolerance; diarrhoea, flatulence and abdominal cramps.
Lactose intolerance can be divided into three clinical syndromes: primary adult lactase deficiency (hypolactasia), congenital lactase deficiency (alactasia), and secondary lactase deficiency.
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 incidence of primary adult lactase deficiency varies based on racial and ethnic distribution. Some people, primarily northern Europeans, have an inherited 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.
Asians, Africans and Native Americans have the highest incidence of lactose intolerance in adults. This is because, traditionally their diets do not contain much lactose, and so the ability of their digestive system to produce lactase has declined.
| Ethnic Group or Race | % of Adults with Lactose Intolerance |
| Northern Europeans | 4-11% |
| Eastern and Southern Europeans | 14-38% |
| Southern and Eastern Indians | 77% |
| Hispanics | 50-80% |
| Africans | 80% |
| Asian and Native Americans | 95-100% |
Congenital Lactase Deficiency
Congenital lactase deficiency 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. It is caused by mutations within the gene that controls lactase production (the LCT gene). Congenital lactase deficiency will occur if a mutation in the gene is inherited from each parent – it is an autosomal recessive inherited condition.
Secondary 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. In some individuals, stomach contents move more rapidly than usual through the small intestine. They too may experience decreased lactose absorption due to insufficient time for lactose digestion to occur.
Secondary causes of lactose intolerance include:
- Coeliac disease
- Chemotherapy or radiation therapy
- Inflammatory bowel disease (IBD)
- Infections caused by parasites
- Overgrowth of bacteria
- Short bowel syndrome
Testing is important to differentiate the various conditions that have similar signs and symptoms, but have different prognoses and treatments. Treatment of the underlying condition may reverse the lactose intolerance.
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
- Diarrhoea
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.
Tests of lactose intolerance are not always needed. If a doctor suspects a patient has lactose intolerance, then a trial of a lactose free diet may be recommended. If the symptoms improve then this is consistent with lactose intolerance (although it is not diagnostic), and further testing will often not be necessary.
Laboratory Tests
The goals of 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.
- Stool acidity (pH) test – rarely performed, usually on infants or children who cannot undergo other types of testing; stool can be acidic with lactose intolerance.
- Additional specialist testing that is rarely requested includes:
- Lactase activity performed on a biopsy from the small intestine
- Genetic testing of the LCT (lactase) gene
Lactose intolerance cannot be prevented or "cured" but it can be managed. Most people who have lactose intolerance can tolerate small amounts of dairy products. Affected individuals should discuss with their doctor or other health care provider actions that can be taken to avoid symptoms. Examples include:
- Eating 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
- Tablets 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 doctor 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 formula feeds that do not contain milk.