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. Worldwide around 68% of the adult population have lactose intolerance.
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:
2. Chemotherapy or radiation therapy
3. Inflammatory bowel disease (IBD)
4. Infections caused by parasites
5. Overgrowth of bacteria
6. 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.