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|
|Descendants of N. Europeans||5%|
|African American adults||79%|
|Mexican American males||55%|
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: