What is "lactose intolerance"

lactose intolerance \lac"tose` in*tol"er*ance\ (l[a^]k"t[=o]s` [i^]n*t[o^]l"[~e]r*ans), n. A physiological condition in which lactose cannot be hydrolyzed in the intestine, leading to uncomfortable gaseousness, cramps, or diarrhea after eating a lactose-containing food such as milk or ice cream. The symptoms may be prevented by ingesting a preparation of beta-galactosidase before or together with dairy products.

n. inability to fully metabolize lactose.

n. congenital disorder consisting of an inability to digest milk and milk products; absence or deficiency of lactase results in an inability to hydrolyze lactose [syn: lactase deficiency, milk intolerance]

Lactose intolerance is the inability of adults and children to digest lactose, a sugar found in milk and to a lesser extent dairy products, causing side effects. It is due to a lactase deficiency, or hypolactasia. In extremely rare cases, mainly limited to Finland, individuals have congenital alactasia, a total absence of lactase caused by a genetic defect, which prevents them from being able to digest lactose from infancy, including breast milk.

Lactose intolerant individuals have insufficient levels of lactase, an enzyme that catalyzes the hydrolysis of lactose into glucose and galactose, in their digestive system. In most cases, this causes symptoms which may include abdominal bloating and cramps, flatulence, diarrhea, nausea, borborygmi (rumbling stomach), or vomiting after consuming significant amounts of lactose. It is common for patients with inflammatory bowel disease to experience gastrointestinal symptoms after lactose ingestion, although the prevalence of lactase deficiency in this population has not been well studied.

Most mammals normally cease to produce lactase (lactase nonpersistence), becoming lactose intolerant, after weaning, but some human populations have developed lactase persistence, in which lactase production continues into adulthood which likely developed as a response to growing benefits of being able to digest the milk of farm animals such as cattle. Research reveals intolerance to be more common globally than lactase persistence, and that the variation has been tied to genetics, but that the largest source of variation has been shown to be based on exposure (e.g., cultures that consume dairy). The frequency of lactose intolerance ranges from 5% in Northern European to more than 90% in some African and Asian countries. Some have argued that this links intolerance to natural selection favoring lactase-persistent individuals, but it is also consistent with a physiological response to decrease lactase production when it is not needed in cultures in which dairy products are not an available food source. Although populations in Europe, India, Arabia, and Africa were first thought to have high frequencies of lactase persistence because of a single mutation, lactase persistence has been traced to a number of mutations that occurred independently.

Healthy people with lactase nonpersistence are able to consume at least one cup of milk without developing symptoms and this tolerance is improved by consuming the milk with a meal (preferably lactose-reduced milk), adding lactase supplements, or replacing milk by yogurt or hard cheeses.