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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 some (rare) cases, babies have'' congenital lactase deficiency'', which prevents them from being able to digest even human 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, 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). However, it is not clear whether digestion needs to be complete to avoid symptoms. 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 lactose production when it is not needed in cultures in which dairy products not an available food source. But this is highly confounded by the fact that polymorphisms are associated with natural non-selective variation in the human genome. Many studies show that individuals with a similar genetic background can have different tolerances to lactose based on exposure more so than genetics and that one can acquire tolerance from repeated exposure,〔 but children who immigrate into dairy rich environments tend to consume dairy products at a rate that is close to that of that population's average. 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. It is important to recognize that the genetic definition of lactose intolerance is not the same as an operative one, in which one experiences symptoms. == Terminology == "Lactose intolerance" primarily refers to a syndrome having one or more symptoms upon the consumption of food substances containing lactose. Individuals may be lactose intolerant to varying degrees, depending on the severity of these symptoms. "Lactose malabsorption" refers to the physiological concomitant of lactase deficiency (i.e., the body does not have sufficient lactase capacity to digest the amount of lactose ingested). Lactase deficiency has a number of causes and is classified as one of three types: * Primary lactase deficiency is genetic, only affects adults, and is caused by the absence of a lactase persistence allele.〔 It is the most common cause of lactose intolerance, as a majority of the world's population lacks these alleles. It is estimated that primary lactase deficiency affects 60% of the world's adult population.〔http://usatoday30.usatoday.com/tech/science/2009-08-30-lactose-intolerance_N.htm〕 * Secondary, acquired, or transient lactase deficiency is caused by an injury to the small intestine, usually during infancy, from acute gastroenteritis, diarrhea, chemotherapy, intestinal parasites, or other environmental causes.〔 * Congenital lactase deficiency is a very rare, autosomal recessive genetic disorder that prevents lactase expression from birth.〔 It is particularly common in Finland. People with congenital lactase deficiency cannot digest lactose from birth, so cannot digest breast milk. It is most common in Finnish infants, occurring in 1 in 60,000 of the population.〔() Lactose Intolerance〕 Lactose intolerance is not an allergy, because it is not an immune response, but rather a sensitivity to dairy caused by lactase deficiency. Milk allergy, occurring in only 4% of the population, is a separate condition, with distinct symptoms that occur when the presence of milk proteins trigger an immune reaction. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「lactose intolerance」の詳細全文を読む スポンサード リンク
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