Lactose Intolerance - Lactase Persistence

Lactase Persistence

Lactase persistence is the phenotype associated with various autosomal dominant alleles prolonging the activity of lactase beyond infancy; conversely, lactase non-persistence is the phenotype associated with primary lactase deficiency (see above). Among mammals, lactase persistence is unique to humans — it evolved relatively recently (in the last 10,000 years) among some populations, and the majority of people worldwide remain lactase non-persistent. For this reason lactase persistence is of some interest to the fields of anthropology and human genetics, which typically use the genetically derived persistence/non-persistence terminology.

Recognition of the extent and genetic basis of lactose intolerance is relatively recent. Though its symptoms were described as early as Hippocrates (460-370 B.C.), until the 1960s the prevailing assumption in the medical community was that tolerance was the norm and intolerance either the result of milk allergy, an intestinal pathogen, or else was psychosomatic (it being recognised that some cultures did not practice dairying, and people from those cultures often reacted badly to consuming milk). There were two reasons for this perception. Firstly, many Western countries have a predominantly European heritage, and so have low frequencies of lactose intolerance, and have an extensive cultural history of dairying. Therefore, tolerance actually was the norm in most of the societies investigated by medical researchers at that point. Secondly, within even these societies lactose intolerance tends to be under-reported: genetically lactase non-persistent individuals can tolerate varying quantities of lactose before showing symptoms, and their symptoms differ in severity. Most are able to digest a small quantity of milk, for example in tea or coffee, without suffering any adverse effects. Fermented dairy products, such as cheese, also contain dramatically less lactose than plain milk. Therefore, in societies where tolerance is the norm, many people who consume only small amounts of dairy or have only mild symptoms, may be unaware that they cannot digest lactose. Eventually, however, it was recognised that in the United States lactose intolerance is correlated with race. Subsequent research revealed that intolerance was the worldwide norm, and that the variation was genetic. However, as yet there is no comprehensive understanding of either the global distribution of lactase persistence, the number of alleles that cause it, or the reasons for its recent selection.