Most people lose the ability to produce lactase in adulthood. Only in some populations is the ability to digest lactose widespread in adults. These figures represent various methods of assessing adult lactose tolerance. Also note that some of the populations (for example Australian Aborigines) may have received the adult-lactase-persistence form of the gene by recent interbreeding.
Sources: Robert D. McCracken, "Lactase Deficiency: An Example of Dietary Evolution,"
Current Anthropology 12 (Oct.-Dec. 1971, pp. 479-517), Norman Kretchner, "Lactose
and Lactase," Scientific American 277 (Oct. 1972, pp. 71-78), and others. Largely lifted from this site.
This table is referred to in the post on Evolution of Adult Lactose Tolerance at Science in Action.
|POPULATION||LACTOSE INTOLERANT ADULTS|
|U.S. & Mexico|| |
|Southern Sudan (cattle herders)||17%|
|Ibo and Yoruba (Nigeria)||99%|
|Masai, East Africa||60%|