Lactase Deficiency: An Example of Dietary Evolution

The ability to assimilate lactose, a primary constituent of milk, is dependent upon the presence of lactase, a genetically controlled enzyme in the small intestine. In most mammals, levels of lactase are high during infancy, but fall rapidly following weaning; adults are lactase-deficient. In man, some populations, including Orientals, Australian Aborigines, and Negroes, are deficient as adults, while others, including Nilotic Negroes and Europeans, maintain consistently high levels of lactase throughout life regardless of individual lactose intake. Ingestion of even small amounts of lactose by lactase-deficient individuals usually gives rise to a variety of symptoms, including cramps, bloating, and diarrhea. It is hypothesized that prior to the domestication of animals and the development of dairying, the normal condition for all men was adult lactase deficiency, but that with the introduction of lactose into the adult diet in certain cultures, new selective pressures were created that favored the genotype for adult lactase production. A substantial body of medical literature dealing with lactase deficiency is reviewed in the light of this hypothesis, and strategies for further research are outlined. Finally, the broader implications of this illustration of the interaction between culture and human population gene frequencies (cultural genetics) are discussed.