THE EVOLUTION OF LACTASE PERSISTENCE: MILK CONSUMPTION, INSULIN-LIKE GROWTH FACTOR I, AND HUMAN LIFE-HISTORY PARAMETERS

For most mammals and humans, production of the intestinal enzyme lactase is a life-history trait that corresponds roughly to the duration of nursing. However, some human populations with long histories of using dairy animals have high frequencies of alleles for lactase persistence (LP), production of lactase throughout the life span. Several hypotheses have been proposed to account for this variation, but little evidence exists for specific fitness advantages of LP. Here I propose the hypothesis that LP alleles allow for milk consumption beyond infancy, which results in higher circulating insulin-like growth factor I (IGF-I) concentrations. IGF-I has been suggested as a mechanism that coordinates a variety of life-history outcomes. Milk is a source of IGF-I, but IGF-I is degraded in the processing of milk into dairy products. Serum IGF-I concentrations rise with milk intake, and less consistently with other dairy product intake. Higher circulating IGF-I may contribute to more rapid growth and larger body size, earlier sexual maturation, or positively impact other physiological systems. There may also be negative pleiotropic consequences that contribute to later life diseases such as cancer. This hypothesis describes a mechanism by which milk consumption could enhance fitness among ancestral dairying populations.