Early influences on human energy regulation: thrifty genotypes and thrifty phenotypes.

Early influences on human ingestive behavior and other aspects of energy homeostasis can be defined according to two very different time scales: the evolutionary time frame responsible for selection of behavioral and metabolic traits embedded within the genome; and the life-course time frame responsible for setting the phenotype. Evolutionary influences: Famine has been a constant threat to human survival leading to the selection of thrifty genes. Thriftiness can take many forms: metabolic (an 'energy-sparing' super-efficient metabolism); adipogenic (a propensity to rapid fat gain); physiologic (an ability to switch off non-essential processes); gluttony (a tendency to gorge when food is available); sloth (a tendency to conserve energy through inactivity); or behavioral (hoarding, meanness, theft, etc). Life-course influences: The nutritional environment of the early embryo can have a major impact on its survival, and its immediate and later physiology. Subsequently, the fetus is sensitive to its nutrient supply that in turn is affected by maternal fuel supply and by the constraints of the utero-placental unit. Adaptive plasticity also continues through infancy. Ingestive behavior in terms of appetite and satiety could theoretically be affected by some of these metabolic adaptations. This paper will describe the key elements of the thrifty genotype and phenotype and review the evidence base relating these early effects to differences in ingestive behavior.