Lasting Biological Effects of Early Environmental Influences* I. Conditioning of Adult Size by Prenatal and Postnatal Nutrition

As commonly used, the phrase "early influences" denotes the conditioning of behavior by the experiences of early life. Early experiences, however, do more than condition behavioral patterns; they also affect, profoundly and lastingly, other biological characteristics such as initial growth rate, efficiency in the utilization of food, anatomic structures, physiologic attributes, maximum adult size, resistance to infection, response to various forms of stimuli, in brief almost every phenotypic expression of the adult. Epidemiologic evidence strongly suggests that some of the most important medical problems in underprivileged countries (and in prosperous ones as well) have their origin in environmental influences that affect human beings during the formative phases of their prenatal and neonatal development. Many effects of such early influences appear irreversible. Among early influences, the most extensively studied so far are those assodated with the nutritional state. Reviews of the literature on this topic will be found in references 1-8. The lasting character of the biological effects of early nutritional influences is strikingly illustrated in the results of investigations carried out in England during the 1960's by McCance and associates (3-5). These investigators compared the growth rate of rats suckled in small litters (3 young per lactating female) with that of comparable animals suckled in large litters (18 young per lactating female). They found that the rats of the latter group (large litters) became much smaller adults than those of the former group, even though the animals of both groups were given the same diet after weaning. An obvious reason for this finding is that the animals raised in small litters enjoyed a nutritional advantage during the lactation period. As McCance and Widdowson pointed out, however, other factors may have played a role in the difference between