How much undernourishment is required to retard brain myelin development.

This study employs a large population of developing rats designed to range continuously from above a normal, average weight to severely undernourished. The purpose of the study is to determine if brain myelin development is vulnerable to mild growth retardation from chronic hunger, or if brain myelin development is altered only after a certain tolerable amount of growth retardation is exceeded. The brains were examined at a landmark age, weaning, since myelination is one of the most vulnerable features of brain development and its rate of synthesis is highest at this age. Brain size was logarithmically related to body weight, and brain growth retardation increased as the severity of food deprivation increased. There was an additional reduction in the concentration of brain myelin. In contrast to brain weight, the reduction in myelin concentration was linearly related to body size over the full range from well nourished to undernourished. From a population perspective, these data indicate growth retardation from undernourishment in any amount slows brain growth and additionally lowers the concentration of brain myelin; however, individuals, in both well nourished and undernourished groups, vary widely. Implications and cautions of extrapolation to human populations are discussed.