Sensitive Periods in Early Flavor Learning

Overview

Flavor is the primary dimension by which young children determine food acceptance. However, children are not merely miniature adults since sensory systems mature postnatally and their responses to certain tastes differ markedly from adults. Moreover, emerging research has revealed that there are sensitive periods during infancy such that early flavor experiences serve to modify later responses to flavors and foods. The proposed study aims to investigate this important issue by using as a model system a class of infant formulas which are hydrolyzed protein based and thus have very pronounced and distinctive flavors which are unpalatable to older-aged infants and adults. This research was initiated because of anecdotal reports by pediatricians that although it is easy to introduce this type of formula to infants during the first months of life, it becomes extremely difficult to do so later in infancy. Indeed, recent studies in the investigators' laboratory provided the first experimental demonstration that infants younger than 4 months of age willingly accept substantial amounts of, and satiate while feeding, a novel, protein hydrolysate formula. In marked contrast, infants older than 4 months reject the protein hydrolysate formula and this rejection occurs within the first minute of a feed, a finding that strongly suggests the sensory qualities of the formula are responsible, at least in part, for this rejection. Moreover, this rejection is not evident when the investigators test older-aged infants with other unfamiliar, but non-hydrolysate, formulas. In other words, the rejection appears to be in response to a particular component or components of protein hydrolysate formulas. This shift in acceptability can be ameliorated by prior exposure. That is, if these formulas are introduced to infants within the first few months of life and are fed continuously, they remain highly acceptable throughout infancy and early childhood. These observations implicate a sensitive period during development, occurring somewhere before 4 months of age, during which exposure to a formula, which is unpalatable to adults and infants over 4 months of age without exposure, renders it acceptable and presumably palatable. To the investigators' knowledge, this is the clearest example of a sensitive period in the development of responses to foods and flavors in humans thus far identified. There is a paucity of information on whether and how the composition of formulas fed to infants influences their short-term feeding behaviors during the first few months of life. The primary objective of this longitudinal study is to determine the period during early infancy when exposure to the casein-hydrolysate formula, Nutramigen, renders it acceptable during later infancy. The study also aims to determine how early sensory experiences with formula impact upon food acceptance during infancy (8-9 months of age) and childhood. The investigators will also explore how variation in the genes that encode for taste receptors influence preferences for foods and other behaviors.