Infant diet, gender and the development of vagal tone stability during the first two years of life.

Postnatal nutrition influences neurodevelopment, but it is not known whether the development of individual differences in physiologic measures is related to variations in early postnatal diet. To address this issue we studied the stability of vagal tone (V)--an index of individual differences in parasympathetic heart rate control-by measuring resting V quarterly during infancy and again at 2 years in 146 breast-fed (BF), 143 milk formula-fed (MF), and 137 soy formula-fed (SF) infants. Stability of V across infancy was more consistently significant for BF than formula-fed infants. Stability was similar for boys and girls in BF and SF groups but was generally higher in boys than girls in the MF group. Significant stability between infancy and 2 years emerged later in SF than other groups and later in boys than girls. Stability generally peaked between 6 and 9 months-a time when postnatal vagal myelination slows and which may represent a pivotal stage in the development of V stability. These findings indicate that infant diet and gender are important modulators of the early development of autonomic state control.