Infant diet, gender and the normative development of vagal tone and heart period during the first two years of life

• Early diet and infant gender modulate autonomic development.
• HP was greater in boys than in girls—particularly in soy-fed infants.
• V was generally greater in girls than in boys and in soy-fed boys than in other boys.
• The time during infancy when rate of change in V slows is diet sensitive.

Relationships between early postnatal diet and the development of cardiac regulation were studied using resting vagal tone and heart period measures obtained quarterly during infancy and at 2 years in 158 breast-fed, 159 milk formula-fed, and 148 soy formula-fed infants. Both measures increased across time for all groups. Heart period was greater in boys than in girls—particularly in soy-fed infants. Higher vagal tone in girls than in boys was not strongly influenced by diet. At 1 and 2 years measures differed among boys (soy-fed > breast-fed) but not among girls. Earlier slowing in breast-fed than in formula-fed infants in the rate of increase in vagal tone during infancy suggests that the timing of this developmental shift is sensitive to early infant diet. Finally, the findings do not indicate atypical development of cardiac activity in soy-fed infants that might be associated with estrogenic compounds in this formula.