Heart rates of breast-fed and formula-fed infants.

Heart rates of 46 breast-fed and formula-fed infants were monitored continuously for approximately 18 h at 1 or 4 months of age. Heart rate differed significantly by age (1 month greater than 4 months; p less than 0.001) and feeding mode (breast-fed less than formula-fed; p less than 0.001). Approximately 58% of the variability in heart rate could be attributed to feeding mode, sex, and age. Heart rate was correlated significantly with energy intake (r = 0.60; p less than 0.001), but not sleeping metabolic rate or total daily energy expenditure. Energy intake accounted for none of the variability in heart rate beyond that explained by feeding mode, sex, and age.