In this partly longitudinal study, we found that salivary cortisol was about 40% higher in breastfed children throughout the first year of life. This finding was consistent after adjustment for infant size, sex, and race, was not accompanied by a significant difference in IL-6, and was true whether the formula-fed infants received cow-milk or soy-based formula.
Saliva is often used for biological assays in infants due to the ease of collection and now is preferred for cortisol analysis. We accounted for possible contamination such as cortisol in maternal milk and oral blood, by taking samples at least one hour after feeding and cleaning subject’s mouth with swabs. The strong correlation between concentrations in saliva and in blood or urine indicated that contamination was not a factor in this study.
Although we did not measure maternal cortisol, we considered its possible role. Previous studies showed a strong positive correlation (p < 0.001) between maternal and breast milk cortisol concentrations, and a fall in maternal plasma cortisol during the breast-feeding.(9
) Using these data, we estimated the amount of cortisol that could be transferred to the infant, and it could account for less that 10% of the differences we observed (data not shown). Infant cortisol might also be affected by a cortisol surge at birth, but when we eliminated data from the first 48 hours of life we obtained the same results.
Our findings contradicted the hypothesis that breastfed infants would have lower levels of cortisol compared with infants in the formula-fed groups. Breast feeding, however, requires more effort on the part of the infant than does bottle-feeding due to the “hydraulics” of the process.(10
) This extra effort may explain in part the higher cortisol levels seen in breastfed infants, but other explanations are possible and we have only limited data on the children in this study. We suspect that the higher cortisol concentrations may actually play a role in the analgesic effects of breast feeding. Cortisol release and blunting of pain are both associated with the stress response. Therefore, the increased cortisol levels in breastfed infants may be related to the pathway for analgesia. Future studies might include assessment of maternal cortisol levels, as well as comparison of cortisol trends in the different feeding groups with levels of inflammatory markers, such as interleukin-6, or of other stress markers, such as urinary catecholamines.
Cortisol during infancy plays an important role in the development of the HPA axis and of the stress response, a role which would have an impact throughout life.(11
) The characteristics of adrenal hormone secretion change markedly during infancy. Disturbances in basal levels may produce psychological dysfunction and are associated with psychopathology in young people.(12
) Although feeding method alone is unlikely to fully explain the observed difference, the results of this study that breastfed infants have higher levels of salivary cortisol than formula-fed infants, may have implications for development of the stress response and resilience later in life.