Human milk cortisol concentration predicts experimentally induced infant fear reactivity: moderation by infant sex

Abstract

Little consideration has been given to the possibility of human infant development being shaped via lactocrine programming, and by breast milk cortisol levels specifically. Despite animal models indicating that glucocorticoid (GC) exposure via lactation might modify brain development and behavior, only one study has reported that milk cortisol levels were positively associated with infant negative affectivity, especially fearfulness and sadness-early emerging risk factors for internalizing difficulties such as anxiety. The aim of the current study was to investigate whether human milk cortisol is associated with mother-reported fearfulness and experimentally induced infant fear reactivity. Mother-infant dyads (n = 65) enrolled in the FinnBrain Cohort Study participated. Breast milk samples were obtained 2.5 months postpartum, and milk cortisol concentrations were ascertained using validated luminescence immunoassay methodology. Infant fear reactivity was assessed using maternal reports 6 months postpartum and in a laboratory 8 months postpartum. There was a significant interaction between infant sex and milk cortisol such that higher milk cortisol was related to higher infant fear reactivity in a laboratory setting in girls (β = 0.36, p = .04) but not in boys (β = -0.15, p = .40). Milk cortisol was not associated with mother-reported infant fearfulness. Results suggest that higher human milk cortisol concentrations are associated with elevated experimentally induced fear in infancy. Findings support lactocrine programming, and suggest that mothers may "communicate" vital information about stressful environments via cortisol contained in breast milk, shaping girls' early emotional reactivity.

Are cortisol concentrations in human breast milk associated with infant crying?

“…The results are not in line with the two previous studies that showed higher breast milk cortisol concentrations to be related to more infant temperamental negativity (Grey et al, ; Nolvi et al, ). Based on these studies, we expected milk cortisol to be related to infant fussing and crying.…”

“…This variation in designs and outcome measures may be behind the differences in results between the three studies. Also, while Nolvi et al () found a relation between milk cortisol and girls’ fear reactivity in a laboratory setting, but no relation between milk cortisol and maternal reported fearfulness, Grey et al () did find a relation between milk cortisol and maternal reported infant negative affectivity. This positive relation between milk cortisol and negative affectivity seemed to be strongest for the fear subscale of negative affectivity (Grey et al, ).…”

“…First, differences in the designs and measures used may explain the differences in results. Although in this study fussing and crying over three 72‐hr periods were assessed within the first 3 months, the Grey et al () study used maternal reports of temperamental negative affectivity (including sadness and fear) at 3 months and the Nolvi et al () study observed laboratory‐induced fear reactivity at 8 months. This variation in designs and outcome measures may be behind the differences in results between the three studies.…”

“…A later study from the same research group on 3‐month‐old infants showed that higher cortisol concentrations in breast milk predicted more reported temperamental negative affectivity (including sadness and fear), but only in female infants (Grey et al, ). Finally, a recent study found that higher breast milk cortisol concentrations at 2.5 months postpartum were associated with higher induced fear reactivity in 8‐month‐old infant girls, but not boys (Nolvi et al, ).…”

“…In sum, only a few studies examined links between human breast milk cortisol and infant behavior and found that higher cortisol concentrations were related to more infant temperamental negativity (Glynn et al, ; Grey et al, ; Nolvi et al, ). The present study aims to longitudinally investigate associations between breast milk cortisol and infant fussing and crying.…”