Breastfeeding and cortisol in hair in children

One of the most important protective health factors for children is breast-feeding, but the mechanisms for this effect are not fully elucidated. Our objective was to assess if the duration of breastfeeding influences cortisol in hair, used as a biomarker for stress in children still at school-age.

ABIS (All Babies in Southeast Sweden) is a prospective population-based child cohort study of 17,055 children born Oct 1st1997- Oct 1st 1999, with the aim to study development of immune-mediated diseases. Questionnaires were answered at birth and then at regular follow-ups, and biological samples were collected. As a biomarker of stress, we measured the child's cortisol in hair collected at 8 years of age, those randomly selected N = 126 children among those with enough hair samples for analyses of hair at 8 years of age. Duration of breastfeeding had been registered as well as psycho-social factors related to breastfeeding and/or stress.

There was a negative correlation (r = - 0.23, p = 0.01) between total duration of breastfeeding and hair cortisol levels at 8 years of age. In a multivariate analysis this association persisted (p = 0.01) even when adjusted for other potential intervening factors like age of mother at delivery and early psychosocial vulnerability in the family, an index based on 11 factors (Multivariate model: df = 5, adj R² = 0.15, F = 5.38, p < 0.01).

Our results show that longer breastfeeding is associated with lower cortisol levels in the child many years later. These associations should be more elaborated in further studies, and these findings also give some implications for public health. Mothers should be encouraged to breastfeed their children also in the modern society, since breastfeeding promotes health in the child. This information could be given via the obstetric departments and later at the well-baby clinics.