Determinants of hair cortisol concentration in children: A systematic review

Background: Several factors are known contribute to hair cortisol concentration (HCC) in adults. However, there is less research on determinants of HCC in children and adolescents. HCC is a valuable tool for medical research pertaining to the hypothalamic-pituitary-adrenal (HPA) axis. This review aims to assess the extent to which established determinants of HCC in adults have been consistently reported in children (birth 18 years) and to identify determinants of HCC specific to this age group. Methods: Eligible studies were identified, selected and appraised as per PRISMA-P guidelines and as detailed in our systematic review protocol, registered on PROSPERO (registration number CRD42017056220). In view of contrasting methods and measures, a meta-analysis could not be done but a qualitative synthesis was performed. Results: Thirty-six studies were included in the analysis. Higher HCC is associated with male sex and anthropometry, particularly increased body mass index and waist circumference. There is preliminary evidence to suggest that socio-economic status is inversely related to child HCC, particularly with reference to caregiver education and income. Of note, most of the studies analysing socio-economic variables were performed in relatively equal societies. Hair wash frequency and use of hair products and treatments do not affect HCC when proximal segments of hair are used. There is conflicting evidence regarding the relationship between HCC and age in children and adolescents. Further investigation is required to better delineate if and how the following are associated with HCC in children: hair colour, hair type, exposure to trauma and stressors, psychiatric illness, atopic illness, steroid use (including topical and inhaled steroids) and perinatal variables. Conclusions: Sex and anthropometry are potential confounders and should be considered for adjustment in hair cortisol research. Hair wash frequency and use of hair products and treatments are not important confounders when proximal hair segments are used. A better understanding of HCC in children in relation to exposure to trauma and stressors is required before it can be used as a biomarker, particularly in terms of vulnerable developmental stages, definition and measurement of stress, and temporal relationship to stressors. Age, SES and other correlates also warrant further investigation.