Hair cortisol and depressive symptoms in youth: An investigation of curvilinear relationships.

• Adolescent hair cortisol levels were not significantly associated with depressive symptoms in the linear model. • Adolescent hair cortisol levels at the low and high end of the sample distribution were associated with increased depressive symptoms. • Significant sex differences in the curvilinear relationship between hair cortisol levels and depressive symptoms were not found. As the prevalence of depression is increasing among youth in the U.S., research on the utility of biomarkers in predicting depressive symptomatology is burgeoning. Hair cortisol may be a useful biomarker as it is a retrospective and longer-term measure of the mean cortisol level. However, studies have yet to examine the relationships between hair cortisol and depressive symptoms in samples of youth, and findings with adult samples are mixed. This study examined hair cortisol as a predictor of depressive symptoms, including the potential for nonlinear relationships. A representative community sample of 432 adolescents aged 11 to 17 years was examined. Depressive symptoms were measured using a 9-item short-form of the Center for Epidemiologic Studies-Depression scale. Hair was cut from the posterior vertex region of the scalp using thinning shears. Hair was washed, minced, ground and assayed with Salimetrics® Cortisol Enzyme Immunoassay Kit. Hair cortisol levels were logged for statistical analysis. In multivariable regression analysis, no significant linear relationship was found in model 1 between hair cortisol and depressive symptoms (b= -0.036, se = 0.02, p = 0.13). In model 2, a marginally significant linear association (b= -0.044, se = 0.02, p-value = 0.06) and a significant curvilinear relationship (b = 0.039, se = 0.01, p-value = 0.005) were found between hair cortisol and depressive symptoms. The results were graphed depicting a u-shaped curve such that hair cortisol levels on the lower and higher end of the distribution predicted depressive symptoms. The findings highlight the need to consider investigation of nonlinear associations between cortisol and depressive symptoms. Longitudinal mechanistic research is needed to elucidate the causal relationships between hypothalamic-pituitary-adrenal axis dysregulation and depressive symptoms as well as a better understanding of the biological mechanisms through which cortisol may contribute to depressive symptoms and psychopathology. [ABSTRACT FROM AUTHOR]