A systematic review of allostatic load in relation to socioeconomic position: Poor fidelity and major inconsistencies in biomarkers employed.

BACKGROUND: The association between disease and socioeconomic position (SEP) is well established. Allostatic load (AL), or physiological 'wear and tear', is a concept that aims to elucidate the biological consequences of stress that may underlie these associations. The primary objective of this paper is to review the biomarkers and methods used to operationalise the concept of AL in studies analysing the association between AL and SEP. METHODS: Four databases (Embase, Global Health, MEDLINE, and PsychINFO) were searched using terms related to AL, biomarkers and SEP. Data extraction focused on the methods used to calculate AL indices. The frequency of pair-wise combinations of biomarkers were used to assess the level of overlap in AL definition between studies. RESULTS: Twenty-six studies analysing the association between AL and SEP were included. There was no consistent method of operationalizing AL across studies. Individual biomarkers and biological systems included in the AL index differed widely across studies, as did the method of calculating the AL index. All studies included at least one cardiovascular- and metabolic-related biomarker in AL indices, while only half of studies included at least one hypothalamic-pituitary-adrenal (HPA) axis biomarker and approximately one third an immune response-related biomarker. All but three studies found evidence of an association between lower SEP and higher AL. CONCLUSIONS: Many studies lacked fidelity to the original concept of AL in which stress was considered central. The considerable variation in biomarkers used makes studies in this review difficult to compare. A more critical approach should be taken in the calculation of AL indices in particular to how far it captures the biological effects of psychosocial stress that may underlie socioeconomic differences in health.