Conceptual and Analytical Overlap Between Allostatic Load and Systemic Biological Aging Measures: Analyses From the National Survey of Midlife Development in the United States

Background Indices quantifying allostatic load (AL) and biological aging (BA) have independently received widespread use in epidemiological literature. However, little attention has been paid to their conceptual and quantitative overlap. By reviewing literature utilizing measures of AL and BA, and conducting comparative analysis, we highlight similarities and differences in biological markers employed and approach toward scale construction. Further, we outline opportunities where both types of indices might be improved by adopting methodological features of the other. Methods Using data from the National Survey of Midlife Development in the United States (N = 2055, age = 26-86), we constructed 3 AL indices: 1 common literature standard and 2 alternative formulations informed by previous work with measures of BA. The performance of AL indices was juxtaposed against 2 commonly employed BA indices: Klemera-Doubal Method Biological Age and Homeostatic Dysregulation. Results All indices correlated with chronological age. Participants with higher AL and older BA performed worse on tests of physical and subjective functioning. Further, participants with increased life-course risk exposure exhibited higher AL and BA. Notably, alternative AL formulations tended to exhibit effect sizes equivalent to or larger than those observed for BA measures, and displayed superior mortality prediction. Conclusions In addition to their conceptual similarity, AL and BA indices also exhibit significant analytical similarity. Further, BA measures are robust to construction using a panel of biomarkers not observed in previous iterations, including carotenoids indexing antioxidant capacity. In turn, AL indices could benefit by adopting the methodological rigor formalized within BA composites, such as applying biomarker down-selection criteria.