Allostatic Load Index

The allostatic load index quantifies the cumulative biological cost of chronic stress by summing dysregulation across multiple physiological systems. McEwen and Stellar introduced 'allostatic load' in 1993 to name the wear and tear the body accrues when stress-response systems are repeatedly or chronically activated, extending the idea of allostasis (stability through change) over time. Seeman, Singer, Rowe, Horwitz, and McEwen operationalized it in the MacArthur Studies of Successful Aging in 1997, scoring older adults on biomarkers spanning cardiovascular, metabolic, neuroendocrine, and immune function and counting how many fell into a high-risk range, typically the worst quartile. The resulting count index predicted later cognitive and physical decline and cardiovascular disease, establishing allostatic load as a measurable marker of cumulative physiological risk that no single clinical test captures.