ReviewAllostatic load biomarkers of chronic stress and impact on health and cognition
Introduction
Chronic psychosocial stress and consequent physiological dysregulations are increasingly viewed as catalysts of accelerated aging and agitators of disease trajectories. Individual differences in the brain's interpretations of and the body's reactions to environmental stressors are nevertheless the ultimate determinants of either vulnerability towards or resilience against stress-related diseases (Lupien et al., 2009, McEwen, 1998b, McEwen, 2009). Health and successful aging can therefore be conceptualized as one's ability to adapt and effectively respond to the dynamic challenges of being alive. Embodying this notion is the allostatic load model, which assesses physiological dysregulations that ensue when normal homeostatic functioning is shifted towards abnormal ranges via the prolonged secretion of stress hormones and the subsequent mal-adaptations this strain exerts on interdependent systems. This review summarizes theoretical developments and studies that incorporate allostatic load algorithms used to predict health and cognitive outcomes throughout lifespan development.
Section snippets
Concept of allostasis
The term allostasis refers to the process whereby an organism maintains physiological stability by changing parameters of its internal milieu by matching them appropriately to environmental demands (Sterling and Eyer, 1988). Traditional homeostatic models define health as a state in which all physiological parameters operate within normal values, while those that do not warrant pharmaceutical intervention. In contrast, allostasis defines health as a state of responsiveness and optimal
Allostatic load model
Allostatic load (AL) represents the ‘wear and tear’ the body experiences when repeated allostatic responses are activated during stressful situations (McEwen and Stellar, 1993). Real or interpreted threats to homeostasis initiate the sympathetic–adrenal–medullary (SAM) axis release of catecholamines and the hypothalamic–pituitary–adrenal (HPA) axis secretion of glucocorticoids that mobilize energy necessary for fight-or-flight responses (Sapolsky et al., 2000). Coordination of allostasis
Older and middle-aged adults
The following sections summarize studies using composite measures of AL in relation to health and cognitive outcomes throughout the lifespan (see Table 1). The MacArthur Studies of Successful Aging have provided the first steps towards an operational definition of AL. A count-based AL index representing the following 10 biomarkers was included in preliminary validation: 12-h urinary cortisol, epinephrine, and norepinephrine output; serum dehydroepiandrosterone-sulphate (DHEA-S), total
Conclusion
Decomposing which protective factors render individuals resilient to age-related health and cognitive declines depends on our conceptual definitions of successful aging and appreciation of individual differences (Lupien and Wan, 2004). Rather than being the absence of pathology or deceleration of senescence, resilience is a state of adaptation to a lifetime of stress and strain. Evidence from AL studies have clearly demonstrated that mal-adapting to stressful environments has serious
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