The term Allostatic load was coined by Bruce McEwen (2000) and refers to the physiological costs of chronic exposure to the neural or neuroendocrine stress response. It is used to explain how frequent activation of the body's stress response, an essential tool for managing acute threats, can in fact damage the body in the long run. Allostatic load is generally measured through a composite index of indicators of cumulative strain on several organs and tissues, but especially on the cardiovascular system.
The hormones and other physiological agents that mediate the effects of stress on the body have protective and adaptive effects in the short run and yet can accelerate pathophysiology when they are over-produced or mismanaged.
Adaptation in the face of stressful situations and stimuli involves activation of neural, neuroendocrine and neuroendocrine-immune mechanisms. This adaptation has been called "allostasis" or "maintaining stability through change", which is an essential component of maintaining homeostasis. The main hormonal mediators of the stress response, cortisol and epinephrine (adrenaline), have both protective and damaging effects on the body. In the short run, they are essential for adaptation, maintenance of homeostasis, and survival “allostasis”. Yet, over longer time intervals, when called upon frequently, they exact a cost “allostatic load” that can accelerate disease processes. Allostatic load can be measured in physiological systems as chemical imbalances in autonomic nervous system, central nervous system, neuroendocrine, and immune system activity as well as perturbations in the diurnal rhythms, and, in some cases, plasticity changes to brain structures.
Four conditions that lead to allostatic load are:
The effects of these forms of dysfunctional allostasis cause allostatic load and this in turn leads over time to diseases. Allostatic load effects can be measured in the body. When tabulated in the form of allostatic load indices using sophisticated analytical methods, it gives an indication of cumulative lifetime effects of all types of stress on the body.
- Repeated frequency of stress responses to multiple novel stressors
- Failure to habituate to repeated stressors of the same kind
- Failure to turn off each stress response in a timely manner due to delayed shut down
- Inadequate response that leads to compensatory hyperactivity of other mediators