The term allostatic load, coined by McEwen and Stellar in 1993, is defined as the physiological consequences of chronic exposure to fluctuating or heightened neural or neuroendocrine response that results from repeated or chronic stress.
It is used to explain how frequent activation of the body's stress response, essential 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 benefits in the short run and yet can accelerate pathophysiology when they are over-produced or mismanaged; this kind of stress can cause hypertension and lead to heart disease. Constant or even irregular exposure to these hormones can eventually induce illnesses and weaken the body's immune system.
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, the resulting “allostatic load” exacts a cost 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:
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; and
Inadequate response that leads to compensatory hyperactivity of other mediators.
The effects of these forms of dysfunctional allostasis cause allostatic load and this, over time, leads 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.