Allostatic load

The allostatic load is "the wear and tear on the body" which grows over time when the individual is exposed to repeated or chronic stress. It represents the physiological consequences of chronic exposure to fluctuating or heightened neural or neuroendocrine response that results from repeated or chronic stress. The term was coined by McEwen and Stellar in 1993.

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.

Reactions
Stress hormones such as epinephrine and cortisol in combination with other stress-mediating physiological agents such as increased myocardial workload, decreased smooth muscle tone in the gastrointestinal tract, and increased coagulation effects have protective and adaptive benefits in the short term, yet can accelerate pathophysiology when they are overproduced 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 increased allostatic load and may, over time, lead to the development of disease. 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.