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What is the Allostatic Load Model?
Allostatic load is "the wear and tear on the body" that accumulates as an individual is exposed to repeated or chronic stress. The term was coined by McEwen and Stellar in 1993. It represents the physiological consequences of chronic exposure to fluctuating or heightened neural or neuroendocrine response that results from repeated or prolonged chronic stress.
The regulatory model of allostasis claims that the brain's primary role as an organ is the predictive regulation or the stabilisation of internal sensations. Allostasis involves the regulation of homeostasis in the body to decrease physiological consequences on the body.  Predictive regulation refers to the brain's ability to anticipate needs and prepare to fulfill them before they arise. Therefore, in this model, the brain is responsible for efficient stimuli regulation.
Part of efficient regulation is the reduction of uncertainty. Humans naturally do not like feeling as if surprise is inevitable. Because of this, we constantly strive to reduce the uncertainty of future outcomes, and allostasis helps us do this by anticipating needs and planning how to satisfy them ahead of time. But it takes a significant amount of the brain's energy to do this, and if it fails to resolve the uncertainty, the situation may become chronic and result in the experience of "allostatic load".
The concept of allostatic load provides that "the neuroendocrine, cardiovascular, neuroenergetic, and emotional responses become persistently activated so that blood flow turbulences in the coronary and cerebral arteries, high blood pressure, atherogenesis, cognitive dysfunction and depressed mood accelerate disease progression." In other words, all of the long-standing effects of continuously activated stress responses are referred to as allostatic load. And allostatic load can even result in permanently altered brain architecture and systemic pathophysiology.
Further, as a result of these physical effects, allostatic load also minimizes an organism's ability to cope with and reduce uncertainty in the future, which cements the entire cycle. There are direct and Indirect effects on health resulting in a higher allostatic load.
Measurement of Allostatic Load
Allostatic load is generally measured through a composite index of indicators of cumulative strain on several organs and tissues, primarily biomarkers associated with the neuroendocrine, cardiovascular, immune and metabolic systems.
Indices of allostatic load are diverse across studies and are frequently assessed differently, using different biomarkers and different methods of assembling an allostatic load index. Allostatic load is not unique to humans and may be used to evaluate the physiological effects of chronic or frequent stress in non-human primates as well.
In the endocrine system, the increase or repeated levels of stress results in the increased levels of the hormone Corticotropin-Releasing Factor (CRH), which is associated with activation of HPA axis. HPA axis is the central stress response system which is responsible for modulating inflammatory responses that occur throughout the body. The prolonged stress levels can also lead to decreases levels of cortisol in the morning and increased levels in the afternoon, leading to greater daily output of cortisol which in the long term increases blood sugar levels.
In the nervous system, structural and functional abnormalities are a result of chronic prolonged stress. The increase of stress levels causes a shortening of dendrites in a neurone. Therefore, the shortening of dendrites causes the decrease in attention. Chronic stress also causes greater response to fear of the unlearned in the nervous system, and fear conditioning.
In the immune system, the increase in levels of chronic stress results in the elevation of inflammation. The increase in inflammation levels is caused by the on going activation of the sympathetic nervous system.  The impairment of cell-mediated acquired immunity is also a factor resulting in the immune system due to chronic stress.
Consequences of High Allostatic Load on Health
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 downstream hormones produced as a result of the stress response, cortisol and epinephrine (adrenaline), have beneficial effects on the body that can become detrimental with excessive activation, such as increased blood pressure and heart rate.
The physiological responses involved in the stress response are widely considered adaptive as they are effective at responding to acute threats to survival across many species. However, in environments of chronic or frequent activation of the stress response, such as exposure to violence or trauma, poverty, war, hypoxia, or low rank in a social hierarchy, the stress response constantly disrupts homeostasis resulting in overexertion of physiological systems.
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
- 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, sometimes with decompensation of the allostatically controlled problem. 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.
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