Allostatic load

[1]The allostatic load model is a cycle of an imbalance in psychological factors such as stress, trauma, abuse, and environmental stressors also contribute to an increase in allostatic load. The allostatic load cycle is maintained by homeostasis. Homeostasis maintains balanced levels of stress by creating responses in the brain to regulate levels such as hormones.

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.[2] The term was coined by McEwen and Stellar in 1993.[3] It represents the physiological consequences of chronic exposure to fluctuating or heightened neural or neuroendocrine response that results from repeated or prolonged chronic stress.[4] The allostatic load model is very similar to Peter A. Levine's concept of accumulated stress, as articulated in his 1977 doctoral thesis at the University of California at Berkeley,[5] as well as in the chapter on Stress in the 1986 text Psychophysiology: Systems, Processes and Applications[6]

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.[7] Allostasis involves the regulation of homeostasis in the body to decrease physiological consequences on the body.[8] Predictive regulation refers to the brain's ability to anticipate needs and prepare to fulfill them before they arise.[7] 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.[9] 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".[9]

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."[9] 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.[9]

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.[9] 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.[10]

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.[10]

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.[8] 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 decreased 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 neuron. Therefore, the shortening of dendrites causes the decrease in attention.[8] 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 ongoing activation of the sympathetic nervous system.[8] The impairment of cell-mediated acquired immunity is also a factor resulting in the immune system due to chronic stress.[8]

Relationship Between Allostatic Load, Allostasis and Homeostasis

The greatest contribution to the allostatic load is the effects of stress on the brain. Allostasis. therefore, is the systems in the body that help achieve homeostasis.[11] Homeostasis is the regulation of physiological processes. The systems in the body respond to the state of the body and also to the external environment.[11] The relationship between allostasis and allostatic load is the concept of anticipation. Anticipation can drive the output of mediators. Examples of mediators include hormones and cortisol. Excess amounts of such mediators will result in an increase in allostatic load, contributing to anxiety and anticipation.[11]

Another relationship between allostasis and allostatic load are the health-damaging and health-promoting behaviours which contribute to allostatic load.[11] Theses behaviours include cigarette smoking, consumption of alcohol, poor diet and physical inactivity.[11]

There are three physiological processes which cause an increase in allostatic load, these include:

  1. Frequent stress: the magnitude and frequency of response to stress is what determines the level of allostatic load which effects the body.
  2. Failed shut-down: the inability of the body to shut off while stress accelerates and levels in the body exceed normal levels, for example, elevated blood pressure.
  3. Inadequate response: the failure of the body systems to respond to challenge, for example, exceeded levels of inflammation due to inadequate endogenous glucocorticoid responses.

The importance of homeostasis, therefore, is to regulate the stress levels encountered on the body to reduce allostatic load.

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.[12]

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, and other chronic health conditions. Constant or even irregular exposure to these hormones can eventually induce illnesses and weaken the body's immune system.[12] Therefore, leading to an increase of the allostatic load.

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.

See also

References

  1. ↑ McEwen, B (2000-02). "Allostasis and Allostatic Load Implications for Neuropsychopharmacology". Neuropsychopharmacology. 22 (2): 108–124. doi:10.1016/S0893-133X(99)00129-3. ISSN 0893-133X. Check date values in: |date= (help)
  2. ↑ Jane Ogden (2004). Health Psychology: A textbook, 3rd edition. Open University Press - McGraw-Hill Education. p. 259. ISBN 0335214711.
  3. ↑ McEwen, BS; Stellar, E (Sep 27, 1993). "Stress and the individual. Mechanisms leading to disease". Archives of Internal Medicine. 153 (18): 2093–101. doi:10.1001/archinte.153.18.2093. PMID 8379800.
  4. ↑ Taylor, S. E. (2006). Their work establishes a general relationship between daily stress, and wide-ranging diseases of the body and mind. Health Psychology. McGraw-Hill Education, pg. 160
  5. ↑ Levine, Peter (1977). Accumulated Stress, Reserve Capacity and Dis-ease (Ph.D. Thesis). University of California at Berkeley.
  6. ↑ Levine, Peter (1986). Chapter on Stress in Psychophysiology: Systems, Processes, and Applications (Coles, Donchin & Porges, eds.). New York: The Guilford Press. ISBN 978-0898626407.
  7. 1 2 Sterling, Peter. "Allostasis: A Model of Predictive Regulation." Physiology & Behavior, vol. 106, no. Allostasis and Allostatic Load, 12 Apr. 2012, pp. 5-15. EBSCOhost, doi:10.1016/j.physbeh.2011.06.004.
  8. 1 2 3 4 5 Danese, Andrea; McEwen, Bruce S. (2012-04). "Adverse childhood experiences, allostasis, allostatic load, and age-related disease". Physiology & Behavior. 106 (1): 29–39. doi:10.1016/j.physbeh.2011.08.019. ISSN 0031-9384. Check date values in: |date= (help)
  9. 1 2 3 4 5 Peters, Achim, et al. "Uncertainty and Stress: Why It Causes Diseases and How It Is Mastered by the Brain." Progress in Neurobiology, 24 May 2017. EBSCOhost, doi:10.1016/j.pneurobio.2017.05.004.
  10. 1 2 Edes, Ashley; Crews, Douglas (January 1, 2017). "Allostatic load and biological anthropology". American Journal of Physical Anthropology. 162: 44–70. doi:10.1002/ajpa.23146.
  11. 1 2 3 4 5 McEWEN, BRUCE S. (1998-05). "Stress, Adaptation, and Disease: Allostasis and Allostatic Load". Annals of the New York Academy of Sciences. 840 (1): 33–44. doi:10.1111/j.1749-6632.1998.tb09546.x. ISSN 0077-8923. Check date values in: |date= (help)
  12. 1 2 McEwen B. S. (2000). "Allostasis and allostatic load: implications for neuropsychopharmacology". Neuropsychopharmacology. 22 (2): 108–24. doi:10.1016/S0893-133X(99)00129-3. PMID 10649824.
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