Harvard physiologist Walter Cannon took Bernard's theory of the milieu intérieur and expanded it to incorporate an evolutionary framework of energy efficiency and preservation.
Cannon coined this concept homeostasis in 1926, demonstrating that the organism's body is a self-governing system of regulation with certain steady-state conditions for optimal functioning.
[2] By the late 20th-century, neurobiologist Peter Sterling and epidemiologist Joseph Eyer noticed generational patterns of chronic stress and its effects on various human physiological mechanisms that could not be easily explained by homeostasis.
They developed the concept of 'allostasis' [from the Greek ἄλλος (állos, "other," "different") + στάσις (stasis, "standing still") to mean "remaining stable by being variable"] to incorporate the body's ability to adjust steady-state conditions based on the perception and interpretation of environmental stressors.
To explain these epidemiological phenomena, Sterling and Eyer suggested social and systemic stress in the setting of advancing capitalism and industrialization to be the main driver of increased morbidity and mortality rates in age-specific cohorts.
Sterling and Eyer proposed the concept of allostasis in 1988 to better explain the process of physiological changes in the individual level that are shaped by large-scale epidemiological patterns.
[3] They noticed a pattern that populations in the United States with the greatest impact of social disruption correlated with higher morbidity and mortality rates.
For instance, the rate of elevated blood pressure (or hypertension) was the highest amongst groups that experienced the most social disruption, namely the unemployed and African Americans.
Sterling and Eyer proposed that there was a mind-brain-body component to permanent physiological changes of the body's internal conditions in the setting of external stress.
In response to stress, the brain directly innervates the thyroid and pancreas for energy regulation, sends signals to the cardiovascular system to increase cardiac output, stimulates the adrenal glands to release cortisol and aldosterone, and releases hormones from the pituitary gland such as ACTH to regulate urine output through the renin-angiotensin-aldosterone system.
Sterling and Eyer theorized that the brain can anticipate stressors to prepare the body to respond adequately to environmental demands through classical conditioning.
Once the stressor is resolved, the body resumes to amounting an adequate immune and inflammatory response, which may explain why it is often seen that a person falls ill after acute stress.
Prediction requires the brain to: (i) collect information across all spatial and temporal scales; (ii) analyze, integrate, and decide what will be needed; (iii) exert feedforward control of all parameters.
Without the ability to prioritize trade-offs between organ systems, the heart and lungs would need to be far larger while much of this costly extra capacity would go unused.
[14] Too much allostasis, also known as allostatic overload, is when the body's attempts to adapt to the environment cause more harm than benefit and can lead to various negative consequences in the form of mental and physical diseases.
Health itself is an allostatic state of optimal anticipatory oscillation, hypothesized to relate to the state of criticality… Diseases are allostatic states of impaired anticipatory oscillations, demonstrated as rigidifications of set points across the brain and body (disease comorbidity).The PAO implications for health extend beyond blood pressure and diabetes to include addiction, depression, and deaths of despair (from alcohol, drugs, and suicide) that have been increasing since 2000,[21] emphasizing that an integrated view of health includes environmental context.
Allostasis proposes a broader hypothesis than homeostasis: The key goal of physiological regulation is not rigid constancy; rather, it is flexible variation that anticipates the organism's needs and promptly meets them.
Using the balance between energy input and expenditure as the basis for applying the concept of allostasis, two types of allostatic overload have been proposed.