Homeostasis as "corrective regulation" is "stability achieved through constancy" whereas allostasis as "predictive regulation" is "stability through change." ...the goal of regulation is not primarily to "defend" all parameters but rather to continuously match them for efficient performance. Allostasis: A clock predicts a cell's individual needs, plus its responsibilities to other cells over the course of the day, and guides metabolism to meet them in a timely fashion. Homeostasis: When needs change on a faster timescale, the clock's predictions may err and require prompt correction by feedback. Allostasis "predicts"; homeostasis "corrects." 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. The model clarifies why the brain should be in charge: it is simply more efficient to predict a need and satisfy it rather than to wait for an error and correct it. For this model of "predictive regulation" to challenge homeostasis it would need a name...we called it allostasis--meaning "stability through change." Whereas homeostasis tends to define "health" as a list of "appropriate" lab values and "disease" as "inappropriate" values, allostasis defines health as the capacity for adaptive variation and disease as a shrinkage or compression of that capacity. Therapeutically, homeostasis emphasizes drugs to clamp down on pesky glucose, blood lipids, and so on. These drugs tend to reduce the capacity for adaptive variation, whereas allostasis emphasizes the opposite, interventions that enhance the capacity for adaptive variation. The allostasis model defines health as the capacity to respond optimally to fluctuations in demand. The homeostasis model uses pharmacotherapy whereas the allostasis model uses system therapy. Pharmacotherapy typically tries to correct a specific parameter....System therapy tries to change the prediction. The goal is to reduce demand for long enough for the system to "believe" the new prediction and readapt. As responses drift back toward the initial mean, response range is maintained. Circuits that share the same receptors as the target circuit may also benefit. This is a "side effect" of system therapy--and it is a good one. The main elements of allostasis: i. Values are not constant but vary according to need (temperature varies diurnally, with stochastic pulses, with infection, and with exercise). ii. Needs are prioritized for urgency and opportunity (cooling vs. feeding), but flexibly, to be reprioritized as conditions change. iii. Control extends beyond the immediate need to coordinate trade-offs for future needs (cool now vs. conserve water and salt for what may come). iv. Each system serves multiple needs (kidney and its regulatory hormones serve fluid/osmotic balance but also serve thermoregulation). v. Control uses behavior to conserve resources (seek shade vs. sweating). vi. Learning improves prediction (don't eat in the heat; take a jacket to the mountains). vii. Anticipation prevents errors (seek water vs. rise in osmolarity). Peter Sterling, What is Health? MIT 2020
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