Allostasis, originally conceptualized to explain persistent morbidity of arousal and autonomic function, is defined as “stability through change.” Allostasis is far more complex than homeostasis and has several unique characteristics (Sterling and Eyer, 1988). Allostasis involves a feed-forward mechanism rather than the negative feedback mechanisms of homeostasis, with continuous re-evaluation of need and continuous readjustment of all parameters toward new set points. An allostatic state can be defined as a state of chronic deviation of the regulatory system from its normal (homeostatic) operating level. Allostatic load was defined as the “long-term cost of allostasis that accumulates over time and reflects the accumulation of damage that can lead to pathological states” (McEwen, 2000).
