Allostasis

Allostasis is the process of achieving stability, or homeostasis, through physiological or behavioral change. This can be carried out by means of alteration in HPA axis hormones, the autonomic nervous system, cytokines, or a number of other systems, and is generally adaptive in the short term (McEwen & Wingfield 2003). Allostasis is essential in order to maintain internal viability amid changing conditions (Sterling & Eyer 1988; McEwen 1998a; McEwen 1998b; Schulkin 2003).

The concept of allostasis was proposed by Sterling and Eyer in 1988 to describe an additional process of reestablishing homeostasis, but one that responds to a challenge instead of to subtle ebb and flow. This theory suggests that both homeostasis and allostasis are endogenous systems responsible for maintaining the internal stability of an organism. Homeostasis, from the Greek homeo, means "similar," while stasis means "stand;" thus, "standing at about the same level." (The term was not coined as "homostasis" or "standing the same" because internal states are frequently being disturbed and corrected, thus rarely perfectly constant.) Allostasis was coined similarly, from the Greek allo, which means "variable;" thus, "remaining stable by being variable" (Sterling & Eyer 1988; Klein 2004). Allostatic regulation reflects, at least partly, cephalic involvement in primary regulatory events, in that it is anticipatory to systemic physiological regulation (Sterling & Eyer 1988; Schulkin 2003). The term Heterostasis is also used in place of Allostasis, particularly where state changes are finite in number and therefore discrete (e.g. computational processes).

Wingfield states:

The concept of allostasis, maintaining stability through change, is a fundamental process through which organisms actively adjust to both predictable and unpredictable events... Allostatic load refers to the cumulative cost to the body of allostasis, with allostatic overload... being a state in which serious pathophysiology can occur... 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 (Wingfield 2003).

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