Allostatis, Allostatic Load, and Stress

Definition of Terms

Stress—a threat, real or implied, to homeostasis. Stressors are those events and experiences that evoke behavioral and physiological response that initially protect and reduce the danger.

Homeostasis—stability of physiological systems that maintain life; used here to apply strictly to a limited number of systems such as pH, body temperature, glucose levels, and oxygen tension that are truly essential for life and are, therefore, maintained over a narrow range.

Allostasis—achieving stability through change. A process that maintains homeostasis (see above), even though the “set points” and other boundaries of control may change with environmental conditions. There are primary mediators of allostasis such as, but not confined to, hormones of the hypothalamic-pituitaryadrenal (HPA) axis, catecholamines, and proand anti-inflammatory cytokines.

Allostatic state—altered and sustained activity levels of the primary mediators (see above) that integrate energetic and associated behaviors in response to changing environments, challenges such as social interactions, weather, disease, predators, pollution, and so forth. Originally proposed for understanding physiological aspects of drug abuse, an allostatic state results in a chronic imbalance in the primary mediators reflecting excessive production of some, and inadequate production of others. Allostatic states can be sustained for limited periods if food intake and/or stored energy such as fat can fuel homeostatic mechanisms (e.g., bears and other hibernating animals preparing for the winter).

Allostatic load and allostatic overload—Allostatic load is the cumulative result of an allostatic state (e.g., fat deposits in a bear preparing for the winter). Within limits, it is part of adaptive responses to seasonal and other demands. However, with additional unpredictable events in the environment, disease, and adverse social interactions, allostatic load can increase dramatically. When allostatic load continues for longer periods and becomes independent of the need for survival, then symptoms of allostatic overload appear. Abdominal obesity is an example of this condition.

...