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

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 “same,” while stasis means “stable;” thus, “remaining stable by staying the same.” Allostasis was coined similarly, from the Greek allo, which means “variable;” thus, “remaining stable by being variable” 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, we can see two types of allostatic overload.

The key difference between allostasis and homeostasis is popularized by Robert Sapolsky's book Why Zebras Don't Get Ulcers:

Homeostasis is the regulation of the body to a balance, by single point tuning such as blood oxygen level, blood glucose or blood pH. For example, if a person walking in the desert is hot, the body will sweat. However he would quickly become dehydrated. Allostasis is adaptation to a more dynamic balance. In our dehydration example, sweat is only a small part of the process. Many other systems will have to adapt their functioning as well, both to reduce water use and to support the other systems that are changing. Kidneys may reduce urine output. Mucous membrane in the mouth, nose and eyes may dry out; urine and sweat output will decrease; the release of arginine vasopressin (AVP) will increase; and veins and arteries will constrict to maintain blood pressure with a smaller blood volume.

This change may be adaptive. If a dehydrated person is lost in the desert and his body acted as it did under normal conditions, he or she would lose too much water too quickly, dehydrate and die. However, maintenance of allostatic changes over a long period may result in allostatic load. If our desert wanderer is rescued, but continues to be stressed and hence does not reinstate normal body function, his systems will quickly wear out. The human body is amazingly adaptable, but it cannot maintain allostatic overload for very long without consequence.

McEwan and Wingfield propose two types of allostatic load which result in different responses:

Type 1 allostatic overload occurs when energy demand exceeds supply, resulting in activation of the emergency life history stage. This serves to direct the animal away from normal life history stages into a survival mode that decreases allostatic load and regains positive energy balance. The normal life cycle can be resumed when the perturbation passes.

Type 2 allostatic overload begins when there is sufficient or even excess energy consumption accompanied by social conflict and other types of social dysfunction. The latter is the case in human society and certain situations affecting animals in captivity. In all cases, secretion of glucocorticosteroids and activity of other mediators of allostasis such as the autonomic nervous system, CNS neurotransmitters, and inflammatory cytokines wax and wane with allostatic load. If allostatic load is chronically high, then pathologies develop. Type 2 allostatic overload does not trigger an escape response, and can only be counteracted through learning and changes in the social structure. ,

The use of adaptogens can help reduce allostatic load. Adaptogens are substances, herbal or pharmaceutical, which are nontoxic in normal doses, produce a nonspecific defensive response to stress, and have a normalizing influence on the body. They normalize the hypothalamic-pituitary-adrenal axis (HPA axis). As defined, adaptogens constitute a new class of natural, homeostatic and allostatic metabolic regulators. Adaptogens have a normalizing effect on the body and are capable of either toning down the activity of hyperfunctioning systems or strengthening the activity of hypofunctioning systems.