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Comment
. 2010 Feb;57(2):105-11.
doi: 10.1016/j.yhbeh.2009.09.011. Epub 2009 Sep 26.

What is in a name? Integrating homeostasis, allostasis and stress

Bruce S McEwen  1 , John C Wingfield
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Comment

What is in a name? Integrating homeostasis, allostasis and stress

Bruce S McEwen et al. Horm Behav. 2010 Feb.
No abstract available

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Figures

Figure 1
Figure 1
The potential interactions of energy demand of a free-living animal going about its daily and seasonal routines, coping with unpredictable perturbations, and the potential energy to be gained from food in the environment (Eg). If Eg declines then starvation is probable unless the organisms can reduce allostatic load so energy demand is below Eg. If Eg is adequate to fuel all aspects of the life cycle (including fattening for migration or hibernation) and no severe perturbations occur, then the life cycle progresses normally. If Eg is even higher and excess calories are available (i.e. above those required for fattening in life history stages such as migration and hibernation) then the individual may be able to store fat for future contingencies such as an unpredictable decline in Eg. In the latter scenario, an individual may be able to mobilize these excess calories to fuel allostatic overload, at least temporarily, until environment conditions improve. Taken from McEwen and Wingfield (2003), Wingfield (2004), Korte et al. (2005).
Figure 2
Figure 2
Although humans are subject to the interactions of Eg and allostatic load as outlined in Fig. 1, there are some additional considerations given here. Declines of Eg below that required for normal daily routines may lead to starvation and is a major issue in many human societies. However, adequate food, but often regarded as restricted calories especially in societies of developed countries, may actually prolong life. Excess calories may lead to pathologies in the long term (McEwen, 2006). This is true of nonhuman animals too (especially in agriculture, zoos) etc. but is probably close to nonexistent in the wild because high Eg and availability of excess calories may not persist for long. The allostasis model provides a useful framework for modeling these concepts within a mechanistic framework
Figure 3
Figure 3
The box summarizes how metabolic (energy) demands from the predictable life cycle (daily and seasonal routines) and the unpredictable can result in anabolism or catabolism depending upon available energy from food (Eg). If Eg is high then the normal life cycle progresses and an individual may be able to store energy such as fat for future contingencies. If Eg is variable (in a temporal sense from day to day, or a spatial sense insofar as available food may be patchy in distribution, or both), then there may be complex shifts from anabolic to catabolic states over short time spans (minutes to hours). If Eg declines below Eo (the sum of all metabolic demands of the individual – e.g. allostatic overload type 1) then energy stores such as fat need to be mobilized until allostatic load is reduced to a manageable level or a new source of Eg is found. Below the box are some mediators that are involved in energy management and metabolism in general. Note that this is not a complete list and does not include other mediators of allostasis such as cytokines that are involved with other, often related, processes particularly from unpredictable events. A point to be made here is that although the relationship of allostatic load to Eg varies in a fairly linear manner, the triggering of mediators of allostasis is not linear and forms a complex network depending upon condition of the individual (age, gender, phenotype, social status, injuries, infection etc.). In many cases the emergency life history stage may be triggered by this network allowing the individual to temporarily suspend the normal life history stage (e.g. breeding, migrating etc.), go into survival mode and reduce allostatic load to a manageable level until environmental conditions improve and the normal life cycle can be resumed.
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