We might be able to recall a time in our adolescence when we could stay up all night and still have energy and eat junk food without gaining any weight. Younger adults recover faster from exercise, while adults recover at a slower rate. What are the physiological mechanisms behind these changes that take place in adolescence to emerging adulthood and beyond? This paper aims to explore and discuss the physiological mechanisms of allostasis, allostatic load, organ reserve, and homeostasis. These physiological mechanisms collectively characterize our capacity for recovery and tolerance to stress. Better understanding of our brain’s physiological mechanisms in response to stress, can help health professionals intervene with sound scientific methods that improve health outcomes related to stress adaptation, cardiovascular disease, recovery, and resilience.
The major endocrine organs are the hypothalamus, pituitary gland, pineal gland, thyroid and parathyroid gland, and adrenal glands make up the major endocrine glands (McGraw hill 2019). Collectively they serve as the physiologic mediators of the stress response, when initiated result in the release of hormones such as catecholamines, glucocorticoids, and cytokines which have an effect on various organ functions and as adaptive response to stress (Kiecolt-Glaser et al.2003,McEwen 2004,Schulkin 2004). “If these mediators are continuously elevated without efficiently shutting off, they would cause tissue damage or receptor desensitization as allostatic load. Suppressed immune function by certain hormones, cardiovascular disease, obesity, and anxious depression showing atrophy of nerve cells in the brain (by cortisol) can be examples of chronic illnesses associated with allostatic load (McEwen 2004,Schulkin 2004).
Young bodies can tolerate temperature changes much better than adults, because of homeostasis and organ reserve. Allostasis and allostatic load are relatively new concepts they both literally mean to remain stable during change. This also implies the physiological responses that occur due to stress (McEwen 2003, Sterling and Eyer (1988). Perceived stress by the brain triggers physiologic and behavioral responses, the physiologic responses lead to allostasis in many systems including the sympathetic-adrenal medullary system, the hypothalamic pituitary adrenocortical axis, and cardiovascular, metabolic, neural, endocrine, and immune systems. Repeated and cumulative allostasis over time causes allostatic load that over activated neural, endocrine, and immune stress mediators results in various diseases (McEwen 1998, 2006). Blood pressure is a biomarker of allostasis. Normally blood pressure fluctuates during the day depending on physical and emotional status (Sterling & Eyer 1988). However, repeated elevated blood pressure (allostatic load) may increase atherosclerotic plaques and stiffness of large arteries leading to greater risk for cardiovascular disease (McEwen 2002,Manuck et al.1995).
The process of allostatic load refers to the point in time when “normal allostatic processes wear out or fail to disengage and therefore, the physiological systems are not able to adapt”(Seeman et al, 2004). “Frequent or chronic challenges produce dysregulation of several major physiological systems, including the hypothalamic–pituitary–adrenal (HPA) axis, the sympathetic nervous system and the immune system “(Shulkin 2004). Reviewed data presented four response patterns 1. Repeated insults lead to allostasis over time. 2. the organism cannot adjust to the stress 3. response pattern of the physiologic systems remains heightened or levels of high levels of activation without sufficient recovery. 4. The primary adaptation mechanisms are inadequate to meet the persistent challenges resulting in the activation of compensatory mechanisms These four types of over actively or inefficiently managed allostatic responses may occur alone or in combination ultimately resulting in chronic illness. (McEwen 2002).
Organ reserve is the additional stored power that each organ can use when needed. In emerging adulthood, organ reserve allows for speedy recovery. Organ reserve is crucial when emerging adults undergo surgery, undergo chemotherapy or radiation (Ahles et al.,2012.) Typically, organ reserve enables emerging adults’ brain and bodies recovery faster from various stressors This is what helps emerging adults to bounce back after insults of hard sports, training, sleep debt, and bad food choices without disrupting body function. This is because organ reserve has been activated, and the body can recover. The opposite is true for older adults. This reserve of organ power shrinks each year of adulthood so that by old age, taking a bad fall, strain-shoveling snow, catching a virus, minor or major surgery, nutritional deficiencies and excessive amounts of high intensity exercise can overwhelm the body. It’s not only the reserve capacity of organs but also the cells metabolic reserves namely the mitochondrial density that are the “functional resilience of a biochemical network” (Ataman et al., 2018, p.177).
Atypical changes, such as inactivity, unhealthy eating, excessive alcohol consumption, and sleep debt accumulate, allostatic load will be reached. The stressors burden regulatory body systems that monitor physiological needs to a point that overall function is negatively impacted (Karlamangla, v 2002). Early childhood exposure to stressors affects their ability to tolerate stress later in life. Our bodies have built in processes for long-term adjustment to stress, which depends on the biological circumstances of experiences in earlier childhood. An example of this is children in Romania during the 1980s in which abortion was forbade. As a result, many children were abandoned and sent to crowded, impersonal, state-run orphanages (Marshall, 2014). Later, these children as adults had serious emotional or behavioral problems. Allostasis takes into consideration normal variations in a dynamic biological system (Carlson & Chamberlain 2005). Homeostasis is a balance between various body reactions that keeps every physical function in sync with every other. “With homeostasis, the internal environment of the organism is maintained relatively steady by homeostatic regulation” (Cannon 1932). An example of homeostasis is when the temperature rises, people sweat to cool themselves.
By mid-and late adulthood, years of accumulated sleep debt, and poor sleep habits impair overall health (McEwen, 2015). A single night of bad sleep for an average adult makes them tired the following day-that’s homeostasis, our bodies mechanism to sustain equilibrium. However, if poor sleep habits continue, over time changes in appetite, mood, circadian rhythm lead the brain and body to attempt to compensate to maintain homeostasis (McEwen, 2002) While stress persists, allostatic load rises and the extra power that organs employ when needed are reduced. “Frequent or chronic challenges produce dysregulation of several major physiological systems, including the hypothalamic–pituitary–adrenal (HPA) axis, the sympathetic nervous system and the immune system (Shulkin 2004).
The brain, hypothalamus and the hippocampus play a primary role in physiological responses after experiencing stress, but they are also a target organ of stress.” The hippocampus, amygdala and prefrontal cortex are known to undergo stress-induced structural remodeling” (McEwen 2007). The hippocampus has been shown to atrophy under stressful events (Duman & Monteggia 2006). Severe and prolonged elevated cortisol levels are correlated with the decrease in size of hippocampus (Lupien et al. 1998). Sapolsky (2003). However, Neuroplasticity may be facilitated to increase the capacity of hippocampus in mediating stress response (Sapolsky 2003). The fact that the brain can reshape itself with new life experiences, exercise, and variability, gives hope that through a facilitated environment by selective methods, resilience can be enhanced, and physiologic outcomes may be changed that improve health outcomes.
Resilience is an example of successful allostasis in which wear, and tear is minimized, and the brain retains considerable resilience in the face of stress (McEwen ,2002). A resilient organism that can adapt to challenging environments will be able to minimize physiological damage (Carlson & Chamberlain 2005). The ability to bounce back from adverse situations and move has been associated with positive outcomes even in situations that could produce pathological conditions (Masten & Coatsworth 1998,Luthar et al.2000). Resilience can be considered as the capacity in which allostasis biomarkers in response to stress can be maintained within normal ranges. The person with a larger adaptive capacity (more resilience) would have more resistance to allostasis converting to the allostatic load than the person who has a relatively small adaptive capacity.
Managing stress levels through the regulation of the autonomic nervous system by various means can help mitigate the stress experienced thus increasing health and performance outcomes. Allostasis and allostatic load help to expand the discussion on the mechanisms behind stress responses and how they contribute to cardiovascular disease and other chronic illnesses associated with neuroendocrine, autonomic nervous system and immune system dysfunctions. Implementing strategies and protocols that increase resilience and treat the physiologic responses to acute and chronic stress are required to fully care for the individual. Therefore, a major goal of practitioners should be to reduce the psychological and resulting physiological burden of chronic stress through social, emotional, cognitive, physical and behavioral therapeutic interventions (McEwen 2007). Prescribing exercises that are multidirectional with variable vectors, help facilitate neurogenesis and synaptogenesis by creating new movement pathways (Sapolsky 2003). This also means knowing when to replace stressful exercise in favor of aerobic low intensity exercise that facilitates increased vagal tone that aids in rest and digestion. This allows individuals adequate time to adapt and therefore reduce allostatic load which would decrease performance and health parameters.
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