Allostatic Load and Homeostasis

As the human life expectancy continues to expand, there has been increasing emphasis on understanding the cumulative effect of everyday experiences on [Page 33]physical health during older adulthood. The wear-and-tear theory, although overly general and unable to account for the “preprogrammed” nature of biological aging, offers a useful view of the aging process. According to this theory, the human body succumbs to the accumulation of wear and tear associated with injuries and physical damage sustained in everyday life. The accumulation of biological damage over time increases the possibility that the stability of physiological functioning within various body systems, including systems essential for survival such as the nervous and cardiovascular systems, may be disrupted.

The stability of body systems, or homeostasis, appears to be particularly challenged during elevated stress. The causes of stress can be physical (e.g., hunger, cold), social (e.g., poor social network, lack of adequate financial resources), or psychological (e.g., perceived threat, negative social interaction). The body adjusts to each episode of stress by initiating a response designed to reestablish homeostasis. For example, shivering when cold is an attempt to achieve homeostasis by warming tissues, and increased heart rate is an attempt to achieve homeostasis by increasing the readiness of cells as the body expects a fight-or-flight response.

The process of reestablishing homeostasis in response to stress has been described more recently asallostasis; that is, maintaining stability through change. Through allostasis, the body systems attempt to match physiological function to the current environmental demands so as to maintain homeostasis. Therefore, allostasis is an essential component in maintaining homeostasis.

Stress, such as physical exertion, a lack of social support, and public speaking, increases environmental demands and challenges the body's systems to respond adequately. Each physiological response to stress (e.g., elevated blood pressure to increase alertness) causes physiological systems (e.g., the cardiovascular system) to use up resources, making them more susceptible to damage. Although a single response to stress is often adaptive, such as when people become more alert during public speaking, accumulation of stress responses over time can lead to exacerbated wear and tear and can increase the chance of system dysregulation (i.e., exaggerated or inadequate physiological response), accelerating physical aging.

The nervous and cardiovascular systems are essential for achieving stability through change (allostasis) during stress. As the brain detects a threat or a stressful event, it signals a cascade of physiological reactions that, in turn, stimulate the brain, the cardiovascular system, and skeletal muscles, increasing the body's readiness for physical response. This physiological cascade occurs within the hypothalamic–pituitary–adrenal (HPA) axis and the sympathetic nervous system. In response to stress, the HPA axis activates the release of glucocorticoids, a type of stress hormone, into the bloodstream, mobilizing energy and increasing physiological readiness of the body for a fight-or-flight action. Although such adaptive responses are essential during a stressful situation, repeated physiological overstimulation of a body system due to stress can bring about dysregulation.

The cumulative impact of responses to daily stress has been referred to asallostatic load; that is, the “price of adaptation.” The physiological response to stress helps people adapt to everyday experiences and stressful situations by mobilizing the resources within body systems, such as the nervous and cardiovascular systems, but it also causes wear and tear in these same body systems as responses accumulate over time. Long-term exposure to stress has been associated with hypertension, increased susceptibility to type 2 diabetes, impaired immune function, and accelerated brain aging, particularly in the hippocampal region. Similarly, higher levels of allostatic load have been associated with an increased risk of heart disease, a faster rate of cognitive and functional decline, and a higher risk of death among older persons.

...