Intrinsic Capacity as a Determinant of Physical Resilience in Older Adults

Abstract

The traditional disease-oriented model of healthcare is inadequate to address the needs of the older population. Greater attention should be given to strategies that promote healthy aging. Recently proposed constructs of intrinsic capacity (IC) and physical resilience (PR) hold great potential to reshape geriatric medicine and aging research. These constructs accentuate the positive health attributes of older people in contrast to the popular frailty construct that is centered on functional deficits. IC was introduced by the World Health Organization (WHO) as a composite of all the physical and mental capacities. WHO has emphasized enhancement of IC throughout the life course so as to maintain functional ability in old age. PR, recently highlighted by the National Institute on Aging, is the ability to successfully cope with stressors. High levels of resilience can result in desirable clinical and functional outcomes after stressors. Therefore, it is important to understand the underlying physiology of PR and the risk factors contributing to diminished PR. The main goal of this article is to explore the potential relationship between IC and PR. Based on a classical theory of aging, we postulate that IC is a determinant of PR and is also a high-level integrative measure of physiologic reserve which is the fundamental factor underlying one's ability to withstand stressors. A major implication of our postulates is that even though IC is only one of the many determinants of PR, it could serve as an important intervenable target for enhancing resilience in older adults.

Keywords: Resilience; aging; frailty; integrated care; physiologic reserve.

Figure 1

Concept model showing the link between intrinsic capacity, physiologic reserve and physical resilience Physical resilience (PR) is a dynamical concept that is defined only under conditions when an organism is exposed to a stressor. The physiological response to the stressor is governed by PR. Long-term outcomes such as physical and cognitive functions, and clinical outcomes will be better for organisms with larger PR. Figure 1 also shows the pre-determinants of PR including age, psychosocial factors, health behaviors, genetics, and diseases. We hypothesize that the impact of these factors on PR is mediated through the intrinsic capacity of the organism. Intrinsic capacity is a high-level integrative measure of the underlying physiologic reserve of the organism, which is the weighted sum of organ-specific reserves (also see Figure 2).

Figure 2

Physiologic reserve as the link between intrinsic capacity and physical resilience (based on the Strehler and Mildvan theory of aging) Darker triangle denotes a robust system, which has a higher physiologic reserve and its integrative function or Intrinsic Capacity (IC), therefore, with higher level of physical resilience(PR). Every time a system is exposed to a stressor it uses some of its physiologic reserve (depending on the magnitude of stressor) to overcome the challenge (that leads to a reduction in IC), therefore will have lower level of PR. Gradual exposure to multiple stressors over time reduces the physiologic reserve/IC/PR greatly leaving the system vulnerable (triangle gets lighter in the figure), ultimately leading to death.