The energetic pathway to mobility loss: an emerging new framework for longitudinal studies on aging

Abstract

The capacity to walk independently is a central component of independent living. Numerous large and well-designed longitudinal studies have shown that gait speed, a reliable marker of mobility, tends to decline with age and as a consequence of chronic disease. This decline in performance is of utmost importance because slow walking speed is a strong, independent predictor of disability, healthcare utilization, nursing home admission, and mortality. Based on these robust findings, it has been postulated that age-associated decline in walking speed is a reliable barometer of the effect of biological aging on health and functional status. Despite the extraordinary prognostic information that walking speed provides, which is often superior to traditional medical information, there is a limited understanding of the mechanisms that underlie age- and disease-related gait speed decline. Identifying the mechanisms that underlie the prognostic value of walking speed should be a central theme in the design of the next generation of longitudinal studies of aging, with appropriate measures introduced and analytical approaches incorporated. This study hypothesized that a scarcity of available energy induces the decline in customary walking speed with aging and disease. Based on work in the Baltimore Longitudinal Study of Aging, examples of measures, operationalized dimensions, and analytical models that may be implemented to address this are provided. The main premise is simple: the biochemical processes that maintain life, secure homeostatic equilibrium, and prevent the collapse of health require energy. If energy becomes deficient, adaptive behaviors develop to conserve energy.

Figure 1. An Extended Model of Aging Energetics

The box represents the total amount of energy available to an individual over 24 hours. The height of the box is determined by an individual’s maximal oxygen consumption (VO₂ max), or the maximum amount of energy an individual can expend during physical activity. Total energy availability can be divided into sections which reflect energy utilization. Section (1) represents the theoretical minimal energy required to maintain life, or resting metabolic rate (RMR). Section (2) depicts extra energy required for unstable homeostasis. In older individuals, this may reflect the energy needed to combat multiple comorbidities as the body attempts to heal itself and/or the extra energy needed to perform physical tasks due to reduced biomechanical efficiency. Section (3) represents the energy used for daily activities, ranging from activities of daily living to volitional exercise. Section (4) represents the energy needed to break down food and maintain body temperature. In older adults, declines in maximal oxygen consumption compress the size of the box, resulting in less energy availability overall. Further, more energy is required to maintain homeostasis and perform daily tasks due to reduced metabolic and biomechanical efficiency which results in reduced energy available for “essential” tasks related to independent living and increased feelings of fatigue (5). These feelings represent a signal to the brain that energy resources are limited and that there is a need to slow down (6).

Figure 2. Energy Constructs

1. Essential energy (submaximal & resting EE*) 2. Potential energy (peak EE – submaximal EE*) 3. Available energy (peak EE – resting EE*) *EE = Energy Expenditure

Figure 3

Figure 3a: Essential Energy and Age Figure 3b: Potential Energy and Age Figure 3c: Available Energy and Age

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Figure 3a: Essential Energy and Age Figure 3b: Potential Energy and Age Figure 3c: Available Energy and Age

Figure 3

Figure 3a: Essential Energy and Age Figure 3b: Potential Energy and Age Figure 3c: Available Energy and Age