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Normal aging

Author
George E Taffet, MD
Section Editor
Kenneth E Schmader, MD
Deputy Editor
H Nancy Sokol, MD

INTRODUCTION

Time modifies many biologic processes. Aging is characterized by progressive and broadly predictable changes that are associated with increased susceptibility to many diseases. Aging is not a homogenous process. Rather, organs in the same person age at different rates influenced by multiple factors, including genetic make-up, lifestyle choices, and environmental exposures. A Danish twin study found that genetics accounted for about 25 percent of the variation in longevity among twins, and environmental factors accounted for about 50 percent [1]. However, with greater longevity (to age 90 or 100), genetic influences became more important.

This topic will present an overview of normal aging. Effects of aging on the endocrine and immune systems are discussed in more detail separately. (See "Endocrine changes with aging" and "Immune function in older adults".)

AGE-ASSOCIATED PHYSIOLOGIC CHANGES

Physiologic rhythms — The organization of rhythmic physiologic processes is altered by aging. Age impacts the circadian pattern of body temperature, plasma cortisol, and sleep, and can cause desynchronization or "internal phase drift." Phase advances can lead to the occurrence of some rhythmic functions (eg, the 24-hour body temperature trough and sleep onset) one to two hours earlier in older adults. The pulsatile secretion of gonadotropins, growth hormone, thyrotropin, melatonin, and adrenocorticotropic hormone (ACTH) are attenuated with age [2]. One source of this dysfunction appears to be neuronal loss in the suprachiasmatic nucleus in the hypothalamus [3]. In addition, age may delay the ability to reset physiologic rhythms to a new photoperiod.

Loss of complexity — Loss of complexity, a concept derived from the field of nonlinear dynamics, may be a general principle of all aging systems [4]. This loss of complexity may result in decreased heart-rate variability, blood-pressure variability, electroencephalographic frequencies, response to auditory frequencies, and response to stress. Age-related loss of complexity may not be immutable, however; as an example, senior athletes show greater heart rate variability than sedentary age-matched controls [5].

Homeostenosis — Homeostenosis refers to the concept that, from maturity to senescence, diminishing physiologic reserves are available to meet challenges to homeostasis. This concept was first recognized by Walter Cannon in the 1940s [6]. Homeostenosis leads to the increased vulnerability to disease that occurs with aging.

                                   

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