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THE IMPACT OF AGING ON MEDICINE
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Aging and old age are among the most significant challenges facing medicine this century. The aging process is the major risk factor underlying disease and disability in developed nations, while older people respond differently to therapies developed for younger adults (usually with less effectiveness and more adverse reactions). Modern medicine and healthier lifestyles have increased the likelihood that younger adults will now achieve old age. However, this has led to rapidly increasing numbers of older people, often encumbered with age-related disorders that are predicted to overwhelm health care systems. Improved health in old age and further extension of human health span are now likely to be generated primarily from increased understanding of the biology of aging, age-related susceptibility to disease, and modifiable factors that influence the aging process.
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Aging is easy to recognize but difficult to define. Most definitions of aging indicate that it is a progressive process associated with declines in structure and function, impaired maintenance and repair systems, increased susceptibility to disease and death, and reduced reproductive capacity. There are both statistical and phenotypic components to aging. As recognized by Gompertz in the nineteenth century, aging in humans is associated with an exponential risk of mortality with time (Fig. 463-1), although it is now realized that this plateaus in extreme old age because of healthy survivor bias. The phenotypic components of aging include structural and functional changes that are separated, somewhat artificially, into either primary aging changes (e.g., sarcopenia, grey hair, oxidative stress, increased peripheral vascular resistance) or age-related disease (e.g., dementia, osteoporosis, arthritis, insulin resistance, hypertension).
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Definitions of aging rarely acknowledge the possibility that some of those biological and functional changes with aging might be adaptive or even reflect improvement and gain. Nor do they emphasize the impact of aging on responses to medical treatments. Old age is associated with increased vulnerability to many perturbations, including therapeutic interventions. This is a critical issue for clinicians—aging would be a less difficult problem if our disease-specific therapies retained their balance of risk to benefit into old age.
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Aging and Disease Susceptibility
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Old age is the major independent risk factor for chronic diseases (and associated mortality) that are most prevalent in developed countries such as cardiovascular disease, cancers, and neurodegenerative disorders (Fig. 463-2). Consequently, older people have multiple comorbidities, usually in the range of 5–10 illnesses per person.
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