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What Is Aging2 - The following was taken from the website h...

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The following was taken from the website http://www.senescence.info/ What Is Aging? Although everyone is familiar with aging, defining it is not so straightforward. In fact, aging can have a positive connotation as in "aging wine." In the context of senescence.info, and unless otherwise noted, the term "aging" refers to the biological process of growing older in a deleterious sense, what some authors call "senescence" ( Williams, 1957 ; Comfort, 1964 ; Finch, 1990 ). Aging is one of the most complex biological processes, whose definition is intrinsically related to its phenotype, as developed below. Demographic Measurements of Aging Aging has been defined as the collection of changes that render human beings progressively more likely to die ( Medawar, 1952 ). Clearly, one hallmark of aging in humans and many other species is an age-related increase in mortality rates (Fig. 1). Figure 1: Mortality rates, expressed in deaths per 100,000 people, as a function of age for the 2002 US population. The black line represents the Gompertz function extrapolated from the mortality rates after maturity. Source: CDC/NCHS , National Vital Statistics System , Mortality .
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Mathematically, aging can be quantified using mortality curves such as those of Figure 1. There are several mathematical functions that can be used ( Wilson, 1994 ; Strehler, 1999 , pp. 103-124). From these equations we can derive the initial mortality rate (IMR), which is the mortality rate independent of aging, often calculated from the mortality rate prior to its exponential increase with age; in this case, IMR = 0.0002/year since that is the mortality rate at ages 10-20. Another important variable taken from the Gompertz equation is the mortality rate doubling time (MRDT) given by MRDT = 0.693/ G ( Finch, 1990 , pp. 22-24). Hence, MRDT = 0.693/0.0800 = 8.66 years. In fact, human populations have a MRDT of about 8 years. This means that after our sexual peak, or roughly age 30, our chances of dying double about every 8 years. Demographic measurements of aging, such as the MRDT, may then serve as estimates of the rate of aging. Changes in the MRDT are expected to reflect changes in the rate of aging, but the same is not true for the IMR ( Finch, 1990 ; Finch and Pike, 1996 ; de Magalhaes et al., 2005a ). For example, the life expectancy at birth increased considerably in the past 100 years: in the US, the life expectancy at birth jumped from 47.3 years in 1900 to 77.3 years in 2002 ( National Center for Health Statistics, Data Warehouse on Trends in Health and Aging ). Still, the rate of aging and the MRDT have remained unaltered for thousands of years ( Finch, 1990 ; Hayflick, 1994 ). What happened was that the IMR, which is not affected by the aging rate, was lowered due to breakthroughs in different areas, such as in the war against infectious diseases, thus lowering mortality rates across the entire lifespan and increasing the life expectancy.
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