FEATURE ARTICLE
Aging: A Biological Perspective
A variety of techniques extend the lives of model organisms, and similar approaches might help human beings stay healthy longer
Robert Arking
Living Longer
As early as 1840, life expectancy started increasing in Sweden. Soon, the trend appeared in other developed countries, too. In the United States, for example, white females in 1900 lived an average of 48 years; by 2000, they lived an average of 87 years. This 39-year increase in average lifespan really took hold by mid-century, largely due to reduced mortality before puberty, which killed 24 percent of the women born in 1900.
Young girls—no longer taken by accidents or infectious diseases—survived to die as old ladies. In the 1920s, extensive public-health measures, including sanitary sewers and clean drinking water, triggered this decreased mortality. Later, antibiotics and improved medical care increased average life spans even more. As the 20th century rolled by, the elderly grew more healthy and mentally independent than their parents at the same ages. In addition, the average person lived longer than ever. Consequently, the probability of some proportion of them surviving for more than a century increased, as well.
Although social and medical interventions helped people live longer, none of the techniques affected the aging process. A healthy 65-year-old in 1900 would be physically indistinguishable from his or her counterpart in 2000. There are simply more 65-year-olds today because the past century's efforts reduced early mortality. If you do not die young, then you can live to be old, but you will still age as humans have throughout history.
Aging involves multiple deleterious biological events that accumulate in different tissues over time and gradually reduce an organism's state of maintenance and function. Calendar time, however, serves as an imperfect measurement of the physiological processes involved in aging. We all know individuals who are the same chronological age but appear to be very different physiological ages. Rather than counting years—or gray hairs, for that matter—modern gerontologists turn to biological markers, or biomarkers, of aging. These physiological parameters indicate an individual's functional level, and some biomarkers, such as insulin levels, correlate with mortality. The presence of such biomarkers depends indirectly on patterns of gene expression, which are induced by a variety of internal or external stimuli. If gene expression remains stable, so does an adult's overall health. In fact, extraordinary stability in gene expression can create a centenarian, but especially unstable expression can trigger premature mortality. If aging is a series of increasingly different molecular and physiological signatures, then senescence comprises the processes that are responsible for the changes in those signatures.
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