Expectancy and Lifespan, Theories of Aging

by National Institute on Aging

(Page 2 of 15)

Why these and other changes occur with advancing age both intrigues and perplexes gerontologists. In fact, senescence is one of nature's least understood biological processes. Gerontologists, for instance, disagree about when senescence begins. Some argue it begins at birth. Others contend it sets in after the peak reproductive years. But clearly, senescence, whether it begins at birth or age 20, 30, or 40, leads to an accumulating loss of bodily functions, which ultimately increases the probability of death, as we get older.

What is the Difference Between Life Expectancy and Lifespan?

When George Washington celebrated his 60th birthday in 1792, he had outlived all of his male ancestors, dating back for several generations. He had outlived a typical Virginian of his era by about 15 years. To achieve this relative "old age," he had survived smallpox, mumps, pneumonia, dysentery, typhoid fever, a staphylococcal infection of the hip, four bouts of malaria, and two nearly fatal encounters with influenza.

When Jeanne Calment was born in 1875, the illnesses that plagued Washington's generation still took many lives, health care was still fairly primitive, and the life expectancy - the average number of years from birth that an individual can expect to live - was still less than 50 years worldwide.

Yet Mme. Calment, perhaps because she was born with a set of extraordinary genes or was simply fortunate enough to elude many of the illnesses that claim so many others, beat the odds and set the benchmark for maximum human lifespan - the greatest age reached by any member of a species.

Life expectancy in the United States rose dramatically in the 20th century, from about 47 years in 1900 to about 73 years for males and 79 years for females in 1999. This increase is mostly due to improvements in environmental factors - sanitation, the discovery of antibiotics, and medical care. Now, as scientists make headway against chronic diseases like cancer and heart disease, some think life expectancy can be extended even further in the 21st century.

As part of this quest, gerontologists are studying a variety of life forms including yeast, fruit flies, nematodes, mice, and primates in search of clues applicable to human aging. As they explore the genes, cells, and organs involved in aging, they are uncovering more and more of the secrets of longevity. As a result, life extension may some day be more than the stuff of myth. In addition, as gerontologists apply their expanding knowledge to medicine, the prevention or retardation of the onset of some age-related diseases and disabilities may become realistic goals.

Why Do We Age?

Gerontologists have proposed many theories to explain the diversity of the aging process in nature. Pacific salmon, for instance, reproduce only once and die within hours of spawning, while at the other end of the spectrum, sea anemones, which reproduce asexually, show few, if any, outward signs of deterioration until the very end of their long lives. Most gerontologists now agree that no single theory can account for this wide spectrum. In fact, with the tools of biotechnology and an influx of new knowledge, all-encompassing theories of aging are giving way to a more diverse perspective.

Aging today is viewed as many processes, interactive and interdependent, that determine lifespan and health, and gerontologists are studying a multitude of factors that may be involved. The rest of this booklet describes what we know and don't know about many of these factors, and where we think scientists are likely to find answers to questions about aging and longevity.

Theories of Aging

Theories of aging fall into two groups. The programmed theories hold that aging follows a biological timetable, perhaps a continuation of the one that regulates childhood growth and development. The damage or error theories emphasize environmental assaults to our systems that gradually cause things to go wrong. Many of the theories of aging are not mutually exclusive. Here is a brief and very simplified rundown of the major theories.

Programmed Theories

Programmed Longevity. Aging is the result of the sequential switching on and off of certain genes, with senescence being defined as the time when age-associated deficits are manifested.

Endocrine Theory. Biological clocks act through hormones to control the pace of aging.

Immunological Theory. A programmed decline in immune system functions leads to an increased vulnerability to infectious disease and thus aging and death.

Error Theories

Wear and Tear. Cells and tissues have vital parts that wear out.

Rate of Living. The greater an organism's rate of oxygen basal metabolism, the shorter its life span.

Crosslinking. An accumulation of crosslinked proteins damages cells and tissues, slowing down bodily processes.

Free Radicals. Accumulated damage caused by oxygen radicals causes cells, and eventually organs, to stop functioning.

Somatic DNA Damage. Genetic mutations occur and accumulate with increasing age, causing cells to deteriorate and malfunction. In particular, damage to mitochondrial DNA might lead to mitochondrial dysfunction.

About the Author

www.nia.nih.gov
NIA, one of the 27 Institutes and Centers of NIH, leads a broad scientific effort to understand the nature of aging and to extend the healthy, active years of life. In 1974, Congress granted authority to form NIA to provide leadership in aging research, training, health information dissemination, and other programs relevant to aging and older people.