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Is there a maximum biological limit to the human life span, somewhere
around 120 years? Or could we live much longer, given the right
conditions? Answers to these and other fundamental questions about
aging may now be within reach. |
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One hundred
and twenty years, as far as we know, is the longest that anyone has
ever lived. A man in Japan, Shirechiyo Izumi, reached the age of 120
years, 237 days in 1986, according to documents that most experts
think are authentic. He died after developing pneumonia.
Long lives
always make us wonder: What is the secret? Does it lie in the genes?
Is it where people live or the way they live -- something they do
or do not do? Eat or do not eat? Most of the scientists who study
aging, gerontologists, say the secret probably lies in all of the
above -- heredity, environment, and lifestyle.
But gerontologists
also ask other and more difficult questions. For example, if the
120-year-old had not finally succumbed to illness, could he have
lived on and on? Or was he approaching some built-in, biological
limit? Is there a maximum human life span beyond which we cannot
live no matter how optimal our environment or favorable our genes?
Whether or
not there is such a limit, what happens as we age? What are the
dynamics of this process and how do they make life spans short,
average, or long? Once we understand these dynamics, could they
be used to extend everyone's life span to 120 or even, as some scientists
speculate, to much greater ages?
And finally
for all of us, the most important question: How can insights into
longevity be used to fight the diseases and disabilities associated
with old age to make sure this period of life is healthy, active,
and independent?
In Search
of the Secrets of Aging describes what we know so far about the
answers to these questions and what we want to know. It gives an
overview of research on aging and longevity, showing the major puzzle
pieces already in place and, to the extent possible, the shapes
of those that are missing.
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Life Span
and Life Expectancy
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This booklet
-- and gerontologists -- talk about two kinds of life span. One is
maximum life span, the greatest age reached by any member of a species.
In humans this is 120 years, we think. The other is average life span,
the average age reached by members of a population. Life expectancy,
the number of years an individual can expect to live, is based on
average life spans.
Average life
span and life expectancy in the United States have grown dramatically
in this century, from about 47 years in 1900 to about 75 years in
1990. This advance is mostly due to improvements in sanitation,
the discovery of antibiotics, and medical care. Now, as scientists
make headway against chronic diseases like cancer and heart disease,
some think it can be extended even further.
Maximum human
life span seems to be another matter. There is no evidence that
it has changed for thousands of years despite fabled fountains of
youth and biblical tales of long-lived patriarchs. However, very
recently, the dream of extending life span has shifted from legend
to laboratory. As gerontologists 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 now be more than the
stuff of myth and the retardation of disease and disability, realistic
goals.
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Aging
Theories
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Gerontology
is often described in terms of its major theories. These fall into
two main groups, one emphasizing internal biological clocks or "programs,"
and the other external or environmental forces that damage cells and
organs until they can no longer function adequately.
Aging processes
can be divided into three general categories -- genetic, biochemical,
and physiological. The rest of this booklet describes what we know
and don't know in each territory and where we think we are likely
to find answers to questions about aging and longevity.
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| 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. For more information, see
Selected Readings.
Programmed Theories
Programmed
Senescence. 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.
Error
Catastrophe. Damage to mechanisms that synthesize proteins
results in faulty proteins which accumulate to a level that
causes catastrophic damage to cells, tissues, and organs.
Somatic
Mutation. Genetic mutations occur and accumulate with
increasing age, causing cells to deteriorate and malfunction.
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Questions:
Selected Readings
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Finch, C.E.,
Longevity, Senescence and the Genome, Chicago:University
of Chicago Press, 1991.
Institute
of Medicine, Extending Life, Enhancing Life: A National Research
Agenda on Aging, Washington, DC: National Academy Press, 1992.
Martin, G.R.,
and Baker, G.T., "Aging and the Aged: Theories of Aging and Life
Extension," Encyclopedia of Bioethics, New York: MacMillan,
1993.
Schneider,
E.L., and Reed, J.D., "Life Extension," New England Journal of
Medicine 313:1159-1168, 1985.
Warner, H.,
Butler, R.N., Sprott, R.L., Schneider, E.L.,eds., Modern Biological
Theories of Aging, New York: Raven, 1987.
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