Questions
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IN
SEARCH OF
THE SECRETS OF AGING
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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.
Life
Span and Life Expectancy
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.
Aging
Theories
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.
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
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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