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Why do we age? summarize 3 theories or hypotheses that explain why our bodies age.( see attachment for materialIn the US and abroad, cosmetic surgery is tremendously popular. Many patients who request cosmetic surgery do so wishing to regain their once youthful appearance. Why do you think Americans are so concerned with the outward signs of aging, most of which are relatively harmless? Is this a positive or a negative social trend? Explain and support your response.
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info aging guides
BIOLOGY OF AGING
THEORIES OF AGING
An introduction to aging science brought to you by the
American Federation for Aging Research
WHAT IS A THEORY
OF AGING?
Theories of aging can be divided
into two categories: those that
answer the question “Why do we
age?” and those that address the
question “How do we age?” Only
a few broad, overarching theories
attempt to explain why we and
nearly all living organisms age.
These theories compete with each
other, making it unlikely that more
than one of them could be true.
Over time, some theories have
fallen out of favor as others have
become more widely accepted.
Other theories, more properly
called hypotheses, are smaller in
scope and address the ­question,
“How do we age?” They attempt
to explain the mechanisms that
affect how we and other ­species
age, and it is likely that a ­number
of them are simultaneously true.
Testing these hypotheses is the
current pursuit of most ­aging
­research. Identification of the
mechanisms that affect ­aging
could lead to interventions that
slow or alter aging. Recent
­research implies that there may be
a limited number of these mechanisms, giving scientists hope that
their efforts may one day lead to
strategies that could help us lead
longer, healthier lives.
MODIFYING THE
COURSE OF AGING
A critical issue in aging research
is whether aging is affected by
one, several, or a multitude of
underlying processes. If there are
­hundreds of different biological
pathways that affect aging, then
odds are slim that science could
ever hope to devise a way of
slowing down how we age or
even understand why aging
happens at all.
2 | Infoaging Guide to Theories of Aging
Identification of the mechanisms that affect aging could lead
to interventions that slow or alter aging.
However, evidence seems to be
pointing to just a few fundamental
processes as the primary culprits
in the scenario of aging. The best
evidence lies in the existence of
single-gene mutations that affect
lifespan in experimental animals,
as well as a well-known environmental intervention called caloric
restriction.
Caloric restriction, in which laboratory animals are maintained
on nutritionally balanced but
sparse diets, containing 30 to
40 ­percent fewer calories than a
normal diet, has been shown to
reliably ­increase the average and
­maximum lifespans of a range
of organisms, including worms,
insects, and rodents. It is currently
under investigation in primates.
By itself, caloric restriction ­retards
almost all of the age-related
changes mice normally undergo,
including the onset of age-related
diseases.
Single-gene mutations that ­extend
lifespan, discovered so far in
roundworms, fruit flies, and mice,
are also a powerful argument
that a finite number of pathways
influence aging. Interestingly, the
genes all seem to affect one of a
few biochemical pathways, such
as energy consumption, stress
resistance, or regulation of what is
called the insulin/IGF-1 neuroendocrine pathway. IGF stands for
insulin-like growth factor.
These findings offer hope that
­researchers may eventually be
able to modify the course of
­aging in humans. However, there
is a ­caveat. Animals ­modified
to live longer often show
­inherent ­defects. Some mutant
­roundworms have reduced ­fertility
and a reduced ability to enter a
dormant state. Mutant Ames dwarf
mice live a long time but are sterile
and inactive. Rodents maintained
on calorically restricted diets are
thin, cold, stunted, and ­sometimes
sterile. It is likely that such animals,
although they survive to a ripe old
age in the laboratory, would never
stand a chance in the wild.
AGING IS NOT A PROGRAMMED
ASPECT OF DEVELOPMENT
Aging is not a programmed aspect
of development. It is the deterioration of what might be thought of
as a survival program. Not long
after Charles Darwin ­published
his groundbreaking theory of
­evolution by natural selection
in On the ­Origin of Species in
1859, ­scientists began to try to
use ­Darwin’s theory to explain
­aging. One of the first was August
­Weismann, who published his
hypothesis in 1891. He proposed
that aging evolved to benefit
­species or groups by eliminating
unfit animals to make room for
the next generation. Although this
idea was popular for decades,
Weismann later rejected it, as
do modern biologists. Evidence
overwhelmingly shows that natural
selection operates to affect the
­reproductive success of individuals,
not the overall survival of groups.
Further argument against a
­ ging
as a programmed aspect of
­development, ordered by a g
­ enetic
blueprint, lies in its v­ ariability.
Although members of a species
­develop into adults in the same
way, even g
­ enetically similar or
identical individuals, raised in
­identical c
­ onditions and eating
identical food, age differently.
Whereas one person (or mouse)
may die of heart failure, another
may succumb to cancer with his or
her heart functioning perfectly.
When scientists discovered that
changing just one gene in the
roundworm, C. elegans, could
significantly extend its lifespan,
some researchers argued that this
showed aging was ­genetically
programmed. However, most
scientists now believe that overstated the case: just because a
gene happens to affect the rate of
aging does not mean that it was
designed by nature to do so. The
majority of scientists now prefer
other theories.
THE CROSS-LINKING/­
GLYCATION HYPOTHESIS
OF AGING
The cross-linking hypothesis is
based on the observation that
with age, our proteins, DNA, and
other structural molecules develop inappropriate attachments
or cross-links to one another.
These ­unnecessary links or bonds
decrease the mobility or elasticity
of proteins and other molecules.
­Proteins that are damaged or
are no ­longer needed are normally broken down by enzymes
called proteases. However, the
­presence of cross-linkages inhibits
the ­activity of proteases. These
­damaged and unneeded proteins,
therefore, stick around and can
cause problems.
One of the main ways cross-­
linking occurs is through a
­process called glycosylation or
­glycation. Glucose molecules
can stick to proteins, then
­transform into brownish molecules
called ­advanced ­glycosylation
­endproducts, or AGEs. When both
of the sticky ends of AGEs adhere
to neighboring proteins, they form
­permanent cross-links that disable
the ­proteins’ ­functions. This is the
same process that causes food to
brown when it is cooked.
Some research supports the
­hypothesis that cross-linking
contributes to aging. Cross-linking
of the skin protein collagen, for
example, has proven at least
partly responsible for wrinkling and
other age-related dermal changes.
Cross-linking of proteins in the
lens of the eye is also believed to
play a role in age-related cataract
formation. Researchers speculate
that cross-linking of proteins in
the walls of arteries or the filtering
systems of the kidney account for
at least some of the atherosclerosis (hardening of the arteries)
and age-related decline in kidney
function observed in older adults.
Another study conducted at the
Bjorksten Institute in Wisconsin
treated brain tissue from young
animals with known cross-linkinducing compounds. That brain
tissue soon looked quite similar to
older brain tissue, with its naturally
cross-linked brain proteins, adding
evidence in support of this theory
of aging.
Recently, scientists have found
evidence that glycation contributes
to the formation of beta-amyloid,
Infoaging Guide to Theories of Aging | 3
the protein that clumps together in
the brains of Alzheimer’s patients.
Somewhat indirect experimental
evidence in support of the crosslinking theory of aging appears
in studies that look at drugs that
­prevent cross-linking, and the
­impact of taking those drugs on
the various components of the
­aging process. Studies done in
China and in the United Kingdom
on the molecule carnosine are
­provocative. Carnosine occurs
in very low concentrations in the
brain and other tissues. In the­
­laboratory, carnosine has been
shown to delay the senescence
or aging of human cells called
fibroblasts. Carnosine works by
­preventing cross-linking of proteins. The more recent ­Chinese
studies suggest carnosine might
be of benefit in delaying the formation of cataracts, in which crosslinking is thought to play a part.
Although many scientists agree
that cross-linking of proteins, and
perhaps the cross-linking of DNA
molecules as well, is a component
of aging, it is likely only one of several mechanisms that contributes
to aging.
THE EVOLUTIONARY
SENESCENCE THEORY
OF AGING
The most widely accepted overall
theory of aging is the evolutionary
senescence theory of aging. Unlike
the earlier programmed theory of
evolution and aging, which tried to
find reasons why evolution might
favor aging, evolutionary senescence theory focuses on the failure
of natural selection to affect latelife traits.
Natural selection, because it operates via reproduction, can have
little effect on later life. In the wild,
predation and ­accidents ­guarantee
that there are always more younger
individuals reproducing than older
ones. Genes and mutations that
have ­harmful effects but appear
only after reproduction is over do
not affect ­reproductive success
and therefore can be passed on
to future generations. In 1952,
Peter Medawar ­proposed that
the inability of natural selection to
influence late-life traits could mean
that genes with detrimental latelife ­effects could continue to be
passed from generation to generation. This theory is called the
mutation accumulation theory.
Many scientists believe that
mitochondrial aging is an
important contributor to
aging in general.
4 | Infoaging Guide to Theories of Aging
A few years later, George
­Williams extrapolated on this
idea by formulating the theory of
­“antagonistic pleiotropy.” Antagonistic pleiotropy means that some
genes that increase the odds of
successful reproduction early in
life may have deleterious effects
later in life. ­Because the gene’s
harmful effects do not appear
until after reproduction is over,
they cannot be eliminated through
natural ­selection. An example of
antagonistic pleiotropy in humans
is p53, a gene that directs damaged cells to stop reproducing
or die. The gene helps prevent
cancer in younger people, but may
be partly responsible for aging
by ­impairing the body’s ability to
renew deteriorating tissues. Because of antagonistic pleiotropy, it
is likely that tinkering with genes to
improve late-life fitness could have
a detrimental effect on health at
younger ages.
Much experimental evidence exists to support the basic premises
of the evolutionary senescence
theory of aging. For example, the
theory predicts that delaying the
age of reproduction should delay
aging, as it would increase the
power of natural selection later
in life. Experiments with fruit
flies in which younger flies were
prevented from mating, ­allowing
only older flies to reproduce,
­confirmed this prediction. Aging
in the fruit fly population was delayed. ­However, these long-lived
flies were less fertile in early life
than normal flies, giving support
to the idea of antagonistic pleiotropy. In experiments with roundworms given a gene mutation that
extended their lifespan, scientists
found that these long-lived worms
­exhibited defects, such as reduced
­ability to enter a protective dauer
stage (a developmental state in
which worm larvae can better
­survive harsh conditions), delayed ­development, and impaired
­reproduction.
In the 1970s, Thomas Kirkwood
added to the evolutionary biology theory of aging with his
­“disposable soma” theory. He
believed that organisms have to
balance the demands of maintaining their body cells, or soma,
and reproducing. Because an
organism invests resources into
­reproduction, over time mutations
and other cellular damage accumulate in the soma because the
body cannot repair all of it. This
idea explains some of the ­disparity
in lifespan between different
types of organisms. Species that
are likely to die due to predation,
such as mice, invest more energy
in reproduction than in maintaining health because an individual
is unlikely to live long anyway.
­Humans, on the other hand, have
few predators and can therefore
allocate more resources to repairing physical damage since they
will be able to reproduce over a
longer period of time.
Research conducted by Steven Austad in the early 1990s
­provides interesting proof of
this idea, namely, that hazardous ­environments favor early
reproduction and short lifespans,
whereas safer environments favor
the ­opposite. Studying Virginia
opossums in South Carolina and
­Georgia, he found that animals
­living on a predator-free island
aged much more slowly and reproduced later than opossums on the
more dangerous mainland.
The disposable soma theory may
also explain why some organisms,
like salmon or certain kinds of
spiders, reproduce only once and
then die. If the animal is likely to
die anyway before the next breeding season, then natural selection would favor allocating all an
animal’s resources to reproduction,
leaving nothing for somatic maintenance.
Although many scientists believe
the evolutionary theory of aging
needs further refinement, most
agree that it is currently the best
explanation for why we and other
organisms age.
THE GENOME MAINTENANCE
HYPOTHESIS OF AGING
Damage to our DNA happens
thousands of times every day in
every cell in our body throughout
our lives. This damage can be
caused by oxidative free radicals,
mistakes in replication, or ­outside
environmental factors such as
radiation or toxins. Mutations
or spontaneous changes in the
structure of our genes that occur
in our egg or sperm cells will be
passed on to future generations, if
those mutations are not so potentially disruptive as to be fatal to
our offspring. Mutations that occur
in the rest of the cells of the body
will only affect that individual and
­cannot be passed on to future
generations. Most of those body
cell, or somatic, mutations will
be corrected and eliminated, but
some will not. Those will accumulate, eventually causing the
cells to malfunction and die. This
process, it has been suggested, is
a crucial component in the aging
process. This theory also encompasses a role for mitochondria, the
­cellular powerhouses, as important ­factors in aging. Mitochondria
create ­damaging free radicals as
a by-product of normal energy
production. Somatic mutations
in the DNA of the mitochondria
­accumulate with age, increasing
free ­radical production, and are
associated with an age-related
decline in the functioning of
­mitochondria. Many scientists
believe that mitochondrial aging is
an important contributor to aging
in general.
Luckily, our bodies have repair
mechanisms to take care of much
of that damage. In fact, many
scientists believe that humans
have long lifespans because we
are much better at repairing our
genome than short-lived animals
like mice. This is related to an evolutionary theory of aging called the
“disposable soma” theory. Defects
in DNA repair seem to be directly
related to aging. Evidence exists
for the decline in DNA repair and
the accumulation of DNA damage
in several different types of cells
taken from elderly subjects. Elderly
patients’ blood and skin cells have
less capacity to repair themselves
than those from young adults.
Indeed, one study that looked
Infoaging Guide to Theories of Aging | 5
in white blood cells found DNA
­damage in two to four percent of
the cells from young adults, but six
times more often in cells from the
elderly. These aging white blood
cells with their higher level of DNA
damage may explain some of the
decline in immune function associated with aging.
In addition, scientists have linked
Werner’s syndrome, a rare disease
of premature aging, to mutations
in the WRN gene. These ­mutations
lead to abnormalities in DNA replication and repair of DNA damage.
Poor capacity for DNA repair is
also linked to the most prevalent
disease of aging, cancer.
Exploring the role of DNA damage
and repair remains a critical area
of aging research.
THE NEUROENDOCRINE
­HYPOTHESIS OF AGING
The neuroendocrine system ­refers
to the complex connections
between the brain and nervous
system and our endocrine glands,
which produce hormones. The
hypothalamus, a structure at the
base of the brain, stimulates and
inhibits the pituitary gland, often
called the “master gland,” which
in turn regulates the glands of
the body (ovaries, testes, ­adrenal
glands, thyroid) and how and
when they release their hormones
into our circulation. As we age, this
system becomes less functional,
and this can lead to high blood
pressure, impaired sugar metabolism, and sleep abnormalities. The
effects that the various hormones
our different glands produce have
on different facets of aging have
been studied extensively.
For a time, aging researchers
working in neuroendocrinology—
the study of hormones regulated
by the brain—thought that laterlife reduction of hormones, such
as the reduction of estrogen that
accompanies menopause, was
responsible for aging. However,
although some late-life functional
changes may be linked to reduced
hormone levels, experimental
evidence in mice from as early as
the 1960s and continuing today
shows the opposite: reduction
in ­hormones can lengthen life.
­Studies in mice whose pituitary
glands were removed showed the
mice lived longer with a delay in
age-related changes.
A flood of recent evidence has
­pinpointed this effect to one
area: the insulin/IGF-1 hormonal
­pathway. IGF stands for insulinlike growth factor, a substance
­activated by growth hormone.
Single-gene mutations in fruit
flies and the roundworm C.
­elegans, widely studied by aging
­researchers, have recently been
tied to the insulin/IGF-1 ­pathway.
In 2002, a study by French
­researchers published electronically in Nature showed a similar
effect in mice. In all the laboratory
Natural substances within our cells called antioxidants sop up and neutralize dangerous
free radicals. But those that escape this cleanup process can damage DNA, proteins, and
­mitochondria.
6 | Infoaging Guide to Theories of Aging
organisms studied, mutations that
reduce the amount of circulating
IGF extend life. In many cases,
however, the long-lived mutants
have defects that could potentially
affect their ability to survive in the
wild, possibly making the IGF-1
pathway’s relationship to aging an
example of antagonistic pleiotropy.
This consistency among ­species
makes scientists optimistic
that the insulin/IGF-1 pathway
may work in a similar fashion in
­humans, and may be an ­excellent
target for interventions that
could affect aging. Interestingly,
this ­evidence flies in the face of
popular support for anti-aging
treatments involving injections of
growth hormone, which increases
circulating IGF-1. Rather than
prolonging life as some ­companies
claim, such treatment may instead
do the opposite. A recent study
of humans who genetically lack
an ability to use growth hormone
found that these people were
­protected against cancer and
the development of adult-onset
­diabetes.
THE OXIDATIVE DAMAGE/
FREE RADICAL HYPOTHESIS
OF ­AGING
Oxidative free radicals are one of
the toxic byproducts of normal cell
metabolism. Natural substances
within our cells called antioxidants
sop up and neutralize these dangerous free radicals. But those
that escape this cleanup process
can damage DNA, proteins, and
mitochondria. This damage, called
oxidative damage, accumulates
over time. Some fruit fly studies
suggest that oxidative damage is
one of the direct causes of aging.
Proponents of the free-radical
hypothesis of aging note that free
radicals can cause DNA damage,
the cross-linking of proteins, and
the formation of age pigments.
Oxidative damage contributes to
many age-related diseases, such

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