Is aging a disease?
The question of whether aging is a disease is a common
On one hand, the medical profession tends to regard
aging and disease separate processes -
with aging being normal and natural - and disease
representing deviations from the normal state.
However - now that aspects of the aging process are showing
signs of being susceptible to anti-aging interventions -
some researchers seem to be keen to rebrand aging as a
collection of disease states.
However, is aging really best
characterised as a disease? This essay examines this
Is senescence a adaptation?
Most previous writers on this subject have assumed
that aging is maladaptive from the point of view
of the individual.
It is not immediately obvious how it can benefit
an individual to have their body disintegrate from
Consequently most previous authors who have
claimed that senescence represents an adaptation
have invoked high-level selection processes to
explain the existence of the phenomenon.
It is plausible that a rapid turnover in members
of the population could help with cases where
organisms are engaged in battles with parasites.
Older individuals tend to have more parasites -
and at some stage they become a hazzard to other
members of the species - as walking reservoirs of
infectious agents. It would benefit the species
to have such individuals die - taking their load
of parasites out of circulation.
Similarly, many species use sexual recombination
as a defense against rapidly evolving parasites.
To work, this mechanism needs a constant influx of
new individuals - and thus the removal of elderly
Supporters of these ideas cite genes which affect
lifespan, heritable differences in lifespans among
closely related creatures, the influence of
hormones on aging - and cases of "catastrophic"
aging as evidence that aging is indeed an
However, a significant problem with these
explanations is that they offer no individual-
level benefit to aging.
Instead they invoke species-level selection (or
other high level selection processes). Such
selection processes have a poor history of
explaining things in biology - and many doubt
whether such explantions are viable.
There are other explanations of the aging
process that don't require high level selection
processes to work:
This states that reproductive and maintenance
processes compete for resources. Reproducing
early clearly has many advantages - and is
consequently somatic tissue maintenance programs
do not receive sufficient investment to support
This proposes that genes that delay the
expression of other deleterious genes are favoured.
More generally, it suggests that alleles may be favoured if
they have beneficial early effects but deleterious later effects.
Under these theories, aging is not necessarily seen
as being adaptive. In particular, it is sometimes seen as
being the product of evolutionary indifference - rather than
something actively selected for.
If a deleterious gene arises through mutation, and another
gene is selected to delay the expression of its
effects, then the resulting senescence isn't really an
adaptation - since another organisms without the deleterious
gene in the first place would be fitter.
Rather senescence there would be seen as a side
effect of deleterious mutations while they are in the
process of being neutralised and before they are completely
disabled or removed from the population.
To the extent that such mechanisms are responsible for
aging, it is not adaptive - and really is worthy of
being regarded as disease to be cured - at least as much as
other diseases caused by inherited, wholly-deleterious
However, the more general case of the antagonistic
pleiotropy theory - and the disposable soma theory -
don't rule out the possibility that aging is
Antagonistic pleiotropy suggests that failure to turn off
developmental processes may be partially responsible for
senescence. If building an off switch for such
processes is more expensive than allowing them to persist,
then the resulting senescence must be seen as being
Similarly, senescence could be the result of a failure to
invest in maintenance and repair equipment, due to the
competition for resources within organisms between survival
and reproductive systems being won by the reproductive
In either of these scenarios, then senescence may well be
selectively advantageous. It might not be possible (beyond
a certain point) to modify the organism so that they are
more long-lived without compromising their
There is good evidence for the "disposable soma" idea - that
organisms face resource tradeoffs between reproductive and
So - I think that even if we totally reject the idea that
senescence benefits the species - we must still
accept the notion that elements of senescence are the direct
consequence of adaptations that influence its rate; that
senescing individuals may well have a higher fitness than
ones that attempt to prolong their lifespans indefinitely -
and thus that aging is part of nature's plan, rather than
representing a deviation from it.
In the light of aging's adaptive qualities, I am inclined to
agree with conventional medicine on the point of whether
aging is a disease.
Some parts of aging probably are
maladaptive - but there are others which are better
considered to be a normal and natural part of
living organisms - and thus need not be regarded as
something necessarily requiring rectifying.
I suspect that those who are labelling aging as a disease
have economic motives. For instance, they appear to think
that it will be easier for them to locate funding to find a
"cure" for aging if it is popularly regarded as as disease.
They are quite likely correct about this - but that doesn't
justify labelling aging as a disease process if parts of it
are actually natural and adaptive processes.
The fact that some elements of aging are adaptive also
suggests that - after a certain point - government funding
of anti-aging medicine will not make much economic sense.
After a certain point it will be less expensive to simply
replace the senesced individual "components" than it would
be to attempt to repair them.
What do the 'Aging as disease' crowd have to say about the issue?
Aging isn't a disease, so obviously it can't be "cured".
Call it what you like. Aging is a biological phenomenon that causes
debilitation and death. These things are undesirable. Thus, it seems pretty
reasonable to me that something which stops getting older from causing
debilitation and death can be called a cure for aging.
The problem with this is that aging cannot be viewed in isolation. It
represents (among other things) pleitropic side effects of devoting energy to
reproduction. So you cannot simply assume that death is undesirable - that
would be assuming what you are trying to prove.
As to the suggestion that it doesn't matter whether aging is classified as a
disease or not - IMO much confusion is caused by thinking of aging as analogous
to a disease that is in need of fixing.
Reliability theory suggests that aging is a natural part of complex systems.
In my view, this suggests that attemping to extend lifespans beyond a certain
point will prove to be misguided.
Often old things really do need throwing out. That is not necessarily
a cause for lamentation - rather it is part of the natural order of things.