A non-adaptive explanation
Firstly, the main theory about
why CR works is the idea that it represents an
adaptation for dealing with famines, that puts the organism
into a "survival mode".
An partial alternative to this explanation has been offered.
It is a good deal more speculative than the conventional
adaptive theory.
The two theories are not mutually exclusive - both could
explain aspects of the observed effect.
The "energy conservation" hypothesis
The "energy conservation" hypothesis proposes that the anti-
aging effects of CR are side effects of activating
an energy-conservation genetic programs - as a result of
resource shortages.
It may be that many of CR's effects make sense in the light
of the hypothesis that their main function
is to conserve energy.
One of the reasons why metabolism in CR'd organisms is
shifted away from growth processes and
into maintenance activities may be simply because
these approaches produce similar results - but a lot of
energy can be saved by avoiding going through a growth
phase.
If it is energetically cheaper to keep existing cells going
than it is to allow them to reproduce (and then killing off
any surplus) then that is what is likely to be favoured
during times of energy shortages.
It is easy to see how this could be the case - since growth
processes are widely believed to be energetically expensive
- and the discarded dead cells are an evident sign of waste.
A similar story could - perhaps - be told about why CR
streamlines organisms' metabolisms and results in cleaner
burning of fuel and more recycling.
Also, this effect may be primarily responsible for the
reduced body temperature of CR'd organisms.
Several of these effects happen to be associated with
increased longevity:
- Reduced cell division:
- In low energy conditions, economic considerations don't favour cell division;
- Reduced cell division leads to less cancer in old age and longer lifespan;
- Decreased body temperature:
- In low energy conditions, there is less inclination to use energy to heat the body;
- Reduced temperatures are associated with longer lifespans;
- Burning less fuel:
- Low energy diets result in reduced combustion of fuel;
- Less burning of fuel results in reduced damage to cells by the resulting exhaust products;
Though ultimately the reason for trying to conserve
energy in the first place is still to live and
reproduce another day, if some CR-induced metabolic change
helps save energy, then that's likely to be the immediate
reason why it happens - rather than because it extends
"health span" by contributing to reduced cancer rates or
resulting in a lower frequency of nuclear mutations.
Teasing out which and how many of CR's effects are explained
by this sort of simple "energy conservation" hypothesis
seems likely to prove to be a substantial theoretical and
experimental challenge.
Other theories
There are also some
other theories about why CR works.
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