The Wayback Machine -

Calorie Restriction 
The "energy conservation" hypothesis

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.

Tim Tyler | Contact