Other theories
Firstly, the main theory about
why CR works is the idea that it represents an adaptive
response to low energy conditions, that prioritises
maintenance activities at the expense of reproductive
ones.
There are a few other theories that attempt to explain why
eating fewer calories might lead to improved health and
greater longevity:
Damage theories
Damage theories suggests that damage to tissues (mainly by
toxic agents) is responsible for much of the aging process.
Toxic calories
This holds that caloric foods are either toxic - or produce
toxins when burned.
Those on calorie restriction consume fewer calories - and
thus have fewer toxic byproducts to deal with.
The toxic byproducts can be free radicals, glucose - or
practically anything that can cause damage in the body.
This theory may have some truth to it.
Rate of Living Theory
This suggests that CR decreases the effective metabolic rate
of organisms - which in turn reduces the rate at which they
age.
This theory is not supported very well by evidence - mainly
since it is difficult to make much of a case for CR'd
organisms having a lowered metabolic rate.
In disucssion of "damage" theories, the "Micro Structure
Exposure Theory" is often mentioned. This points out that
the intensity of toxin the damage depends on the size and
composition of the body - e.g. on how many vulnerable
structures there are.
While CR practitioners consume fewer toxins, they also have
fewer tissues in a number of areas. Consequently, food-
borne toxins might perhaps be expected to be more
concentrated in the tissues CR folk can tend to lose (e.g.
fat tissue) - and less concentrated in those tissues most
preserved (e.g. brain tissue).
Resource allocation theories
Resource composition
It may be that a nutrient spectrum low on energy - but high
on other nutrients - "naturally" favours maintenance
activities - simply because that is the sort of nutrient
profile that maintenance activities require.
This is a weak theory. It is unsupported by any evidence
that I know of - and passes the explanatory "buck".
However it is not impossible, and hasn't really been refuted
- and it might yet explain some aspects of CR.
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