Two main modern theories successfully account for most aspects
of the aging process:
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 theory 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.
These theories are complementary:
Antagonistic pleiotropy theory
...describes one way in which problems can come to occur
more frequently in older organisms;
Disposable soma theory
...offers an underlying economic reason why not all
such problems are eventually fixed by natural selection.
Effects of dietary energy restriction
In the context of these theories, dietary energy restriction is
best seen as a sort of resource shortage.
Resource shortages happened to our ancestors - in the form
of seasonal food shortages and famines - and it appears that
part of our genetic program consists of a stratgey for
dealing with them.
Organisms in conditions of reduced energy respond by:
Burning existing tissues for fuel
Fat is intended as an energy store. It's designed to be
burned off in times of resource shortages.
Muscles are also rarely critical tissues. They can be reduced in
size - and the resulting resources can be recycled.
Reduce organs of digestion
Stomach and liver tissues are not needed so much - since
less food is being consumed. Any surplus tissue can also
get burned as fuel.
Expending less energy
Having fewer tissues to support means less energy expended heating and
Some sorts of non-essential activity can be reduced.
However, activity relating to finding food can be stimulated
- and the overall resulting activity level may not be
Growth is rarely an essential process - but it can come with
a substantial energetic burden. Cutting out most growth processes
Reproduction is another non-essential process. Sometimes,
it may be better to live until the end of the famine than
attempt to reproduce in the middle of it. Any resulting
offspring would be likely to be compromised anyway - as a
result of suffering from resource shortages at the start of
Resource shortages also activate programs related to hunger
The result is that restricted organisms are placed in a
substantially altered physiological state.
Why dietary energy restriction retards aging
So far, none of this explains why dietary energy restriction might be
able to retard some aspects of the aging process.
The main theory that attempts to account for that is as follows:
What seems to be happening is that those metabolic programs
responsible for maintenance activities in organisms are being
allocated more resources.
However this is happening in response to a resource shortage.
This might seem like a bit of a paradox - how can it be that
a resource shortage triggers greater
resource expenditure in some areas?
What seems to be happening is that organisms facing resource shortages
react by going into an altered physiological state - a survival
mode - where allocated resources are channelled away
from the reproduction-related activities that normally preoccupy
organisms - and into activities that promote survival in the
face of the current resource shortage.
Restricted orginsims try to survive until the end of their famine -
and hope to refeed themselves and reproduce again when it is over.
They want to be as viable as possible at the end of the famine - and
so do their best with whatever resources they have available to ensure
they are in good reproductive health at the end of it.
The process of attempting to live until the end of a resource shortage
- and being in as positive a state as possible at the end of it - is
likely to result in something similar to retarding the aging process.
Being in an reproductively-viable state at the end of a famine may not
present exactly the same challenge that living to a ripe old
age does - but there are enough similarities for simulated famines to
be effective at both retarding the aging process - and prolonging life.
Since it is not intuitively obvious how a smaller resource pie can
result in some processes getting more of the pie, I sometimes find it
useful to explain the phenomenon using an analogy:
Imagine you are going on a holiday, and have a limited budget to
spend. You might typically spend your money relaxing and having a good
time. However, imagine also that you have to buy a ticket home again -
and then consider the effect of budget cuts. Initially reductions in
the budget might result in similar behaviour, but for a shorter
duration - you pack your bags and go home earlier. However, consider
the effect of budget cuts which mean that you can no longer afford the
ticket home. Suddenly what resources you have start getting spent on
activities that would not have received attention before. You might
wire home for funds. You might try and get a job. Or you might gamble
the money in a casino - hoping to increase your funds enough to be able
to afford the vital ticket.
This shift in the recipients of resources that occurs when they are
limited is very much like the survival mode which results from dietary
Reversed polarity disposable soma
When reproduction is not a very realistic possibility, the
main evolutionary force that is responsible for the aging process
in the first place - the diverting of resources from maintenance
activities into reproductive ones (i.e. the disposable soma theory) -
has its polarity reversed.
Suddenly, what pays off is diverting resources away from
reproductive activities and towards maintenance pathways - to
avoid wasting resources on offspring that would be doomed anyway -
and instead allow the current environmental challenge to be survived.
Our ancestors' genes figured this puzzle out - and have a genetic
program to deal with the situation. This mechanism produces a
switch-around in the resource-allocation system - and it can be
activated by strongly restricting intake of dietary energy.
Paying the costs
Looking at the effects of dietary energy restriction on humans, it
isn't difficult to see how it might result in reduced fertility.
Females exhibit reduced body fat stores, and may even exhibit Amenorrhea - a
cesation of menstruation. Males turn into weak-looking stick figures,
that most females do not find sexually attractive.
However, nontheless, there are some people willing to undergo the
process for various reasons. Some can't have kids in the first place.
Some value their future existence highly. Others just want a dose of
the health and vitality the regime brings.
I hope an understanding of some of the reasons why dietary energy
restriction is effective at extending the lifespans of many species of
animal will help illuminate the decisions of those contemplating
embarking on the diet.
This is not the only explanation that has been offered about why
dietary energy restriction works.