Why organisms age

			Disposable soma
			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 indefinite survival.
			Antagonistic pleiotropy
			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

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
			Reduce fat Fat is intended as an energy store. It's designed to be burned off in times of resource shortages. Reduce muscles 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
			Reduced tissues Having fewer tissues to support means less energy expended heating and maintaining them; Reduced activity 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 reduced. Reduced growth Growth is rarely an essential process - but it can come with a substantial energetic burden. Cutting out most growth processes saves energy. Reduced reproduction 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 their development.

Resource shortages also activate programs related to hunger and stress.

The result is that restricted organisms are placed in a substantially altered physiological state.

Why dietary energy restriction retards aging

The main theory that attempts to account for that is as follows:

Survival mode

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.

Vacation 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 energy restriction.

Reversed polarity disposable soma

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

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.

Other theories

Click here for more theories.