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.

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 allocation of resources is channelled away from the reproduction-related activities that normally preoccupy organisms - and into activities that promote survival in the face of the current challenge.

Restricted orginsims try to survive until the end of their resource shortage - 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 a productive state at the end of it.

The process of attempting to live until the end of a resource shortage - and being in as good a state as possible at the end of it - is likely to result in something similar to retarding aging.

Being in an 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 retarding aging and prolonging life.

Reversing the polarity of the disposable soma theory

Suddenly, what pays off is diverting resources away from reproductive activities and towards maintenance pathways - to allow the current challenge to be survived.

Our ancestors' genes figured this puzzle out - and have a genetic program to deal with the situation. This effects the switch around in the resource-allocation system - and can be activated by strongly restricting calorie intake.

Other theories