Weindruch and Walford summarize their findings in this important area by listing the characteristics, which they feel their experience allows them to predict, in relation to 'thrifty' and place, 'burner' mice (they see no reason why these same findings should not apply to humans). They predict that the thriftiest 20 per cent of a group of 100 mice (or people) to whom dietary restriction is applied will be those with the highest body weights while on the dietary restriction programme.

The 'thrifty' mice (people) will have:

• a longer life span
• higher body weights (during and after, but not necessarily before dietary restriction)
• lower oxygen consumption
• lower body temperature
• fewer signs of ageing
• less free radical activity

The most likely 'burner' mice (people) would be the 20 per cent of this group with the lowest body weights during dietary restriction, and they would have all these characteristics in reverse.

It is clear that an 'efficient' (thrifty) organism will make the most of the limited food it receives on a restriction programme, and will demonstrate this by maintaining body weight throughout the regime, whereas an 'inefficient' (burner) organism would lose weight and become relatively slim when restricted.

Weindruch and Walford point out that animals become 'thriftier' in their use of energy when on dietary restriction, and that they continue to be thrifty afterwards. One of the major findings in life extension research has been the revelation that core body temperature drops with dietary restriction. The warmer the environmental temperature the less energy the organism (fruit fly, mouse or human) has to generate and expend in order to produce adequate body temperature. Just what effect the external environmental temperature has on the benefits or otherwise of dietary restriction and life extension I cover in Chapter 9.

Making protein
The manufacture of new protein by cells is seen to decline steadily with advancing age, but whether this is because of relative changes in the efficiency of DNA/RNA activity is not certain. One of the features seen during dietary restriction is that of a more efficient synthesis by cells of protein. There is disagreement between experts on whether this improved synthesis of protein is simply a sign of increased youthfulness following (or accompanying) dietary restriction, or whether it is actually a part of the process which produces life extension. One argument which supports the idea that this might be an actual cause of the life extension phenomenon is that it is now known that the changes in protein synthesis (when dietary restriction is applied) are not uniform, they are selective. That is to say some proteins are produced very much more efficiently and abundantly, while at the same time all other proteins are synthesized at only a moderately increased rate, during dietary restriction.

For example, a particular protein called EF-1 ('elongation factor') is seen to be increased dramatically with dietary modification, and this particular protein is known to decline in efficient production before general signs of ageing occur (such as an overall drop in protein synthesis) in a variety of species. Some of these protein synthesis changes might be the result of more RNA being produced (the template sent from the DNA blueprint in the cell nucleus to show the cell what protein to make), or might it simply be the case that the messenger RNAs start binding more efficiently with ribosomes to form new proteins?