Is appearing more youthful the same as life extension?
It is important to remind ourselves that life extension does not necessarily mean the same thing as looking younger, or having a more youthful immune system, or a more efficient ability to synthesize proteins. Such features are important, of course, but they do not automatically lead to life extension, although they may well accompany it when it is achieved. The significance of this thought will become more apparent when we look at other methods being tried in the quest for life extension, including use of growth hormone (see Chapter 7). For, while growth hormone stimulation, or its actual injection into the body, has a 'youthening' effect on animals and humans, as yet there is no evidence of it affecting the length of life.

There is a distinction to be made, therefore, between methods which make us feel (and perhaps look) younger but don't actually extend life span, and those which may not have this particular effect but which do improve our potential longevity.

The evolution factor
Weindruch and Walford quote the work of Dr J. Totter, who has suggested that there is an evolutionary process at work when dietary restriction is operating (either in natural settings or experimentally). Dr. Totter believes that when there exists a scarcity of food, energy is diverted from reproductive functions and basic metabolic activity, towards muscular activity, in order to enhance the chances of survival as food is sought. When food is freely available again, breeding (reproduction) is resumed and the basic metabolic rate increases.

There is evidence in human populations to support this hypothesis, and as Weindruch and Walford point out: 'It makes good evolutionary sense not to have babies when food is scarce, to divert reproductive energy to personal survival, and to outlive scarcity.' What makes even more evolutionary (survival of the species) sense, they maintain, is the fact that dietary restriction seems to have a rejuvenating effect on reproductive function, but only once food is again freely available. They point to what happens to rats following 10 weeks of a 50 per cent reduction in calorie intake. Young animals cease their menstrual cycles during the dietary restriction period (evidence of diversion of energy to muscular activity away from reproductive function?) and resume their cycles and ability to reproduce when full feeding is restored. More remarkable still is the effect on animals who have already ceased their cycles due to age, and who are found, after a 50 per cent calorie restriction for ten weeks (once full feeding is restarted), to recommence menstruating and to become fertile once more.

What happens to basic metabolic rate during dietary restriction?
In general, the more energy used to maintain body functions, in relation to body weight, the shorter the life expectancy of the organism. In other words, the better the energy efficiency the greater will be life expectancy.

As we have seen, in numerous examples, the single most important feature of dietary restriction is not fat, or protein, or carbohydrate restriction, nor additions of particular nutrients, but quite simply calorie restriction combined with a diet adequate in all other respects. This means that life extension is achieved when we modify energy intake, which must also mean that, in any search for how life extension techniques work, we must look closely at the energy mechanisms of the body.