Weindruch and Walford believe that there are strong arguments in support of the idea that some of the genes in DNA, which are specifically linked to the ageing process, are affected beneficially by dietary restriction. Some of these help immune function while others are related to improving oxidation protection functions as well as the elongation factor mentioned above.

DNA looks after itself
Part of this selective upgrading of certain functions, during dietary restriction, seems to be the way in which DNA makes sure that it is itself protected more efficiently (for example, ensuring repair where damage has occurred) especially in those sites where genes exist which influence ageing. In this way dietary restriction causes DNA to produce more 'physiologically useful' messenger RNAs and, in short-lived, disease-prone breeds of animals, to produce fewer cancer-initiating RNA messengers. Both of these effects will increase life expectancy.

What about immune function and hormones?
Chapter 7 looks at the remarkable 'youth enhancing' results obtained by the use of growth hormone, and I also elaborate on other hormonal theories of aging. At this point I suggest that we should note simply that dietary restriction and fasting change hormonal patterns beneficially, and that the overall effect of a better functioning hormonal system makes for a healthier and better balanced individual (animal or human), with 'younger' characteristics, but not necessarily with a longer life expectancy. This is much the same point as made in relation to the immune system, which becomes increasingly efficient with dietary restriction (and fasting). This effect is bound to increase life expectancy simply by keeping disease processes under better control (or by helping to avoid them altogether). However, this is not the same as saying that enhanced immune function (whether resulting from dietary restriction or anything else) produces life extension.

For example, where immune function has been made more efficient by means such as the injection of thymus gland extracts into elderly animals, there has certainly been a rejuvenating effect, but there is no evidence of increased life span. As Weindruch and Walford put it: 'As part of the life extension induced by dietary restriction the immune system is kept "younger" longer by a mechanism whose ultimate origin lies elsewhere.'

Overview
So, how does dietary restriction influence life extension? Despite heroic research our understanding of the aging process remains only partial, with extremely strong views being held as to the relative importance of one or other aspect of it. The influence of dietary restriction on all of the various contenders for the 'major' influence on aging seems to make it a universally applicable technique, whether we are looking at how cells function in terms of self-repair and protein synthesis; or energy production and use; or the influence of the immune and endocrine (hormonal) systems; or the build up of altered structures (cross-linkage) and toxins, often related to free radical activity.

It might well be that aging is the result of a decline in efficiency brought about by wear and tear, a gradual overload which our self-repairing mechanisms ultimately fail to deal with. Or aging might be the result of a built-in obsolescence factor, or indeed it might be the result of a combination of these and other as yet unidentified elements.