Further studies have shown that dietary restriction patterns started in early adult life, or even middle age, allows rats and mice to extend their average life expectancy by about 90 per cent as much as would have been gained if the restrictions had started in infancy. This shows clearly that the major drawbacks of delayed puberty and stunted growth, which occur when starting the restricted eating pattern early, can be avoided with negligible loss of benefit by beginning the process later in life.
In these particular studies the longest life span reached by individual mice was 50 months and by rats 46 months. The average life extension achieved in whole groups of mice was 39 months when restriction started as adults, and 43 months when started as infants, as against around 36 months for those animals fed freely throughout life For rats the best average life extension achieved by groups was 35 months for adult onset dietary restriction and 38 months for early onset dietary restriction, as compared with 31 months for those animals fed ad lib throughout life.
When disease prone, normally short-lived animals were exposed to dietary restriction techniques the results were dramatic in reducing the levels of ill-health (auto-immune conditions affecting the kidneys, for example) with the dietary restriction animals showing increased activity and greatly outliving their contemporaries on a normal diet. The implications for humans (see Chapter 4) of such results is stressed by the researchers.
The most recent studies described by Weindruch and Walford involved mice with a tendency towards a variety of late-life tumors. They divided the animals into groups, which began dietary restriction at either 1, 5 or 10 months, as well as a control group in which the diet was not altered. They found that those on unrestricted diets lived an average of 26 months, whereas the various dietary control groups averaged a 31 to 33 months life span. The longest surviving free-fed mouse lived for 34 months, whereas the longest restricted-diet mouse lived for 41 months. They also found that the mice which were started on dietary restriction at one month old did only marginally better than did the adult onset dietary restriction mice.
Don't overfeed children
Weindruch and Walford point out that efforts to improve life extension, using dietary restriction on adult animals, are more successful with those animals which have not been overfed early in life, before the beginning of the dietary restriction. The animals which do best on dietary restriction patterns of eating, introduced in adult life are those which have matured slowly, and which remain slightly above average weight during the dietary period (indicating energy efficiency) but which had not been above average weight early in life. Being above average body weight did not lead to a longer life if dietary intake remained unrestrained, only if dietary restriction patterns were implemented. From this the message for parents is clear: keep the food intake (and type of food) of children optimal without allowing early obesity to appear.
Pearson and Shaw, writing in Life Extension (Nutri Books, 1983) give their version of why we should take seriously the likelihood that animal experiments such as those outlined above apply to people as well:
Animal studies can be considered relevant to people when: (1) the experiment affects a system in the animal that is like a similar system in humans, and (2) the method of bringing about these changes in the animal involve biochemical pathways (a chain of chemical reactions) found in both people and animals.