Weindruch and Walford state: 'Restriction beginning about 20 years of age in humans would, in our view, be free of potential childhood drawbacks, yet afford the greatest extension of life span consistent with safety.' This view is one this book supports, and upon which many of its recommendations are based. It is, however, not a recommendation for anyone to apply dietary restriction methods to themselves or their family. Any personal experimentation is taken to be at own risk, with careful consideration of current state of health, and after discussion with a suitably qualified health professional taking account of the advice and cautions given in this book.

Is it ever too late to start?
Not according to an experiment conducted in 1955 involving 60 people of average age 72, living in a religious institution for the elderly. AU were healthy and none was under the age of 65 at the outset (E. Vallejo, Review of Clinical Experiments (1957) 63:25). The diet which the experimental group received contained a well balanced and fairly hearty 2,300 calories on odd days of the month, with a drastic reduction on even days, on which they received one liter of milk and around 11b of fresh fruit (hardly an ideal restriction pattern, but adequate for experimental purposes). Sixty other people at the institution continued to receive the 2,300 calorie diet daily throughout the study so that they could be compared with the experimental group. The study lasted for three years, during which time those on the restricted diet spent a total of 123 days in the infirmary compared with a total of 219 days for the full-diet group, and only six of the dietary restriction group died as against 13 of the full diet group.

These results suggest that it is not too late to reap benefits by starting dietary restriction strategies, even moderate ones, fairly late in life. Animal studies confirm that middle and advanced age interventions produce significant benefits in life extension and health terms, if 'undernutrition without malnutrition' methods are applied correctly.

Don't start too quickly or cut the diet too much!
The keys to the successful achievement of life extension, when dietary restriction is applied to middle aged and elderly animals, lie in: (a) the speed with which the introduction of the regime is achieved, and (b) the degree of severity of the dietary regime. In early studies, involving dietary restriction of adult animals, results had been poor (hence Mann's negative opinion about applying it to humans) because the dietary changes were made too rapidly or the regime itself was too severe. Once Weindruch and Walford realized that adult animals would respond well by staying healthy and living longer if calorie reductions were made slowly and the reduction in calorie intake was not too drastic, compared with previous intake, results began to improve and life extension was achieved with regularity.

As a general guideline, then, the older a person is when starting a dietary restriction programme, the more slowly should the process be introduced and the less severe the degree of restriction aimed for. I give guidelines in the next chapter for what might be attempted at different ages.

How can you know if the diet is too severe?
Weindruch and Walford discuss the amazing way in which animals will adapt to dietary change. In general, if calorie intake is increased, their metabolic rate rises and they become less 'efficient' in their use of energy, a tactic which allows them to retain their normal body weight. When calorie intake is reduced they become more efficient in their use of energy, and again tend to retain their body weight at around their individual normal level. This is a phenomenon which glimmers have had to contend with to their eternal frustration.