There is no question whatever that dietary restriction tactics lengthen life and reduce disease incidence in species after species of animals that have been tested, and there is no reason whatever to suppose that this will not also apply to humans. Indeed, there is strong evidence already available that says it does so.
Many investigators believe that dietary modification does not lengthen life unnaturally, but rather that it allows humans and animals to reach something close to their normal potential life span; although there remains some controversy over what is and what is not 'normal'. Some argue that since the life expectancy of animals in the wild is shorter than that achieved under 'ideal' laboratory conditions, what the experiments are achieving is an extension beyond the natural term. However, another way of looking at it would be to say that if optimal conditions prevailed in the wild (adequate food and no dangers from predators and climatic extremes, for example), this would be even more 'natural' than laboratory conditions, no matter how ideal, and would probably lead to enhanced life spans so far achieved only under laboratory conditions.
What happens in the laboratory cannot be thought of as natural, but the results obtained surely point to ways in which everyday life and habits might be modified to produce the benefits of a longer, healthier life. It is, after all, hardly natural that we purify our drinking water. In just this sort of way modifications to our eating patterns might become the norm, and while seemingly 'unnatural' such changes might allow our true potential to emerge.
Places where restricted calories (and longevity) am the norm
Several population groups have long been reputed to be remarkably long-lived. These include the South American people of Vilcabamba, the central European Caucasian peoples and the Hunza people of the Himalayas. Apart from all being resident in inhospitable mountainous regions their diets seem similar in a number of respects, including the central issue of being lower in overall calorie content than 'normal' (usually around half that normally consumed by active adults in Europe and the US - 1,600 to 1,900 calories per day as against 3,300 for US males of all ages).
The truth of the claims of longevity of such populations is, admittedly, open to doubt, owing to the lack of well-documented legal records of births and deaths, but there is one population group where this is not the case, and where dietary restriction of calories has long been the norm, with quite startling and well recorded results in terms of longevity and health.
Human evidence from Japan
There exists a near perfect example of dietary restriction in action amongst humans. People living on the Japanese island of Okinawa come very close to living on a diet which fulfill the requirements of dietary restriction, as successfully applied to animals in life extension research, and doctors Weindruch and Walford (Retardation of Aging and Disease by Dietary Restriction, Charles Thomas, Springfield, Illinois, 1988) examined a particular group in detail in order to compare them with the studies they had conducted on animals. The evidence they accumulated is compelling.
There are excellent and accurate legal records of births and deaths on Okinawa, which have been kept since 1872, and this allows us to know exactly how long people are living. We are also fortunate that for many years the Japanese Ministry of Welfare and Health has evaluated the dietary habits of different households, chosen randomly, in all areas of the country. This provides us with another useful source of information as we compare the life expectancy of one group against another, especially as it is now known that the people of Okinawa produce centenarians up to 40 times more often than do other Japanese population groups.