The idea that calorie-restricted diets can lengthen lifespan has been scientifically proven many times with dozens of animal species, and it has been around for several decades. Since 1935, when the first experiments were performed with laboratory rats, researchers found that lab animals consistently lived longer when fed calorie-restricted diets instead of an unlimited choice of food.
Ray Walford picked up on the studies and formulated that the life-lengthening properties of a calorie-restricted diet would also be effective in humans, and came up with his book Beyond the 120-Year Diet and preached of the virtues of CRON: Caloric Restriction with Optional Nutrition. What he makes very clear in his philosophy is this: calorie restriction is not simply avoiding overeating; it is the elimination of 20 to 40 percent of the calories deemed by various health organizations as being necessary to function on a daily basis.
Although calorie-restriction has been proven to prolong the lifespans of many groups and species of laboratory animals, the diet has its critics. Opponents include Dr. Shinichi Nakagawa of the University of Otago, who in his work Aging Cell mentions that while caloric restriction works on laboratory animals, it seems to have no effect on animals that do not live in laboratories. Furthermore, opponents claim that studies have not yet been done on humans, therefore it is simply not known if people on calorie-restricted diets live longer.
Another point researchers mention is that in lab animals, a calorie restricted diet has much more effect on females than on males; the lack of proteins may slow or shut down reproduction processes such as gestation and egg-laying. The effects on human females are not yet known, and studies to test the effectiveness of such diets in a human population will need to take place over multiple decades.
As one critic pointed out, many feel that by only consuming 60% of the recommended calories necessary for proper functioning, life may be prolonged but it may not be worth living. Furthermore, many experts in the field of nutrition and health feel that strict diets will ultimately be detrimental to humans as they will not be receiving all the needed nutrients for a healthy lifestyle.