This approach, called calorie restriction, involves eating about 30 percent fewer calories than normal while still getting adequate amounts of vitamins, minerals and other nutrients. Aside from direct genetic manipulation, calorie restriction is the only strategy known to extend life consistently in a variety of animal species.

How this drastic diet affects the body has been the subject of intense research. Recently, the effort has begun to bear fruit, producing a steady stream of studies indicating that the rate of aging is plastic, not fixed, and that it can be manipulated.

In the last year, calorie-restricted diets have been shown in various animals to affect molecular pathways likely to be involved in the progression of Alzheimer’s disease, diabetes, heart disease, Parkinson’s disease and cancer. Earlier this year, researchers studying dietary effects on humans went so far as to claim that calorie restriction may be more effective than exercise at preventing age-related diseases.
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In 1935, Dr. Clive McCay, a nutritionist at Cornell University, discovered that mice that were fed 30 percent fewer calories lived about 40 percent longer than their free-grazing laboratory mates. The dieting mice were also more physically active and far less prone to the diseases of advanced age.

Dr. McCay’s experiment has been successfully duplicated in a variety of species. In almost every instance, the subjects on low-calorie diets have proven to be not just longer lived, but also more resistant to age-related ailments.

“In mice, calorie restriction doesn’t just extend life span,” said Leonard P. Guarente, professor of biology at the Massachusetts Institute of Technology. “It mitigates many diseases of aging: cancer, cardiovascular disease, neurodegenerative disease. The gain is just enormous.”
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“Calorie restriction has a powerful, protective effect against diseases associated with aging,” said Dr. John O. Holloszy, a Washington University professor of medicine. “We don’t know how long each individual will end up living, but they certainly have a longer life expectancy than average.”

Researchers at Louisiana State University reported in April in The Journal of the American Medical Association that patients on an experimental low-calorie diet had lower insulin levels and body temperatures, both possible markers of longevity, and fewer signs of the chromosomal damage typically associated with aging.

These studies and others have led many scientists to believe they have stumbled onto a central determinant of natural life span. Animals on restricted diets seem particularly resistant to environmental stresses like oxidation and heat, perhaps even radiation. “It is a very deep, very important function,” Dr. Miller said. Experts theorize that limited access to energy alarms the body, so to speak, activating a cascade of biochemical signals that tell each cell to direct energy away from reproductive functions, toward repair and maintenance. The calorie-restricted organism is stronger, according to this hypothesis, because individual cells are more efficiently repairing mutations, using energy, defending themselves and mopping up harmful byproducts like free radicals.