Caloric restriction has consistently produced health benefits for animals.
In the new study, scientists tracked 76 adult rhesus monkeys from the Wisconsin National Primate Research Center starting in 1989. Half the animals were fed a typical diet of lab chow, and the rest got a version with a higher concentration of vitamins and minerals to make up for the 30% reduction in chow quantity.
Over the course of the study, the monkeys that ate the regular diet were three times more likely to die of an age-related disease than their counterparts on caloric restriction. Fourteen deaths in the control group were attributable to age-related diseases, compared with five such deaths among the animals that ate 30% fewer calories, according to the study.
The rates of cardiovascular disease and pre-cancerous cell growths were twice as high in the control group compared with the reduced-calorie group.
The researchers also noted that although five of the control monkeys became diabetic and 11 were classified as pre-diabetic, all the calorie-restricted animals remained diabetes-free.
Brain scans revealed significantly less atrophy of gray matter in the monkeys that ate less.
They even looked less wrinkled and flabby.
In all, the monkeys on caloric restriction "appear to be biologically younger than the normally fed animals," the researchers wrote in their report.
Scientists aren't sure why eating less slows the aging process, but theories abound.
There's evidence from mice that caloric restriction induces the body to activate fewer genes related to inflammation, which many scientists suspect plays a key role in aging.
Another theory holds that starved organisms hunker down in maintenance mode, shutting down activities such as reproduction that put wear and tear on the body.
Or perhaps caloric restriction reduces body temperature, thus limiting production of dangerous free radicals that gradually break down the body by damaging tissues and DNA.
"It's all speculation," said Dr. Sergei Romashkan of the National Institute on Aging, who is overseeing a clinical trial on caloric restriction in people.
Authors of the monkey study won't be able to calculate how much caloric restriction extended the animals' average life span -- or whether it boosted their maximum life span -- until all the animals have died.
That could take 10 to 15 years, said senior author Richard Weindruch, a medical professor at the University of Wisconsin-Madison.
