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Eating about 25 percent fewer calories for six months could tip the scales in favor of a longer life, according to a US research study published in the Journal of the American Medical Association.

Joggers in Washington DC
© AFP/File Tim Sloan

The finding indicates calorie restriction affects at least two key "markers" of longevity: fasting insulin levels and body temperature.

The team from the Pennington Biomedical Research Center reported on a six-month study of men and women between 25 and 50 who were placed on a calorie-restriction diet that lowered their daily caloric intake by about 25 percent.

The researchers compared those on calorie restriction to subjects who either had not been on a diet, had cut calories by about 12.5 percent and increased the energy they burned through exercise by a like amount, or had spent six months on a standard low-calorie diet of about 1,800 to 2,000 calories per day until they had lost 15 percent of their body weight.

The study, called the Comprehensive Assessment of the Long-Term Effects of Reducing Intake of Energy (CALERIE), found that all subjects who dieted or increased their exercise lost weight and body fat.

But those on a calorie restriction diet ended the study with lower fasting insulin levels and lower core body temperatures. They also had less oxidation damage to their DNA, thought to be a marker of aging at the biochemical and cellular level.

"Longer-term studies are required to determine if these effects are sustained and whether they have an effect on human aging," the researchers wrote.

The research team was led by Dr. Leonie Heilbronn of Louisiana State University in Baton Rouge, Louisiana.

"This study has laid the groundwork for future research into the long-term effects of calorie restriction in humans to see whether it really can extend lifespan," John Holloszy, professor of medicine at Washington University in St. Louis, Missouri, said in a statement.

Holloszy, who was among several researchers more than a decade ago to demonstrate that a strict calorie-reduced diet in mice and rats increased their longevity by about 30 percent, said the latest finding further revealed diet's role in aging.

"It's becoming clear from studies with the CRONies (Calorie Restriction with Optimal Nutrition adherents) -- and from this brief, prospective study -- that calorie restriction does change some of the markers we associate with aging," he said.

Holloszy and Luigi Fontana, assistant professor of medicine at Washington University, plan to launch a second phase of the CALERIE study, to look at the effects of calorie restriction over the course of two years.

"We know people on calorie restriction will lose weight," Fontana said. "But this study isn't a weight-loss study. We're hoping to learn more about whether calorie restriction can alter the aging process."

Fontana and his colleagues wrapped up a study on low-calorie diets in January.

They found that after an average of six years of eating a low-calorie yet nutritionally balanced diet, "people's hearts functioned like the hearts of much younger people," the university said in a statement.

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