Cutting the number of calories you eat every day could prolong your life, suggests a new study.

Eating 300 to 500 fewer calories per day has been shown to reduce the aging process by lowering a thyroid hormone called triiodothyronine. This hormone has been shown in previous studies to slow metabolism and the aging of tissues in the body.

Researchers at Saint Louis University studied sedentary, non-smoking 50-60-year-old men and post-menopausal women to see whether levels of the hormone could be lowered when caloric intake was reduced. The participants did not have heart disease, diabetes, lung disease, uncontrolled high blood pressure nor malignancy and were average or slightly above average in weight.

They were assigned to 12 months of caloric restriction. Their hormone levels were measured at the onset of the study and then at 12 months.

Volunteers were also recruited to lose weight through exercise, to see whether fat reduction — rather than calorie reduction — would lower the triiodothyronine levels.

Researchers added a control group for comparison.

While participants in both the calorie-restriction group and exercise group lost weight and body fat, only those in the calorie-reduction group saw their triiodothyronine levels drop.

Though researchers caution that further study is needed to prove a definitive link between a decrease in the hormone and a slowing of the aging process in humans, they suggest cutting back on calories is good for one's health.

"There is plenty of evidence the calorie restriction can reduce your risks for many common diseases including cancer, diabetes and heart disease," said Edward Weiss, an associate professor of nutrition and dietetics at Saint Louis University.

"And you may live to be substantially older."

The study is published in the June 2008 issue of Rejuvenation Research.