MONDAY, July 14 (HealthDay News) -- Cutting just 300 to 500 calories a day from your diet could be the key to slowing the signs of aging and living longer, according to a new study.

Studies have long shown that reducing calorie intake slows the aging process in rats and mice. A popular theory is that fewer daily calories decreases production of the thyroid hormone triiodothyronine (T3), which then slows metabolism and tissue aging.

A new study, by Saint Louis University researchers, found this hormone decrease occurs when humans regularly skip rich desserts or substitute a turkey sandwich for a Big Mac and fries every day.

"Our research provides evidence that calorie restriction does work in humans like it has been shown to work in animals," study lead author Edward Weiss, associate professor of nutrition and dietetics at Saint Louis University's Doisy College of Health Sciences, said in a prepared statement. "The next step is to determine if this in fact slows age-related tissue deterioration. The only way to be certain, though, is to do a long-term study."

The findings, published in the June 2008 issue of Rejuvenation Research, are based on a study of healthy but sedentary, non-smoking, 50- to 60-year-old men and post-menopausal women. For a year, the volunteers participated in either: a calorie-restriction group that cut their daily calorie intake by 300 to 500 calories per day; a group that stayed on their regular diet and exercised regularly; or a group that maintained its normal routine.

While those in the calorie-restriction and exercise groups both lost body fat mass, only those in the calorie restriction group also had lower levels of the thyroid hormone.

Although a long-term study is still needed to determine if reducing T3 levels through calorie restriction does indeed slow the aging process, Weiss said cutting back on calories is a good idea.

"There is plenty of evidence the calorie restriction can reduce your risks for many common diseases including cancer, diabetes and heart disease," Weiss said. "And you may live to be substantially older."

Weiss warned that while cutting calories, people need to maintain a healthy diet by eating nutrient-rich foods. He noted that long-term slowing of the metabolism could also make people more prone to weight gain over time.

The key to maintaining a healthy weight, Weiss said, is keeping a consistent diet and exercising regularly.

More information

The Calorie Restriction Society has more about calorie restriction.