"Our study provides evidence that calorie restriction does work in humans like it has been shown to work in animals. The next step is to determine if this slows age-related tissue deterioration," lead author Edward Weiss said.

In the study, Weiss of Saint Louis University and his colleagues wanted to know if calorie reduction would lower T3 levels in humans.

To determine if the lowered levels of T3 were a result of calorie restriction and just not decreases in fat mass in general, the researchers recruited volunteers to lose weight through exercise, the 'Rejuvenation Research' reported.

The participants included sedentary, non-smoking, 50- to 60-year-old men and post-menopausal women with average or slightly above average body man index values. They're in good health and did not have cardiovascular disease, diabetes, lung disease, uncontrolled hypertension and evidence of malignancy.

The volunteers were randomly assigned to one of three groups -- a calorie-restriction group, an exercise group or a control group -- and followed for one year.

Those in the calorie restriction group lost weight by reducing their daily calorie intake by 300 to 500 calories per day. The volunteers in the exercise group maintained their regular diet and exercised regularly.

The participants in both the calorie-restriction and exercise groups experienced similar changes of body fat mass.

However, only volunteers in the calorie restriction group also experienced lower levels of the thyroid hormone.