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15-06-2006 | Thyroid | Article

Calorie restriction alters T3 levels, maybe ageing process


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Study findings show that eating a low calorie, yet nutritionally balanced diet lowers concentrations of triiodothyronine (T3).

The team, led by Luigi Fontana from Washington University School of Medicine in St Louis, Missouri, USA, also found that calorie restriction decreases the circulating concentration of tumor necrosis factor (TNF)-α and suggest that the combination of lowered T3 and TNF-α levels may slow the ageing process.

They hypothesize that such a curtailing of the ageing process may occur by reducing the body's metabolic rate as well as oxidative damage to cells and tissues.

For their study, published in an advance online publication by the Journal of Clinical Endocrinology and Metabolism, the researchers examined 28 members of the Calorie Restriction Society who had been eating a calorie-restricted diet for an average of 6 years.

The participants consumed an average of about 1,800 calories per day, and consumed at least 100% of the recommended daily amounts of protein and micronutrients.

The data from these volunteers was compared with that from 28 individuals who were sedentary and ate a standard Western diet of about 2400 calories per day. A third group of individuals was also included who ate a standard Western diet – approximately 2800 calories per day – but they also did regular endurance training.

Evaluation of serum thyroid hormones revealed that T3 levels were significantly lower in the calorie-restricted group compared with the sedentary and exercising groups, at 73.6 versus 91.0 and 94.3 ng/dl, respectively.

Serum total and free thyroxine, reverse T3, and thyroid-stimulating hormone levels were similar across all groups, indicating that individuals on a calorie-restricted diet were not suffering from hypothyroidism.

Intriguingly, body fat levels did not affect serum T3 concentrations, with similar levels of fat among the three groups.

The scientists propose that as previous studies have suggested, energy deprivation may modulate serum T3 concentration by reducing the conversion of T4 to T3 via lowering the activity or concentrations of iodothyronine deiodinases.

"The difference in T3 levels between the calorie restricted group and the exercise group is exciting because it suggests that calorie restriction has some specific anti-aging effects that are due to lower energy intake, rather than to leanness," concludes Fontana.