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Effect of zinc supplementation on Basal metabolic rate and thyroid hormones of late-middle aged and older population

Animal and human studies have shown that zinc (Zn) deficiency is associated with decreases in circulating thyroid hormone levels, in the ratio T3/T4 and in Basal Metabolic Rate (BMR). It is postulated that iodothyronine deiodinase expression is Zn sensitive and that the production of T3 is therefore reduced in Zn deficiency. No recent studies have addressed or confirmed this hypothesis in human subjects.

The objectives of this work were to investigate whether Zn supplementation might affect thyroid hormone levels and consequently affect the BMR. A sub-sample of 70 middle-aged (aged 55-70y) volunteers (35 men and 35 women recruited in Clermont-Ferrand, France) and 108 older (aged 70-85y) volunteers (56 men and 52 women recruited in Rome, Italy) was selected for BMR measurement and body composition; thyroid hormone levels were measured on all volunteers (n=387) participating in the survey. Volunteers were assigned to receive placebo (0mg) or 15mg or 30mg of Zn per day for six months.

At the beginning and at the end of the supplementation period, serum Zn levels, BMR, body composition and thyroid hormone levels were determined. At baseline Italian older volunteers had a significantly lower FFM than middle-aged French volunteers (-7% P<0.01). A negative correlation between BMR and age (men, r-0.64; women, r-0.62; both P<0.0001) was observed: BMR was significantly (P<0.00001) lower in Italian elderly volunteers (4.03±0.46kJ/min and 3.29±0.42kJ/min for men and women, respectively) than in middle-aged French volunteers (4.84±0.45kJ/min and 3.87±0.38kJ/min for men and women respectively), even after adjustment for FFM (-12%). No correlation has been observed between BMR and thyroid hormones both in French and Italian subjects. Total T4 (TT4) concentrations were lowest in middle-aged population (-10%; P<0.0001). A moderate negative correlation has been found with TT4 and red blood cell Zn (r=-0.12, P<0.02; slope -0.026).

However, after three and six months of supplementation there was no significant effect of Zn on serum concentrations of thyroid hormones. The results of this study confirm the age related decline in BMR, which cannot entirely be explained by body composition or thyroid hormones differences. This study also shows that older individuals have a moderately higher serum concentration of total T4, suggesting an age-related impairment in the normal mechanisms that regulates energy expenditure. Although a possible role of Zn was postulated at baseline no effect was found after six months of Zn supplementation.

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