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Basal metabolic rate and thyroid hormones of late-middle aged and older human subjects

This report describes baseline data on basal metabolic rate (BMR), thyroid hormone levels and body composition of middle aged and older people participating in the ZENITH project and the correlation of thyroid hormone levels with Zn status. A multi-centre prospective intervention study employing a randomised double blind design was performed in Clermont-Ferrand, Theix (France), Coleraine (Northern Ireland), Grenoble (France) and Rome (Italy).

BMR has been measured on a sub-sample of 70 middle-aged volunteers (35 men and 35 women recruited in Clermont-Ferrand, France, aged 55-70 y) and 108 older volunteers (56 men and 52 women recruited in Rome, Italy, aged 70-85 y). Thyroid hormone levels were evaluated in the entire group of ZENITH volunteers (n 387). BMR was measured by indirect calorimetry. Fat free mass (FFM) was derived by four skinfold thickness using Durnin and Womersley's equations. Concentrations of thyroid hormones (total T3 and T4) were measured using a competitive immunoassay with an enhanced chemiluminescence endpoint. 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<0.000001) 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). The results confirm an age-related decline in BMR not entirely explained by body composition or thyroid hormones differences.

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