Objective: This paper 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 zinc status. Design: A multicentre prospective intervention study employing a randomised double blind design. Setting: Clermont-Ferrand, Theix (France), Coleraine (Northern Ireland), Grenoble (France), Rome (Italy). Interventions: BMR has been measured on a subsample 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 thicknesses using Durnin and Womersley's equations. Concentrations of thyroid hormones (total T3 and T4) were measured using a competitive immunoassay with an enhanced chemiluminescence end point. Results: 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 +/- 0.46 kJ/min and 3.29 +/- 0.42 kJ/min for men and women, respectively) than in middle-aged French volunteers (4.84 +/- 0.45 kJ/min and 3.87 +/- 0.38 kJ/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 (T-174) concentrations were lowest in middle-aged population (-10%, P<0.0001). A moderate negative correlation has been found with TT4 and red blood cell zinc (r= -0.12, P<0.02, slope -0.026). Conclusions: The results confirm an age-related decline in BMR not entirely explained by body composition or thyroid hormones differences.