A low resting metabolic rate (RMR) has been proposed as a possible cause for the increased body fat commonly seen in women compared with men. Absolute RMR is higher in men, but whether RMR adjusted for lean body mass (LBM) remains higher is unresolved. The objective of the present study was to determine whether RMR adjusted for various body composition factors differed between healthy adult men and women. Thirty men (28·3±8·0 years, BMI 23·7±2·1 kg/m2) and twenty-eight women (28·7±6·9 years, BMI 22·2±1·9 kg/m2) were included in the analyses. RMR was measured by open-circuit indirect calorimetry for 60 min. Extracellular water (ECW) was measured by corrected Br- space and total body water (TBW) by 2H dilution. LBM was estimated as TBW/0·732. Intracellular water (ICW) was calculated as TBW-ECW, and body cell mass (BCM) as ICW/0·732. Men were heavier and had higher BMI, LBM, BCM and ECW, but less fat mass. Absolute RMR was higher in men than women (7280±844
v.5485±537 kJ/d, P<0·0001). This difference became non-significant when RMR was adjusted for LBM by ANCOVA (6536±630 v.6282±641 kJ/d, P=0·2191), but remained significant when adjusted for BCM (6680±744 v.6128±756 kJ/d, P=0·0249). Fat mass explained a significant amount of variation in RMR in women ( r2 0·28, P=0·0038), but not in men ( r2 0·03, P=0·3301). The relationships between body fat and the various subcompartments of BCM and RMR require further elucidation.