Resting energy expenditure (REE) and components of fat-free mass (FFM) were assessed in 26 healthy nonobese adults (13 males, 13 females). Detailed body composition analyses were performed by the combined use of dual-energy X-ray absorptiometry (DEXA), magnetic resonance imaging (MRI), bioelectrical impedance analysis (BIA), and anthropometrics. We found close correlations between REE and FFMBIA ( r = 0.92), muscle massDEXA( r = 0.89), and sum of internal organsMRI( r = 0.90). In a multiple stepwise regression analysis, FFMBIA alone explained 85% of the variance in REE (standard error of the estimate 423 kJ/day). Including the sum of internal organsMRI into the model increased the r 2 to 0.89 with a standard error of 381 kJ/day. With respect to individual organs, only skeletal muscleDEXAand liver massMRI significantly contributed to REE. Prediction of REE based on 1) individual organ masses and 2) a constant metabolic rate per kilogram organ mass was very close to the measured REE, with a mean prediction error of 96 kJ/day. The very close agreement between measured and predicted REE argues against significant variations in specific REEs of individual organs. In conclusion, the mass of internal organs contributes significantly to the variance in REE.
The Age-Related Decline in Resting Energy Expenditure in Humans Is Due to the Loss of Fat-Free Mass and to Alterations in Its Metabolically Active Components
