Effect of organ and tissue masses on resting energy expenditure in underweight, normal weight and obese adults.
Level 4 - case-series / case-control
Cross-sectional physiological study
PubMed 14647174 · doi:10.1038/sj.ijo.0802526
What was done
A cross-sectional study evaluated 57 healthy adults (35 females, 22 males, aged 19–43 years; 14 underweight, 25 intermediate weight, and 18 obese). Masses of the brain, internal organs, skeletal muscle, bone, and adipose tissue were measured using magnetic resonance imaging (MRI) and dual-energy X-ray absorptiometry (DXA). Resting energy expenditure (REE) was measured by indirect calorimetry (REEm) and compared against calculated REE based on detailed MRI/DXA organ masses (REEc1) or a simplified DXA-only approach (REEc2) using constant organ-specific metabolic rates.
What was found
Calculated REE (REEc1) showed high agreement with measured REE across the entire fat-free mass (FFM) range of 28–86 kg. Prediction errors were -17 ± 505 kJ/day in intermediate-weight, -145 ± 514 kJ/day in underweight, and -141 ± 1058 kJ/day in obese participants (differences not statistically significant). Regressing REEm on FFM produced an intercept of 1.6 MJ/day, which fell to 0.5 MJ/day after adjusting for the muscle-to-organ mass ratio. Muscle mass and liver mass together explained 81% of the variance in REEm. The simplified DXA-derived calculation also matched measured values closely (REEc2: 5.82 ± 1.51 MJ/day vs REEm: 5.72 ± 1.87 MJ/day; difference not significant).
Why it matters
This study demonstrates that organ-specific metabolic rates remain largely constant across different body sizes, showing that apparent non-linearities between FFM and REE are primarily driven by differences in the proportion of high-metabolic-rate organs versus muscle tissue.
Limits
The sample size was modest (n = 57) and restricted to healthy adults aged 19–43, limiting generalizability to older or metabolically diseased populations. The study design was cross-sectional, so direct within-individual changes during weight loss or gain were not assessed, and organ-specific metabolic rates were assumed from historical values rather than measured directly.
Cited by
- supports Muscle tissue burns more calories at rest than fat tissue, meaning having more muscle increases resting metabolic rate.