Metabolically active components of fat-free mass and resting energy expenditure in humans: recent lessons from imaging technologies.
Level 5 - mechanism / opinion, no new human data
Narrative review synthesizing previously published imaging and calorimetry data without systematic search methodology
PubMed 12120418 · doi:10.1046/j.1467-789x.2002.00057.x
What was done
The authors reviewed published literature evaluating imaging technologies (MRI, CT, and DEXA) for assessing metabolically active components of fat-free mass (FFM) and resting energy expenditure (REE). The review synthesized data from 116 in vivo measurements of organ masses combined with REE measurements, as well as data from 244 obese and non-obese individuals with regional lean body mass (LBM) assessed by DEXA. REE was also modeled by multiplying individual organ masses by constant tissue-respiration rates.
What was found
MRI- or CT-derived organ masses explained approximately 5% to 10% of the interindividual variance in REE. REE reconstructed from individual organ masses multiplied by constant respiration rates showed high correlation with measured REE, with small, non-significant differences of 83 to 96 kJ d-1. In the 244 subjects with regional DEXA data, regional LBM distribution accounted for the non-linear relationship between REE and body mass.
Why it matters
Measuring specific organ masses through imaging provides a more precise biological explanation for resting metabolic rate variation than bulk fat-free mass alone, improving understanding of energy expenditure across obesity, malnutrition, and disease.
Limits
The total synthesized evidence base was small (116 organ mass measurements). REE reconstruction relied on constant organ-tissue respiration rate assumptions rather than direct in vivo organ respiration measurements. The authors noted a lack of standardized imaging protocols and limited reference data.
Cited by
- context Lean body mass accounts for approximately 70% to 80% of the variance in basal metabolic rate and total energy expenditure.