Contribution of individual organ mass loss to weight loss–associated decline in resting energy expenditure

Abstract

Background:Weight loss leads to reduced resting energy expenditure (REE) independent of fat-free mass (FFM) and fat mass (FM) loss, but the effect of changes in FFM composition is unclear.Objective:We hypothesized that a decrease in REE adjusted for FFM with weight loss would be partly explained by a disproportionate loss in the high metabolic activity component of FFM.Design:Forty-five overweight and obese women [body mass index (in kg/m2): 28.7–46.8] aged 22–46 y followed a low-calorie diet for 12.7 ± 2.2 wk. Body composition was measured by magnetic resonance imaging, dual-energy X-ray absorptiometry, and a 4-compartment model. REE measured by indirect calorimetry (REEm) was compared with REE calculated from detailed body-composition analysis (REEc) by using specific organ metabolic rates (ie, organ REE/mass).Results:Weight loss was 9.5 ± 3.4 kg (8.0 ± 2.9 kg FM and 1.5 ± 3.1 kg FFM). Decreases in REE (8%), free triiodothyronine concentrations (8%), muscle (3%), heart (5%), liver (4%), and kidney mass (6%) were observed (all P< 0.05). Relative loss in organ mass was significantly higher (P< 0.01) than was the change in low metabolically active FFM components (muscle, bone, and residual mass). After weight loss, REEm – REEc decreased from 0.24 ± 0.58 to 0.01 ± 0.44 MJ/d (P= 0.01) and correlated with the decrease in free triiodothyronine concentrations (r= 0.33, P< 0.05). Women with high adaptive thermogenesis (defined as REEm – REEc < 0.17 MJ/d) had less weight loss and conserved FFM, liver, and kidney mass.Conclusions:After weight loss, almost 50% of the decrease in REEm was explained by losses in FFM and FM. The variability in REEm explained by body composition increased to 60% by also considering the weight of individual organs.