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Contribution of individual organ mass loss to weight loss–associated decline in resting energy expenditure^1,2,3

¹ From the Institute of Human Nutrition and Food Science, Christian-Albrechts University, Kiel, Germany (AB-W, KG, WL, BH, US, and MJM); the Division of Medical Physics, Clinic for Diagnostic Radiology, University Medical Center Schleswig–Holstein, Kiel, Germany (EK and C-CG); the Clinic for Diagnostic Radiology, University Medical Center Schleswig–Holstein, Kiel, Germany (MH); and Merck & Company, Rahway, NJ (SBH).

² Supported by Deutsche Forschungsgemeinschaft (DFG Mü 714/8-3).

³ Address correspondence to MJ Müller, Institut für Humanernährung und Lebensmittelkunde, Agrar- und Ernährungswissenschaftliche Fakultät, Christian-Albrechts-Universität zu Kiel, Düsternbrooker Weg 17-19, D-24105 Kiel, Germany. E-mail: mmueller{at}nutrfoodsc.uni-kiel.de.

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.