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No evidence of mass dependency of specific organ metabolic rate in healthy humans^1,2,3

¹ From the Institute of Human Nutrition and Food Science, Christian-Albrechts University, Kiel, Germany (WL, AB-W, BH, and MJM); the Division of Medical Physics, Clinic for Diagnostic Radiology, University Medical Center Schleswig-Holstein, Kiel, Germany (EK and C-CG); and the Clinic for Diagnostic Radiology, University Medical Center Schleswig-Holstein, Kiel, Germany (MH)

Background: In humans, resting energy expenditure (REE) can be calculated from organ and tissue masses using constant specific organ metabolic rates. However, interspecies data suggest allometric relations between body mass and organ metabolic rate with higher specific metabolic rates in mammals with a smaller body mass.

Objective: The objective was to compare the accuracy of REE prediction with the use of either constant or body mass–dependent specific organ metabolic rates.

Design: Healthy subjects (79 women, 75 men) within the normal range of fat mass (FM) expected for a healthy body mass index and aged 18–78 y were stratified into tertiles of body mass. Fifty subjects were grouped as tertile 1 (<66.3 kg), 52 as tertile 2 (66.3 to 77.2 kg), and 52 as tertile 3 (>77.2 kg). Magnetic resonance imaging was used to assess the volume of 4 internal organs (brain, heart, liver, and kidneys). REE was measured by indirect calorimetry (REE_m) and compared with REE calculated from previously published constant (REE_c1) and body mass–dependent organ metabolic rates (REE_c2).

Results: REE_m increased significantly with weight tertile (tertile 1: 5536 ± 529 kJ/d; tertile 2: 6389 ± 672 kJ/d; tertile 3: 7467 ± 745 kJ/d; P < 0.01). The deviation REE_m–REE_c1 did not differ between weight tertiles (tertile 1: 66 ± 382 kJ/d; tertile 2: 167 ± 507 kJ/d; tertile 3: 86 ± 480 kJ/d; NS) and showed no relation with body mass (r = –0.05, NS). By contrast, REE_m–REE_c2 increased with increasing weight tertile (tertile 1: –45 ± 369 kJ/d; tertile 2: 150 ± 503 kJ/d; tertile 3: 193 ± 482 kJ/d; P < 0.05) and correlated significantly with body mass (r = 0.16, P < 0.05).

Conclusion: Our data do not support a lower specific organ metabolic rate in humans with a larger body mass than in those with a smaller body mass.