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Small organs with a high metabolic rate explain lower resting energy expenditure in African American than in white adults^1,2,3

¹ From the Obesity Research Center (DG, QH, and SH) and the Department of Cardiology (NK), St Luke’s–Roosevelt Hospital, New York, NY; the Institute of Human Nutrition and the Department of Medicine, College of Physicians and Surgeons, Columbia University, New York, NY (DG, QH, and SH); the Division of Cardiology, North Shore University Hospital, Manhasset, NY (LB); and the Institute of Human Nutrition, University of Southampton, Southampton, United Kingdom (ME)

Background:African Americans have a lower resting energy expenditure (REE) relative to fat-free mass (FFM) than do whites. Whether the composition of FFM at the organ-tissue level differs between African Americans and whites and, if so, whether that difference could account for differences by race in REE are unknown.

Objective:The objectives were to quantify FFM in vivo in women and men at the organ-tissue level and to ascertain whether the mass of specific high-metabolic-rate organs and tissues differs between African Americans and whites and, if so, whether that difference can account for differences in REE.

Design:The study was a cross-sectional evaluation of 64 women (n = 34 African Americans, 30 whites) and 35 men (n = 8 African Americans, 27 whites). Magnetic resonance imaging measures of liver, kidney, heart, spleen, brain, skeletal muscle, and adipose tissue and dual-energy X-ray absorptiometry measures of fat and FFM were acquired. REE was measured by using indirect calorimetry.

Results:The mass of selected high-metabolic-rate organs (sum of liver, heart, spleen, kidneys, and brain) after adjustment for fat, FFM, sex, and age was significantly (P < 0.001) smaller in African Americans than in whites (3.1 and 3.4 kg, respectively; ± SEE difference: 0.30 ± 0.06 kg). In a multiple regression analysis with fat, FFM, sex, age, and race as predictors of REE, the addition of the total mass rendered race nonsignificant.

Conclusions:Racial differences in REE were reduced by >50% and were no longer significant when the mass of specific high-metabolic-rate organs was considered. Differences in FFM composition may be responsible for the reported REE differences.

Key Words: Race • ethnicity • African Americans • whites • metabolism • organs • tissues • fat-free mass • resting energy expenditure • magnetic resonance imaging

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