Energy metabolism in African Americans: Potential risk factors for obesity

Abstract

Background: Recent reports have identified a lower resting metabolic rate in African Americans than in whites, but most studies included only females and used short-term measurements with ventilated-hood systems. Objective: Our objective was to compare 24-h measurements of energy metabolism between African American and white women and men using a respiratory chamber. Design: Thirty-eight African American (x̄ ± SD: 32 ± 7 y of age, 24 ± 10% body fat) and 288 white (31 ± 7 y of age, 26 ± 12% body fat) subjects spent 24 h in a respiratory chamber for measurement of 24- h energy expenditure (24EE), sleeping metabolic rate (SMR), 24-h respiratory quotient (24RQ), and substrate oxidation rates. Results: After adjustment for sex, age, and body composition (by hydrodensitometry), African Americans had lower SMR (-301 ± 105 kJ/d; P < 0.01) and higher 24RQ (0.014 ± 0.004; P < 0.001) than whites, whereas 24EE was similar. A sex-specific analysis, using a subset of 38 whites with an equal sex distribution and similar age and body weight, revealed that African American women had lower SMR (-442 ± 182 kJ/d; P < 0.05) and lower 24EE (-580 ± 232 kJ/d; P < 0.05), but similar 24RQ values compared with white women. African American men tended to have lower SMRs than white men (-355 ± 188 kJ/d; P = 0.07), but had higher 24RQ values, accounting for a 992 ± 327-kJ/d lower 24-h fat oxidation rate (P < 0.005). Conclusions: These data not only confirm the findings of a lower metabolic rate in African American than in white women, but also suggest that fat oxidation is lower in African American men than in white men.