Differences in resting metabolic rates of inactive obese African-American and Caucasian women

OBJECTIVE: To compare resting metabolic rates (RMR) of African-American (n = 25) and Caucasian (n = 22) premenopausal (35 ± 1 y, Mean ± s.e.m.) women who are obese (95.2 ± 2.9 kg, body mass index (BMI) = 34.7 ± 0.9, % body fat = 45.2 ± 0.9), inactive and free from metabolic disorders or medications that would affect heart rate or RMR. MEASUREMENTS: RMR and respiratory exchange ratio (RER) by indirect calorimetry, body composition by plethysmography, maximal aerobic capacity (VO(2max)) and girth measurements. RESULTS: Group mean comparisons were made with a Student's t-test or an ANCOVA, which controlled for individual differences in body weight and lean body mass (LBM). Significance was set at P < 0.05. Groups were not significantly different in age, height, weight, BMI, % body fat, fat mass, RER, VO(2max), resting heart rate, maximal heart rate; or chest, waist, hip, arm, thigh or calf circumferences. After adjusting for body weight, RMR (IO₂/min) for African-Americans (0.254 ± 0.007) was significantly lower (9%) than for Caucasians (0.277 ± 0.008). After RMR (IO₂/min) was adjusted for LBM, an even larger difference (-12%) persisted for African-Americans (0.250 ± 0.008) compared to Caucasians (0.281 ± 0.008). Predicted RMR (kJ/d) for the African-Americans was the same as measured RMR, whereas Caucasian women expended about 13% more energy than predicted. When controlling for LBM, the partial correlation between VO(2max) and RMR was r = 0.51 when VO(2max) was expressed as I/min, and r = 0.56 when VO(2max) was expressed as ml O₂/kg/min, both highly significant (P < 0.000). CONCLUSION: The lower prevalence of obesity in Caucasian women may be due in part to a higher RMR as well as an under estimation of RMR in weight control therapy. Fitness level (VO(2max)) as well as LBM are significant predictors of RMR for both races.