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

Abstract

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 (l O₂/min) for African-Americans (0.254±0.007) was significantly lower (9%) than for Caucasians (0.277±0.008). After RMR (l O₂/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 l/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.