Low fitness partially explains resting metabolic rate differences between African American and white women.

BACKGROUND High levels of obesity among African American women have been hypothesized to be partially resultant from a lower resting metabolic rate compared with white women. The aim of the current study was to determine if differences in cardiorespiratory fitness and moderate-to-vigorous physical activity are associated with differences in resting metabolic rate among free-living young adult African American women and white women. METHODS Participants were 179 women (white women n = 141, African American women n = 38, mean age = 27.7 years). Resting metabolic rate was measured using indirect calorimetry, body composition using dual energy x-ray absorptiometry, cardiorespiratory fitness via maximal treadmill test, and moderate-to-vigorous physical activity using an activity monitor. RESULTS African American women had higher body mass index, fat mass, and fat-free mass compared with white women but lower levels of cardiorespiratory fitness. No differences were observed between African American and white women in resting metabolic rate when expressed as kcal/day (1390.8 ± 197.5 vs 1375.7 ± 173.6 kcal/day, P = .64), but African American women had a lower resting metabolic rate when expressed relative to body weight (2.56 ± 0.30 vs 2.95 ± 0.33 mL/kg/min, P < .001). After statistical adjustment for differences in body composition between groups using linear regression models, African American women had a lower resting metabolic rate compared with white women (1299.4 ± 19.2 vs 1400.4 ± 9.2 kcal/day, P < .001). The addition of cardiorespiratory fitness reduced the differences among groups by 25%. The addition of moderate-to-vigorous physical activity did not improve the model. CONCLUSIONS The present study confirms that African American women have a lower resting metabolic rate compared with their white peers, and low cardiorespiratory fitness explained 25% of this difference. Variables associated with resting metabolic rate, such as cardiorespiratory fitness, represent possible points of tailored interventions designed to address high levels of obesity seen in certain demographic groups.