CLINICAL RESEARCH STUDY
Low Fitness Partially Explains Resting Metabolic Rate Differences Between African American and White Women Robin P. Shook, PhD,a Gregory A. Hand, PhD, MPH,a Xuewen Wang, PhD,a Amanda E. Paluch, MS,a Robert Moran, PhD,b James R. Hébert, ScD,b,c,d Damon L. Swift, PhD,e Carl J. Lavie, MD,f,g Steven N. Blair, PEDa,b a Department of Exercise Science and bDepartment of Epidemiology and Biostatistics, Arnold School of Public Health, University of South Carolina, Columbia; cDepartment of Family and Preventive Medicine, University of South Carolina, Columbia; dSouth Carolina Cancer Prevention and Control Program, Arnold School of Public Health, University of South Carolina, Columbia; eDepartment of Kinesiology, Center for Health Disparities, East Carolina University, Greenville, NC; fJohn Ochsner Heart and Vascular Institute, Ochsner Clinical School, University of Queensland School of Medicine, New Orleans, La; gDepartment of Preventive Medicine, Pennington Biomedical Research Center, Louisiana State University System, Baton Rouge, La.
ABSTRACT 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 ¼ 141, African American women n ¼ 38, mean age ¼ 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-tovigorous 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 ¼ .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. Ó 2014 Elsevier Inc. All rights reserved. The American Journal of Medicine (2014) 127, 436-442 KEYWORDS: Fitness; Physical activity; Race; Resting metabolic rate; Women
African American females have the highest prevalence of overweight (82.1%) and obesity (58.6%) of any racial group in the US,1 are more likely to gain weight,2 have a harder Funding: See last page of article. Conflict of Interest: See last page of article. Authorship: See last page of article. Requests for reprints should be addressed to Robin P. Shook, PhD, Public Health Research Center, 921 Assembly St., Columbia, SC 29201. E-mail address:
[email protected] 0002-9343/$ -see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.amjmed.2014.02.003
time losing and maintaining weight loss,3 and are more likely to regain weight4 compared with white women. A majority of studies have reported lower levels of resting metabolic rate in African American women compared with white women, ranging from 81 to 274 kcal/day.5,6 While these values may seem trivial, small differences in energy balance (eg, a positive energy balance due to insufficient levels of energy expenditure) may result in long-term clinically relevant changes in energy storage (eg, adiposity).7
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Race, Resting Metabolic Rate, and Fitness
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Many,8-12 but not all,13-15 studies have found a low resting race in academic and journalistic writing, we have adopted metabolic rate to be predictive of subsequent weight gain. the categories utilized in the 2010 US Census, “Black or The primary cause of lower resting metabolic rate apAfrican American” and “White,”33 (self-identified by parpears to be due to individual differences in fat-free mass ticipants, and only one category could be selected). compartments, with African American women possessing Dual-energy x-ray absorptiometry (DXA) provided mealarger levels of skeletal muscle mass and bone mass and surements on bone mineral density, fat mass, and fat-free mass, lower levels of residual mass both whole body and various re(including the brain, liver, heart, gions (eg, arms, legs) using a Lunar CLINICAL SIGNIFICANCE and kidneys) compared with white DPX system (version 3.6; Lunar women.16-21 Fat-free mass is not Radiation Corp, Madison, Wis). African American young women have a Skeletal muscle mass was estienergetically homogenous; for lower resting metabolic rate than white mated from appendicular lean soft example, the metabolic rate of bone young women. tissue mass using the following and skeletal muscle (approximately Low cardiorespiratory fitness levels linear regression equation: 2.3 and 13 kcal/kg of tissue/day, respectively) are drastically lower among African American women explain Skeletal mass ¼ (1.13 appendiccompared with other organs, inapproximately 25% of this difference in ular lean soft tissue) (0.02 cluding the brain, kidneys, heart, resting metabolic rate. age) þ (0.61 sex) þ 0.97 and liver (approximately 330 kcal/kg Low levels of moderate to vigorous of tissue/day).22-24 Most analyses physical activity did not explain any where sex ¼ 0 for females.34 This only measure fat-free mass, rather additional difference in resting metathan the individual compartments equation was developed (n ¼ 321) bolic rate between groups. described above, and may not accuand validated (n ¼ 93), with ethnirately identify sources of variation cally diverse men and women using Variables associated with resting metain resting metabolic rate among magnetic resonance imaging (MRI) bolic rate (eg, cardiorespiratory fitness), groups.5,18,25 and DXA. Correlation between represent possible points of tailored skeletal mass derived from the Another potential cause of low interventions designed to address the equation and MRI were high (r ¼ resting metabolic rate levels in obesity epidemic. 0.96, P < .0001).34 Residual mass, African American women may be low cardiorespiratory fitness, as representing brain, liver, kidneys, unfit sedentary individuals often heart, gastrointestinal tract, and other have a 5%-20% lower resting metabolic rate compared with organs and tissues, was then calculated using the following those with high levels of cardiorespiratory fitness and equation21: 26-28 moderate-to-vigorous physical activity. Indeed, previous Residual mass ¼ body weight fat mass skeletal analyses from the National Health and Nutrition Examination mass bone mass Survey (NHANES) and elsewhere indicate that African Americans have lower levels of cardiorespiratory fitness Cardiorespiratory fitness was assessed by maximal compared with Whites, by 5%-10%, particularly among treadmill test (Modified Bruce protocol) with respiratory women.29-31 It is possible that the low levels of cardioregases sampled using a TrueOne 2400 Metabolic Measurespiratory fitness in African American women may augment ment Cart (Parvo Medics, Salt Lake City, Utah). Resting existing low levels of resting metabolic rate due to differmetabolic rate was measured via indirect calorimetry using a ences in metabolically active tissues. ventilated hood and an open-circuit system (also the TrueOne The aim of the current study was to compare resting 2400 Metabolic Measurement Cart). An initial stabilization metabolic rate between African American and white women, period of 15 minutes was followed by a 30-minute data adjusting for differences in body compartments and levels of collection period. Participants arrived for a morning visit cardiorespiratory fitness and moderate-to-vigorous physical (1.5 condition). All women were eumenorrheic. Given that no to 3.0 METs; Moderate, >3.0 to 6.0 METs; Vigorous, widely accepted nomenclature exists for classification of
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>6.0 METs. Due to low amounts of time spent in vigorous activity among participants, all activity >3.0 METs also was summed to identify moderate-to-vigorous physical activity. Energy intake was measured using interviewer-administered 24-hour dietary recalls using the Nutrient Data System for Research software (NDSR, Version 2012; University of Minnesota, Minneapolis, Minn). Before data collection, study participants underwent a brief training period (10-15 minutes) to estimate portion sizes of commonly eaten foods. Three interviews were conducted on randomly selected, nonconsecutive days over a 14-day sampling window. Statistical significance for comparison between groups was tested using t-tests. Linear regression analyses were performed to determine relationships among covariates and potential confounders and effect modifiers with the dependent variable resting metabolic rate expressed as kcal/day. Analysis of covariance was used to compare adjusted resting metabolic rate between African American and white women. Linear modeling results are presented as least squares (ie, multivariable-adjusted) means with standard errors, and were adjusted for multiple comparisons using the Tukey-Kramer procedure. Statistical significance was set at P < .05 (2-sided) for all analyses. All aforementioned analyses were performed using SAS 9.3 (SAS Institute Inc., Cary, NC). Due to differences in sample size between the groups, a post hoc power analysis was performed using G*Power 3 (University of Kiel, Germany), which yielded a power of 0.78 to detect differences between group means based on an effect size of 0.5.
RESULTS Participant characteristics are presented in Table 1, including 179 individuals (141 white and 38 African American; mean age of 27.7 years). African American women had higher body mass index, body weight, body fat percentage, fat mass, and fat-free mass compared with white women; fatfree mass was then compartmentalized into bone mass, Table 1
skeletal mass, and residual mass, with the latter including visceral organs, which have higher rates of metabolic activity compared with fat, bone, and skeletal muscle. Skeletal mass was significantly higher in African American women compared with white women in kilograms, but lower relative to total body mass. There were no differences among groups for residual mass in kilograms, but African American women had a lower proportion of residual mass when expressed relative to total body mass. African American women had higher absolute bone mass, with no statistically significant difference when expressed relative to body mass. There was no difference in resting metabolic rate between groups of women when expressed as kcal/day, but when expressed relative to body mass (mL/kg/min), African American women were significantly lower than white women (Table 2). Peak oxygen consumption also was lower among African American women, both absolute and relative to body mass. Compliance with the activity monitor was excellent, with 23.2 0.75 hours of daily wear time and no significant difference between groups. There was no difference in total daily energy expenditure between the groups. African American women spent significantly more time in sedentary and light activity, and less in moderate-to-vigorous physical activity (Table 3). Energy intake and diet composition (percent of total kcals for each macronutrient) also did not differ between groups, except for percent of kcals from alcohol (Table 3), consistent with previous findings.36 Due to statistically significant differences in body compartments among groups, linear models were created to adjust resting metabolic rate (Table 4), including the covariates race and age, which have previously been shown to influence resting metabolic rate, and 4 body compartments (skeletal muscle mass, residual mass, fat mass, and bone mass) were added individually in subsequent models. Adjusted resting metabolic rate values were calculated for each group from each model. The initial model, which included race, age,
Participant Anthropometric Characteristics Overall and by Race All (n ¼ 179) (Mean SD)
Age (years) Body mass index (kg/m2) Height (cm) Total body mass (kg) Fat mass (kg) Fat mass (%)* Fat-free mass (kg) Skeletal mass (kg) Skeletal mass (%)* Residual mass (kg) Residual mass (%)* Bone mass (kg) Bone (%)*
27.7 25.5 165.8 70.2 25.4 35.0 45.0 21.3 30.7 21.0 30.4 2.7 3.9
3.8 4.3 6.2 12.4 9.6 8.1 5.6 3.1 4.3 2.7 4.2 0.4 0.5
*Percentages (%) based as a fraction of total body weight.
White (n ¼ 141) (Mean SD) 27.3 24.6 166.2 68.0 23.7 33.8 44.5 20.9 31.1 20.9 31.2 2.7 4.0
3.4 3.9 5.8 11.3 9.1 8.2 5.1 2.7 4.4 2.5 4.0 0.4 0.5
African American (n ¼ 38) (Mean SD) 29.0 28.8 164.6 78.3 31.7 39.8 47.0 22.6 28.9 21.3 27.4 3.0 3.9
4.8 4.1 7.3 13.0 8.7 6.0 6.9 3.8 3.4 3.3 3.2 0.5 0.6
P value Between-Group Differences .06
