International Journal of Obesity (1998) 22, 215±221
ß 1998 Stockton Press All rights reserved 0307±0565/98 $12.00
Differences in resting metabolic rates of inactive
obese African-American and Caucasian women
JN Forman, WC Miller, LM Szymanski and B Fernhall
Exercise Science Programs, The George Washington University Medical Center, Washington DC, USA
OBJECTIVE: To compare resting metabolic rates (RMR) of African-American (n ˆ 25) and Caucasian (n ˆ 22) premenopausal (35 1 y, Mean s.e.m.) women who are obese (95.2 2.9 kg, body mass index (BMI) ˆ 34.7 0.9, % body
fat ˆ 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 (VO2max) 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). Signi®cance was set at P < 0.05. Groups were not
signi®cantly different in age, height, weight, BMI, % body fat, fat mass, RER, VO2max, 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 AfricanAmericans (0.254 0.007) was signi®cantly lower (9%) than for Caucasians (0.277 0.008). After RMR (l O2/min) was
adjusted for LBM, an even larger difference (712%) 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 VO2max and RMR was r ˆ 0.51 when VO2max was expressed as l/min, and r ˆ 0.56 when VO2max was
expressed as ml O2/kg/min, both highly signi®cant (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 (VO2max) as well as LBM are signi®cant predictors of
RMR for both races.
Keywords: obesity; racial differences; metabolic rate; African-American women; Caucasian women; metabolism
Introduction
Recent data indicate that the prevalence of obesity in
the US has risen during the past years.1 This is of
concern because it is generally accepted that obesity is
a risk factor for several diseases including non-insulin-dependant diabetes mellitus (NIDDM), gallbladder
disease, cardiovascular disease, hypertension, cancer
and musculoskeletal disorders. Age-adjusted prevalence of diabetes and hypertension are approximately
double for African-American women compared to
Caucasian women and death rates from these diseases
are 2.5 and 1.5 times higher among African-American
women, respectively.2±4 Additionally, death rates
from heart disease are 2±3 times higher in AfricanAmerican women than in Caucasian women aged
30±55 y.3
One possible explanation for the high prevalence of
debilitating diseases in African-American women is
the fact that 49% of African-American women are
overweight, compared to only 34% of Caucasian
Correspondence: Wayne C Miller PhD, The George Washington
University, Exercise Science Programs, 817 23rd Street NW,
Washington, DC 20052, USA.
Received 3 April 1997; revised 8 August 1997; accepted
17 October 1997
women.1 Furthermore, a recent study has indicated
that African-American women are approximately 50%
more likely than Caucasian women to experience a
major weight gain (10 kg) over a period of 10 y and
60% more likely to become obese.5
Some researchers have hypothesized that differences in socioeconomic status,6 genetics,7,8 parity,7,9
participation in physical activity,10±18 body image19
and cultural attitudes toward obesity,7,19 may be
responsible for the higher prevalence of overweight
for African-American women compared to Caucasian
women. It has been shown that obese African-American women are less likely to lose weight than
Caucasian women10 and are likely to regain lost
weight.19 Several researchers have reported lower
levels of physical activity10±17 and higher rates of
sedentary lifestyle4,20 in African-American women
compared to Caucasian women. However, it has
been reported that African-Americans and Caucasians
employ similar weight loss practices21 including calorie reduction and/or increasing physical activity.
Although it is apparent that cultural and behavioral
differences exist between races,10±19 it is still important to determine if racial metabolic differences exist
that may contribute differentially to adiposity levels.
For instance, after adjusting for body composition,
age, and gender; low relative resting metabolic rate
(RMR) has been shown to be a predictor of weight
Racial differences in RMR of inactive obese women
JN Forman et al
216
gain in Pima Indians.22 A comparative study of
metabolic rates between African and European men
revealed a reduced (17±20%) RMR for the Africans,
whether expressed in absolute terms, per total body
weight (TBW), or per lean body mass (LBM).23
Results from this latter study may not be generalizable
since there were only 8±9 men per group, and these
men were young and of average weight.
Only a few studies examining metabolism between
obese Caucasian and African-American women have
been published.24±26 In the ®rst study, the AfricanAmerican women tended to have a lower RMR than
the Caucasian women, though these differences were
not statistically signi®cant.24 One reason why these
researchers did not ®nd signi®cant metabolic differences between races may be because of the small
sample sizes (n ˆ 14 or 15 per group). Furthermore,
these researchers did not control for the menstrual
cycle and there is strong support for the signi®cant
¯uctuation of up to 10% in metabolic rate between
phases of the menstrual cycle.27±31 Although the
subjects in the second study were all postmenopausal
women, no differences were found between races
for resting energy expenditure (REE).25 AfricanAmerican women in the third study, however, had a
lower REE than the Caucasians; in spite of the sample
being a mix of premenopausal and postmenopausal
women.26 Consequently, when examining the effect
of race on metabolic rates of women, it is critical to
control for age, body size and composition, phase of
the menstrual cycle and daily activity levels, as these
can all signi®cantly affect RMR. Therefore, the purpose of this study was to determine if RMR is
different in obese African-American women compared to a closely-matched group of obese Caucasian
women.
Subjects
Forty-seven women (25 African-American and 22
Caucasian) volunteered as subjects. According to
self-report, none of the African-American women
were of other ancestry (for example Jamaican). All
subjects were premenopausal (age, 35 1 y, Mean
s.e.m.) with at least 10 menstrual cycles per year.
They were non-smokers, sedentary, de®ned as exercising less than once per week for the previous six
months, and obese (95.2 2.9 kg, body mass index
(BMI) ˆ 34.7 0.9, % body fat ˆ 45.2 0.9). All
subjects were free from metabolic disorders and
medications that would affect heart rate or RMR.
Prior to participation, each subject signed an informed
consent form approved by the Medical Center Institutional Review Board in compliance with the Department of Health and Human Services Regulations for
Protection of Human Research Subjects.
Testing protocol
Subjects reported to the laboratory within 60 min of
waking and after a 12 h fast. Following 30 min of
quiet supine rest in a dimly lit room, RMR was
measured for 30 min through indirect calorimetry
(Sensormedics Vmax System; Yorba Linda, CA)
utilizing a ventilated hood system. The coef®cient of
variation for RMR measurements using this system in
our laboratory is 4%. Measurements were taken
during the early follicular phase of the menstrual
cycle (days 1±7) to control for any possible variation
in metabolic rate, with the majority of testing being
conducted on day 4 or 5 of the menstrual cycle.
Menstrual cycle phase was determined by self-report
of the subjects, with the ®rst day of menstrual bleeding noted as day 1 of the cycle and the beginning of
the follicular phase. Menstrual cycle control is important since RMR,27,28 sleeping metabolic rate
(SMR)28±31 and total daily energy expenditure
(TDEE)28,29,31 have been shown to be signi®cantly
higher (10%) during the luteal phase than during the
follicular phase. Predicted metabolic rates were determined by the Harris-Benedict equation.32
Following the RMR measurement, body composition was determined by an air displacement 1plethysmograph, referred to as the BODPOD Body
Composition System (Life Measurement
Instruments,
1
Concord, CA, USA). The BODPOD consists of a
dual-chambered plethysmograph that provides a densitometric means of body composition analysis
wherein the subject is tested in the front chamber
and instrumentation is housed in the rear chamber of a
®berglass capsule. In contrast
to hydrostatic weighing
1
(HW), the BODPOD uses air displacement rather
than water displacement to determine body volume.
Similar to HW, once body volume is determined,
standardized equations for calculating body density
and % body fat from body density are used. Therefore,
the only difference between the two methods is how
body volume is measured±not how % body fat is
calculated.
Although a speci®c % body fat equation for
African-American men has been derived,33 the Siri
equation34 produced % body fat values for the AfricanAmerican women in this study that were equivalent
to the values produced by the race-speci®c equation.
Hence, we elected to remain with the more familiar
Siri equation for calculating % body fat for both
groups.
1
Research has shown the BODPOD to be a highly
reliable and valid method for determining body composition in comparison
to HW. When compared to
1
HW, the BODPOD had a test-retest coef®cient of
variation equal to HW, correlated with the HW
technique (r ˆ 0.96), and had a 95% con®dence interval of 70.6±0% body fat.35 This new method for
body composition analysis was chosen because it is
quick, relatively easy to administer and can accom-
Racial differences in RMR of inactive obese women
JN Forman et al
modate populations such as the obese more comfortably than HW.35
Subjects rested quietly in a seated position for
approximately 3±5 min while heart rate was monitored in order to determine resting heart rate (RHR).
Maximal aerobic capacity (VO2max) was determined
on a motorized treadmill (Quinton 18±60; Quinton
Instruments, Seattle, WA) using the Bruce protocol.36
Subjects walked slowly for 2±3 min to become comfortable with walking on a treadmill, after which the
speed and gradient were increased every 3 min until
exhaustion. Heart rate was recorded each minute
during the treadmill test, using a heart rate monitor
(Polar CIC, Inc, Port Washington NY) while respiratory gases were measured continuously and displayed
in 20 s averages using the Quinton Q-plex System.
Blood pressure was assessed once per stage, beginning with stage two. Rating of perceived exertion
(RPE) was assessed during the last 30 s of each
stage. VO2max was determined by calculating the
highest 20 s measurement of VO2, with respiratory
exchange ratio (RER) greater than 1.1 and heart rate
within 10 bpm of predicted maximal heart rate
(MHR).
Girth measurements were taken for the chest, arm,
hip and calf at maximal protrusion. The thigh was
measured six inches above the knee and waist was
measured at the umbilicus since there was no narrowing of the torso for most subjects.
Statistics
All group values are reported as mean s.e.m. An
analysis of covariance (ANCOVA), adjusting for body
weight and LBM was used to compare RMR between
groups. Other comparisons between groups were
examined with a Student's t-test. The strength of
relationship between two variables was made with a
Pearson Correlation Coef®cient. Signi®cance was set
at the P < 0.05 level.
Results
The two groups were not signi®cantly different in age,
height, weight, BMI, % body fat, fat mass; or chest,
arm, waist, hip, thigh, and calf circumferences (Table
1). Although % body fat was not different between the
two groups, African-American women had signi®cantly more LBM than Caucasian women (Table 1).
RER and RHR were similar between races, as were
metabolic measurements during maximal exercise
(MHR; VO2max, l O2/min).
When adjusted for TBW, RMR for AfricanAmericans was signi®cantly lower than Caucasians
by 9% (Table 2). An even larger difference (712%)
217
Table 1 Characteristics of the subjects
Age (y)
Height (cm)
Weight (kg)
BMI (kg/m2)
% Body fat
Fat mass (kg)
Lean mass (kg)
Girth measurements (cm):
Chest
Arm
Waist
Hip
Thigh
Calf
RER
RHR (bpm)
MHR (bpm)
VO2max (l O2/min)
AfricanAmericans
(n ˆ 25)
Caucasians
(n ˆ 22)
34 1
164.6 1.3
99.0 3.2
36.1 1.3
44.8 1.2
45.2 3.1
53.7 1.7*
36 2
165.4 1.5
91.1 3.6
33.2 1.1
45.6 1.3
42.3 2.6
48.8 1.2
111.8 2.8
38.4 1.0
115.8 3.3
124.7 3.0
70.6 2.0
42.9 1.0
0.75 0.02
73 2
183 3
2.23 0.13
114.0 2.3
37.1 0.8
107.4 2.5
120.9 2.0
66.8 1.5
42.4 1.0
0.74 0.02
74 2
182 2
2.41 0.09
Values are mean s.e.m. *Signi®cantly different from
Caucasians. BMI ˆ body mass index; RER ˆ respiratory
exchange ratio; RHR ˆ resting heart rate; VO2max ˆ maximal
aerobic capacity.
Table 2 Adjusted metabolic measurements of sedentary obese
women
African-Americans
(n ˆ 25)
RMR (l O2/min)a
(l O2/min)b
RMR (kJ/d)a
(kcal/d)a
RMR (kJ/d)b
(kcal/d)b
Predicted RMR (kJ/d)
(kcal/d)
0.254 0.007*
0.250 0.008*
7222 184*
1726 44*
7100 201*
1697 48*
7247 180
1732 43
Caucasians
(n ˆ 22)
0.277 0.008
0.281 0.008
7816 192**
1868 46**
7945 209**
1899 50**
6971 159
1666 38
Values are mean s.e.m. RMR ˆ resting metabolic rate.
Adjusted for individual differences in body weight.
Adjusted for individual differences in lean body mass.
*
Signi®cantly different from Caucasians. **Signi®cantly different
from predicted energy expenditure of the same group.
a
b
for African-Americans compared to Caucasians was
found when RMR was adjusted for LBM. Correspondingly, when RMR was expressed as kJ/d or kcal/d, the
adjusted mean values were different between races.
While predicted RMR for the African-Americans was
the same as measured RMR, Caucasian women
expended more energy than was predicted (Table 2).
RMR (l O2/min) was signi®cantly (P < 0.000)
related to body weight (r ˆ 0.69) and LBM
(r ˆ 0.61) in these obese women (Figure 1, Figure
2). The data in these ®gures indicate that at all values
of body weight and LBM, obese African-American
women would be expected, on average, to have lower
RMR than obese Caucasian women.
RMR (ml O2/kg/min) was also signi®cantly related
(r ˆ 0.51±0.56, P < 0.000) to VO2max (l/min; ml
O2/kg/min) for both groups. When this relationship
was tested for group differences, the large Sample Z
test37 revealed no racial differences in correlation
coef®cients.
Racial differences in RMR of inactive obese women
JN Forman et al
218
Figure 1 The relationship between resting metabolic rate (RMR)
and body weight in obese African-American and Caucasian
women. Filled circles represent African-American women,
open circles represent Caucasian women.
Figure 2 The relationship between resting metabolic rate (RMR)
and body weight in obese African-American and Caucasian
women. Filled circles represent African-American women,
open circles represent Caucasian women.
Discussion
Obesity is believed to be caused by a positive energy
balance over time, where energy intake exceeds
energy expenditure. The higher prevalence of obesity
in African-American women compared to Caucasian
women4 suggests that there may be metabolic differences between the two races. The data generated from
this study indicate that RMR, adjusted for body
weight and LBM, of obese African-American
women is respectively 9±12% lower than that for a
comparable group of obese Caucasian women. This
metabolic difference cannot be attributed to differences in age, BMI, body composition, daily activity
levels, menstrual cycle phase or by ®tness level of
these women.
Although metabolic rates differ between men and
women and between obese and lean individuals,28
Geissler and Hamod Aldouri23 observed a highly
signi®cant effect of race on RMR, which is in accordance with our observation. Speci®cally, both absolute and relative RMR were 17±20% lower for
African men (n ˆ 9) compared to European men
(n ˆ 8). Although these men were lean (68 kg), the
signi®cant effect of race on RMR found by Geissler
and Hamod Aldouri23 is consistent with the ®ndings
of our study involving obese women. It should be
noted that there is no difference in obesity prevalence
between African-American and Caucasian men4
though African-American men appear to be more
metabolically ef®cient than Caucasian men.23
Other than a couple of abstracts, only a few studies
have addressed the issue of metabolic differences
between obese African-American and Caucasian
women.24±26 Foster et al26 found that obese AfricanAmerican women had RMR that were 5% lower than
Caucasian women, whether RMR was expressed in
absolute terms or adjusted for individual differences
in body weight. When RMR was adjusted for LBM,
the difference between races became 8%.26 These
racial differences found by Foster et al26 were slightly
smaller than the 9±12% found in our study. The
variation in magnitude of racial differences in RMR
between studies might be due to the fact that ®tness
level and activity patterns were not controlled for in
the previous study. Furthermore, menstrual status of
the women in both groups of this earlier study was
variable. Approximately 25% of the Caucasian
women in the study were postmenopausal compared
to 9% of the African-Americans, while menstrual
status was undetermined in 6% of the Caucasian
women.26
While looking at postmenopausal women only,
Nicklas et al25 found no difference in RMR between
obese African-Americans and Caucasians, when RMR
was adjusted for individual differences in LBM.
However, these authors also reported that RMR was
related to plasma leptin levels in obese postmenopausal African-American, but not Caucasian women.
Thus, it is not yet understood how factors such as
leptin, menstrual status and phase of the menstrual
cycle relate to RMR in the different races; although it
is known that there is a signi®cant ¯uctuation of up to
10% in metabolic rate between different phases of the
menstrual cycle.27±31
Kushner et al24 reported that African-American
women tended to have a lower RMR than Caucasian
women, though the difference was not statistically
signi®cant. Although these authors did not control for
phase of the menstrual cycle in their RMR measurements, their lack of statistically signi®cant differences
between races could also be due to small sample sizes.
When we ran a power analysis on the data from their
study, we found that if Kushner et al24 had doubled
their sample size from 14 or 15 per group to 31 per
group, they would have found that the African-American women had signi®cantly lower RMR than the
Caucasian women. Therefore, it seems that when age,
body size and composition, phase of the menstrual
cycle, ®tness and activity levels are equivalent, as in
the current study; there is a small, but signi®cant
difference in RMR between obese African-American
and obese Caucasian women.
Racial differences in RMR of inactive obese women
JN Forman et al
While a strong relationship between LBM and
RMR has been established,38 studies evaluating the
relationship between VO2max and RMR are con¯icting. A study including men and women of various
®tness levels revealed that VO2max, did not affect
RMR independent of LBM.38 In contrast, it has
been shown that aerobically-trained men (VO2max ˆ
4.3 0.6 l/min) have an increased RMR over
untrained men (VO2max ˆ 3.6 0.6 l/min), independent of LBM, suggesting there is a relationship
between RMR and ®tness level.39 Other researchers
have reported a similar ®nding where a higher RMR
was found in trained vs untrained individuals, again
indicating a link between VO2max and RMR.40
Furthermore, when these latter researchers controlled
for LBM and VO2max., the age-related decline in RMR
was no longer signi®cant. According to these results,
loss of LBM alone does not fully explain the agerelated decline in RMR among men because VO2max
was an independent factor contributing to this
decline.40
Since the studies cited above included only subjects
of average weight, it would be meaningful to investigate a possible relationship between these variables
in the obese women studied presently. In our study,
stepwise regression analysis revealed that VO2max and
LBM were both signi®cant predictors of RMR,
whether RMR and VO2max were expressed in absolute
terms (l/min) or relative to body weight (ml O2/kg/
min). When controlling for LBM, the partial correlation between VO2max and RMR was r ˆ 0.51 when
VO2max was expressed as l/min and r ˆ 0.56 when
VO2max was expressed as ml O2/kg/min. This relationship is particularly interesting considering these subjects were sedentary, untrained obese women.
Consequently, VO2max is related to RMR independent
of LBM and exercise training.
Commonly, dietitians, nutritionists, and weight
reduction counsellors prescribe speci®c dietary
intakes for overweight persons with the intention of
creating a negative or less positive energy balance, so
that theoretically weight loss will occur. Unfortunately, measuring energy expenditure directly is
time-consuming and expensive. A more practical
method is to use prediction equations to estimate
metabolic rate at rest and during physical activity so
that TDEE can be calculated. To do this, these
professionals need only readily available information
such as body weight, body composition, age, gender,
or a combination of these to utilize the prediction
equations. Once TDEE is determined, energy intake
can be manipulated to create a change in energy
balance which would promote weight reduction or
control.
As expected, predicted RMR did not differ between
races in our study since these predicted rates were
based on measurements which were similar in both
groups, including age, height and weight.32 However,
when comparing predicted RMR to measured RMR,
racial differences were found. RMR for obese Cauca-
sian women were signi®cantly under predicted, while
the predicted RMR for African-Americans did not
differ from their measured RMR. This discrepancy
between predicted RMR and measured RMR is in
contrast with what has been found previously. One
study has shown that the Harris-Benedict equation
actually over predicted RMR in obese women.41
However, no details were given with regard to the
racial makeup of the subject sample. Furthermore,
homogeneity among RMR testing conditions for the
subjects in this study were not documented.
Another earlier study found that there was a tendency for RMR to be under predicted in older women,
but this discrepancy between predicted and measured
RMR was not signi®cant (P ˆ 0.07).42 It is also hard
to interpret the results of this study because the racial
makeup of this subject sample was not described in
the study, and the women in the study were all normal
weight. Moreover, the women in this study were
heterogeneous with regard to menstrual status, most
of the women (88%) being postmenopausal.
Thus it appears that in a homogeneous group of
obese premenopausal Caucasian women, the HarrisBenedict equation will under predict true RMR by
about 628 kJ/d (150 kcal/d). Theoretically, this difference can add up to 4400 kJ per week (> 1000 kcal/
week) and corresponds to over 7 kg body weight by
the end of one year. For Caucasian women, this error
may supplement weight loss caused by the prescribed
energy de®cit or enhance weight control by preventing further weight gain, but it probably does not
explain the degree of obesity nor the difference in
weight loss success between races.9±19,43,44 Accordingly, prescribed energy intakes can be estimated
accurately for obese African-American women using
the Harris-Benedict equation, but it appears that a
more accurate equation is needed for obese Caucasian
women.
Conclusion
In summary, the results from this study indicate that
there are signi®cant differences in adjusted RMR
between inactive, obese African-American and Caucasian women. It is likely that the lower RMR for
African-American women contributes to the high
prevalence of obesity in the African-American
female population. Thus, when examining the metabolism of premenopausal women, it is critical to
control for racial differences in metabolic rates.
Due to the metabolic differences found in this
study, it is particularly important to control for race
when creating energy expenditure prediction equations for obese women. The current discrepancy in
the obesity prevalence between the African-American and Caucasian races may be attributed in part to
metabolic differences found between obese African-
219
Racial differences in RMR of inactive obese women
JN Forman et al
220
American and Caucasian sedentary women. Although
cultural and environmental factors may interfere with
the weight loss/control efforts of African-American
women,9±19,43,44 metabolic ef®ciency in an environment with a surplus of food can lead to high obesity
rates which contribute to life-threatening disease. It
will be important to determine if exercise training,
behavior modi®cation or other metabolic manipulations have a favorable effect on RMR or weight control
in the obese African-American female population.
Acknowledgements
The authors wish to thank Dana Brown, Kirsten
Ambrose, Angie Lyden, Kevin Rohan, Jennifer
Tooley, Michele Roylance and Connie Strine for
their technical assistance in the laboratory.
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