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1 Human Performance Laboratory, Department of Exercise and Sports Science, and Departments of 2 Biochemistry and 3 Physiology, East Carolina University, Greenville, North Carolina 27858
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ABSTRACT |
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The goal of this study was to determine
whether differences in physical activity-related fat oxidation exist
between lean and obese African-American (LAA and OAA) and lean and
obese Caucasian (LC and OC) premenopausal women. Lean AA (28.4 ± 2.8 yr, n = 7), LC (24.7 ± 1.8 yr,
n = 9), OAA (30.9 ± 2.2 yr, n = 11), and OC (34.1 ± 2.5 yr, n = 9) women
underwent preliminary assessment of peak aerobic capacity
(
O2 peak). On a subsequent testing day,
participants exercised after an 8-h fast on a cycle ergometer at 15 W
(~40%
O2 peak) for 10 min and then
for 10 min at ~65%
O2 peak. Fatty
acid oxidation was determined using the average respiratory exchange
ratio and O2 consumption during minutes 5-9
of the exercise session. Percent body fat and fat-free mass were lower
(P < 0.05) in LAA (25.8 ± 2.8% and 48.3 kg) and
LC (26.4 ± 2.0% and 45.8 ± 1.7 kg) than in OAA (41.2 ± 1.3% and 58.8 ± 3.3 kg) and OC (39.3 ± 2.7% and 58.6 kg) women. Fat oxidation among the groups was analyzed statistically
using analysis of covariance with fat-free mass and
O2 peak as covariates. During exercise
at 15 W, fat oxidation was as low in LAA (0.134 ± 0.024 g/min) as
in OAA (0.144 ± 0.026 g/min) and OC (0.156 ± 0.020 g/min)
women: all these rates of fat oxidation were lower than in LC women
(0.200 ± 0.021 g/min, P < 0.05, LC vs. all other
groups). Fatty acid oxidation during higher-intensity exercise (65%
O2 peak) was higher in LC than in OC
women but was not statistically different between African-American and Caucasian groups. Fatty acid oxidation was therefore lower during low-intensity physical activity in OAA, LAA, and OC than in LC women.
body composition; obesity; respiratory exchange ratio
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INTRODUCTION |
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THE PREVALENCE OF OBESITY in the United States is greater in African-American than in Caucasian individuals (22, 23). Obese African-American women also suffer from higher incidences of hypertension, Type 2 diabetes, and morbidity than obese Caucasian women (12). Although environmental factors such as socioeconomic status and diet may influence the greater prevalence of obesity in African-American women, the higher incidence of obesity and subsequent adverse outcomes are not entirely due to environmental factors (5, 20, 25, 31, 32). This suggests that inherent physiological and biochemical differences underlie the increased obesity in African-Americans.
Resting energy expenditure and resting fat oxidation have been shown to
be depressed with obesity and may be lower in African-American than in
Caucasian women (6, 7, 14, 15, 26, 27). This decreased
resting energy expenditure could contribute to the increased rate of
weight gain in African-American than in Caucasian women
(5). In addition, there has been evidence of lower fat
mobilization during weight loss in African-American than in Caucasian
women. African-American women have been shown to have a slower rate and
a smaller degree of weight loss, consistent with the hypothesis that
there are inherent physiological and biochemical differences between
the races (11, 24, 28). Although reduced resting energy
expenditure or fat oxidation has been demonstrated in African-American
compared with Caucasian individuals (14, 18, 27), there
has been only one investigation of fat oxidation in these groups during
exercise. Chitwood et al. (8) recently demonstrated that
fat oxidation during treadmill exercise at 65% of maximal
O2 uptake (
O2) was lower in
lean premenopausal African-American than in lean Caucasian women.
However, there have been no investigations of fat oxidation in obese
groups of African-American and Caucasian premenopausal women with the lower peak aerobic capacity (
O2 peak)
in African-American than in Caucasian women taken into account
(18). There have also been no investigations in these
populations of fat oxidation during low-intensity exercise, which is
more representative of normal daily activities.
We therefore investigated fat oxidation in groups of lean and obese
African-American and Caucasian women. We analyzed the data using
analysis of covariance to control for differences in our groups with
respect to fat-free mass and
O2 peak per kilogram fat-free mass. The purpose of this investigation was to
determine whether differences in fat oxidation exist in obese and lean
African-American women compared with corresponding groups of Caucasian women.
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METHODS |
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Subjects.
African-American (n = 18) and Caucasian
(n = 18) premenopausal women participated in the study
after giving informed consent according to the East Carolina University
Institutional Review Board. African-Americans were individuals who
self-reported only African-American parents and grandparents.
Caucasians were individuals who self-reported only Caucasian parents
and grandparents. All subjects were sedentary, in that they reported
exercising less than one time per week over the previous 6 mo. Through
personal history questionnaires, it was determined that the subjects
were weight stable over the past 4 wk, had normal menses over the past year, were normotensive, had no prior history of cardiovascular or
metabolic disease, and were not taking medications known to affect
respiratory exchange ratio (RER) or
O2.
These groups were further subdivided into lean African-American
(n = 7) and lean Caucasian [n = 9, body mass index (BMI)
28 kg/m2] women and obese
African-American (n = 11) and obese Caucasian (n = 9, BMI > 28 kg/m2) women.
Subject characteristics are presented in Table
1.
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Protocol.
Participants underwent preliminary assessment of
O2 peak on a cycle ergometer. On a
subsequent testing day, participants exercised after an 8-h fast on a
cycle ergometer at 15 W for 10 min and then for 10 min at ~65%
O2 peak (not significant between
groups). Fat oxidation (mean ± SE in g/min) was determined using
the average RER and
O2 during
minutes 5-9 of the exercise session. The subjects were
studied in their habitual state, in that diet was not controlled.
Body composition and anthropometric determinations. Residual volume was determined by the O2 dilution method as described by Wilmore (33). Body density was determined by hydrostatic weighing, with percent body fat calculated using residual volume and body density calculated using the equations of Brozek et al. (3).
O2 peak.
O2 peak was determined on an
electronically braked cycle ergometer. The test was incremental,
beginning at 20 W and increasing in exercise intensity by 20 W every 3 min. Respiratory gases were analyzed continuously and averaged over
20-s intervals using a Sensormedics 2900 Metabolic Measurement Cart
(Anaheim, CA). The subjects exercised until they could no longer
maintain a cadence of 40 rpm. Achievement of
O2 peak was determined by attainment of
two of the following criteria: 1) plateau in
O2 with increased exercise intensity,
2) RER > 1.1, and 3) heart rate greater
than age-predicted maximal heart rate.
Exercise protocol.
Within 1 wk of the
O2 peak
determination, participants exercised after an 8-h fast on a cycle
ergometer at 15 W (~40%
O2 peak) for
~10 min and then for 10 min at 65%
O2 peak. Respiratory gases were
analyzed continuously and averaged over 20-s intervals using a
Sensormedics 2900 Metabolic Measurement Cart. Fat oxidation was
determined using the average RER and
O2 during minutes 5-9 of the exercise session. The
day-to-day variation (coefficient of variation) for fat oxidation in
these groups was 9.5%.
Statistics.
For each variable a two-way [race (African-American or Caucasian) × body composition (obese or lean)] ANOVA was performed. For fat
oxidation, analysis of covariance was performed with covariates of
fat-free mass and
O2 peak because of
the known effect of these variables on rates of fat oxidation. When
significant results were found, Newman-Keuls post hoc analysis was
used. Values are means ± SE. The level of significance was set at
P < 0.05.
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RESULTS |
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Body composition and aerobic capacity.
Lean African-American and lean Caucasian groups had lower body weight,
BMI, body fat, and fat-free mass than respective obese groups (Table
1).
O2 peak was not different among any of the groups, although
O2 peak per
kilogram fat-free mass was higher in lean Caucasians than in both obese
groups (P < 0.05).
Submaximal power output.
The power output was set at 15 W for all groups for the low-intensity
exercise. The power output for the higher intensity exercise (~65%
O2 peak) was lower in obese
African-American women (38.3 ± 5.2 W) than in lean
African-American (56.0 ± 4.9 W), obese Caucasian (64.7 ± 7.7 W), and lean Caucasian (62.2 ± 3.2 W) women
(P < 0.05, obese African-American vs. all other groups).
RER.
RER was lower in lean Caucasian than all other groups of women during
exercise at 15 W (Table 2;
P < 0.05). RER was lower in lean Caucasian than in
obese Caucasian and lean African-American women during exercise at
~65%
O2 peak (Table 2;
P < 0.05).
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Submaximal
O2.
Submaximal
O2 was higher in obese
Caucasian and obese African-American women than in corresponding lean
groups of women during exercise at 15 W (Table
3; P < 0.05). There were
no differences in submaximal
O2 during
exercise at 65%
O2 peak (Table 3).
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Fat oxidation.
The rate of fat oxidation in lean African-American women was 70%
(P < 0.05) of that in lean Caucasian women during
exercise at 15 W (Fig. 1A).
During exercise at 65%
O2 peak, there was an interaction (race × body composition) in the rates of fat oxidation (P < 0.05). The rate of fat oxidation in
obese Caucasian women was 45% (P < 0.05) of that in
lean Caucasian women during exercise at 65%
O2 peak, although there was no
difference in lean and obese African-American women (Fig.
2A). Fat oxidation rate in
lean African-American women was 50% of that in lean Caucasian women
during exercise at 65%
O2 peak
(P = 0.1; Fig. 2A). Similar results were
obtained when fat oxidation rates were expressed per kilogram fat-free
mass (Figs. 1B and 2B).
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DISCUSSION |
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The data demonstrate higher rates of fat oxidation during exercise
in lean premenopausal Caucasian than in lean African-American women,
although no significant differences were observed between obese
Caucasian and obese African-American women. During low-intensity physical activity (~40%
O2 peak),
there was a higher rate of fat oxidation in lean Caucasian women than
in all other groups (obese Caucasian as well as lean and obese
African-American women; Fig. 1A). Although the data
presented in Fig. 1A are expressed as rates of total fat
oxidation per minute, covariance analysis was performed to control for
differences in our groups with respect to fat-free mass and aerobic
capacity. The findings of lower rates of fat oxidation during cycle
ergometer exercise at low intensity in African-American than in
Caucasian women were therefore not due to a lower
O2 peak per kilogram fat-free mass in the African-American women.
Fat oxidation provides the majority of energy production at rest and
during exercise below ~65%
O2 peak
and is therefore an important regulator of adipose mass (4,
10). Reduced fat oxidation results in increased fatty acid
storage as triglyceride when diet and exercise level are held constant.
The resultant excess storage of fat has been implicated in insulin
resistance, hypertension, and the increased risk of obesity-related
diseases (2, 9, 16). The lower fat oxidation rate in
African-American than in Caucasian women may be one of the underlying
causes of the higher rates of obesity in this population. Although
socioeconomic status, diet, physical activity, and other environmental
factors have also been implicated in the higher rates of obesity in
African-American than in Caucasian women, the rates of obesity remain
higher when these factors are controlled (5, 20, 25, 31,
32).
Our findings of a 30% lower fat oxidation rate in lean
African-American than in lean Caucasian women during low-intensity exercise (~40%
O2 peak) are in
agreement with those of a previous publication (8) in
which RER was higher (reflective of lower fat oxidation) in lean
African-American than in lean Caucasian premenopausal women during
higher-intensity treadmill exercise (65% maximum
O2). We also have demonstrated a higher RER in lean African-American than in lean Caucasian women in the present study during exercise at 65%
O2 peak, although the calculation of
fat oxidation from RER and
O2 data did
not yield a statistically significant difference in fat oxidation between these two groups. However, the mean fat oxidation rates in the
present study were 65% lower in lean African-American than in lean
Caucasian women during this higher intensity (~65%
O2 peak) cycle ergometry exercise (Fig.
2A; P = 0.1). These data suggest differences
that are similar in magnitude to the ~65% difference in fat
oxidation rate between lean African-American and lean Caucasian women
that can be calculated from RER data of Chitwood et al. (8). Our findings at the lower exercise intensity (15 W,
~40%
O2 peak) extend the previous
findings of Chitwood et al., in that this is the first study to
document a lower fat oxidation in lean African-American than in lean
Caucasian women during low-intensity physical activity. The 15-W
exercise intensity elicits a
O2 in these
groups that would be comparable to a walking pace. This decreased lipid
oxidative disposal at intensities of activity close to everyday
physical activity levels would promote fat storage in African-American women.
Reduced fat oxidation has been shown by some (6, 19, 21), although not all (17, 29, 30), authors to be related to increased adiposity. Findings presented in the present study of lower rates of fat oxidation in obese than in lean Caucasian women are in line with previous reports demonstrating reduced fat oxidation with obesity from our group and others, although our present data may have been influenced by the age of the lean (24.7 yr) and obese (34.1 yr) Caucasian women (P = 0.05). The novelty of the present findings was that fat oxidation during low-intensity exercise was reduced to a similar extent in obese African-American, lean African-American, and obese Caucasian women (Fig. 1). There may therefore be a similar defect in fat oxidation in these groups. These results are striking, suggesting that fat oxidation is already reduced in lean African-American women to a level equal to that in obese Caucasian women and is not further reduced with increasing adiposity in this group. These findings support the hypothesis of a racial difference between African-American and Caucasian women that may have an underlying genetic component.
The reduced fat oxidation per se could lead to an increased
partitioning of fat to storage. The energy requirement for converting carbohydrate to triglyceride for storage has been approximated to be
25% greater (13) than the energy requirement for direct incorporation of fatty acids into triglyceride. Reduced fatty acid
oxidation would therefore favor the more efficient method of energy
storage, thereby increasing the propensity for obesity. The difference
in fat oxidation between lean Caucasian and African-American women in
the present study was 3.7 g/h at ~40%
O2 peak and 4.2 g/h at ~65%
O2 peak. This reduced fat oxidation during physical activity, combined with the previously reported reduction in resting fat oxidation (14, 18, 27), could
result in a substantial accumulation of body fat over the decades of life. With the assumption of only 1 h/day of activity
40%
O2 peak, there would be a difference of
~1.5 kg of fat per year, or 15 kg of fat per decade, more oxidized
during physical activity in a Caucasian than in an African-American
woman. It is not possible to determine the relative contribution of
activity-related and resting fat oxidation from this study, because we
did not measure the amount of time each day spent in physical activity,
rather than at rest, in these subjects. Furthermore, additional studies are necessary to determine whether the defect resulting in reduced fat
oxidation during physical activity is the same as that resulting in
reduced resting fat oxidation in lean African-American compared with
lean Caucasian women. The lack of difference in fat oxidation at the
same absolute (15 W) and relative (~65%
O2 peak) exercise intensities in obese
Caucasian and obese African-American women in the present study
suggests that reduced fat oxidation is not responsible for the
previously reported (11, 24, 28) slower rate of weight
loss in obese African-American than in obese Caucasian women.
There was a surprising difference in power output between the obese
African-American and the obese Caucasian women during the higher
intensity exercise. The 42% lower power output in obese African-American than in obese Caucasian women is not consistent with
the similar
O2 in these groups at 65%
O2 peak. Efficiency was apparently
lower in the obese African-American than in the obese Caucasian women
and was also lower than in all other groups. This phenomenon was not
apparent when the subjects exercised at the lower power output set at
15 W. The higher
O2 at 15 W in obese
than in lean women in this study was likely due the necessity of moving
the larger leg mass of the obese women at this very low power output.
There are, to our knowledge, no previously published data regarding
efficiency in African-American compared with Caucasian individuals,
although resting
O2 has been shown to be
lower in African-American than in Caucasian women (8).
The lack of blood or muscle biopsy data in this study limits the findings to the data obtained using whole body indirect calorimetry. Previous authors (8) reported that plasma insulin remains higher in African-American than in Caucasian individuals during exercise, which may have suppressed fat oxidation in African-American women. The lower slow-twitch fiber distribution in African-American than in Caucasian individuals might also contribute to the lower fat oxidation in African-American women (1). There are numerous other potential sites of limitation in fat oxidation, including transport of fatty acids into the muscle cell (lipoprotein lipase and putative fatty acid transporters), transport and activation of the fatty acids within the muscle cell (fatty acid-binding proteins and fatty acyl-CoA synthase), or transport of fatty acids into the mitochondria (carnitine palmitoyl transferases and regulatory molecules such as malonyl-CoA). There are no reported data, to our knowledge, comparing African-American and Caucasian women with respect to any of these potential sites of limitation.
In summary, fatty acid oxidation rates are lower in lean
African-American than in lean Caucasian premenopausal women. This difference is evident during low-intensity (40%
O2 peak) physical activity, in addition
to the previously reported difference in RER at 65%
O2 peak in lean African-American and
lean Caucasian women (8). Fatty acid oxidation during
low-intensity physical activity is reduced to a similar degree in obese
Caucasian, lean African-American, and obese African-American
premenopausal women compared with lean Caucasian premenopausal women.
These findings support the hypothesis of inherent physiological or
biochemical differences between African-American and Caucasian women
that may have an underlying genetic component.
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ACKNOWLEDGEMENTS |
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The authors are grateful for the technical assistance of Gwen Cunningham.
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FOOTNOTES |
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This study was supported by a grant from the North Carolina Institute of Nutrition.
Address for reprint requests and other correspondence: R. C. Hickner, 371 Ward Sports Medicine Bldg., East Carolina University, Greenville, NC 27858 (E-mail: Hicknerr{at}mail.ecu.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 14 September 2000; accepted in final form 3 January 2001.
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H. Barakat, J. Davis, D. Lang, S. J. Mustafa, and M. M. McConnaughey Differences in the Expression of the Adenosine A1 Receptor in Adipose Tissue of Obese Black and White Women J. Clin. Endocrinol. Metab., May 1, 2006; 91(5): 1882 - 1886. [Abstract] [Full Text] [PDF] |
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J. F. Bower, J. M. Davis, E. Hao, and H. A. Barakat Differences in transport of fatty acids and expression of fatty acid transporting proteins in adipose tissue of obese black and white women Am J Physiol Endocrinol Metab, January 1, 2006; 290(1): E87 - E91. [Abstract] [Full Text] [PDF] |
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C. J. Tanner, H. A. Barakat, G. L. Dohm, W. J. Pories, K. G. MacDonald, P. R. G. Cunningham, M. S. Swanson, and J. A. Houmard Muscle fiber type is associated with obesity and weight loss Am J Physiol Endocrinol Metab, June 1, 2002; 282(6): E1191 - E1196. [Abstract] [Full Text] [PDF] |
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