Department of Medicine, University of Vermont, Burlington, Vermont 05405; Division of Gerontology, Baltimore Veterans Affairs Medical Center, Baltimore 21201; and Division of Geriatric Medicine and Gerontology, Department of Medicine, Johns Hopkins University, Baltimore, Maryland 21224
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ABSTRACT |
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Low rates of daily energy expenditure, increased energy intake, or a combination of both contribute to obesity in African-Americans. We examined whether African-Americans have lower rates of free-living daily energy expenditure than Caucasians. One hundred sixty-four (>55 yr) volunteers (37 African-American women, 52 Caucasian women, 28 African-American men, and 47 Caucasian men) were characterized for total daily energy expenditure, resting metabolic rate, and physical activity energy expenditure from the doubly labeled water method and indirect calorimetry. Absolute total daily energy expenditure was lower in women than men but was not different between African-Americans and Caucasians. However, we found race and gender differences in total daily energy expenditure after controlling for differences in fat-free mass. Total daily energy expenditure was 10% lower (P < 0.01) in African-Americans compared with Caucasians due to a 5% lower resting metabolic rate (P < 0.01) and 19% lower physical activity energy expenditure (P = 0.08). Moreover, total daily energy expenditure was 16% lower (P < 0.01) in women compared with men due to a 6% lower resting metabolic rate (P = 0.09) and a 37% lower physical activity energy expenditure (P = 0.06). Low rates of energy expenditure may be a predisposing factor for obesity, particularly in African-American women.
metabolic rate; aging; obesity; activity
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INTRODUCTION |
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OBESITY OCCURS when energy intake chronically exceeds energy expenditure. The prevalence of obesity is increasing rapidly in the United States and other industrialized nations (12). The prevalence of obesity in African-American women is estimated to be twice that of Caucasian American women. Obesity contributes to many adverse health outcomes, including type II diabetes, as well as an increase in cardiovascular and all-cause mortality. The associated rate of hypertension and mortality due to heart disease, stroke, and diabetes in African-American women is 1.5 to 2.0 times that of Caucasian women (6, 13, 19). Moreover, the prevalence of obesity-related risk factors in African-Americans increases with advancing age due to unfavorable changes in body composition and a sedentary lifestyle (16-18, 20, 25, 26, 32). The annual economic costs of obesity in the United States from medical expenses and loss of income are reported to exceed sixty-eight billion dollars (35).
The etiology of race and gender differences in obesity is complex, with behavioral, environmental, and genetic factors contributing to the development of obesity. An individual may become obese with a "normal" energy expenditure if intake is excessive, or with a normal energy intake if energy expenditure is inappropriately low. A low resting metabolic rate (RMR) is one factor that predisposes individuals to gain body weight (29). Some studies show a lower RMR in younger African-Americans compared with Caucasians (2, 9, 15) and in women compared with men (1, 7, 23, 24). Collectively, these findings suggest that African-American women may be particularly susceptible to obesity because of a low resting energy expenditure.
Body weight, however, is regulated by the dynamic balance between energy intake and daily energy expenditure in free-living individuals. No studies, to our knowledge, have examined the effects of race on daily energy expenditure in free-living African-Americans. We hypothesized that African-Americans would have a lower total daily energy expenditure than Caucasians. To address this issue, we used the doubly labeled water method, indirect calorimetry, and dual-energy X-ray absorptiometry (DEXA) to examine the effects of race on total daily energy expenditure and its components (RMR and physical activity energy expenditure) in a relatively large sample of African-American and Caucasian men and women.
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METHODS |
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Study populations. Apparently healthy volunteers >55 yr of age were recruited by newspaper advertisements. All participants were in generally good health defined by the following criteria: 1) no clinical evidence of coronary heart disease as assessed by normal resting and exercise stress test electrocardiogram, 2) a resting blood pressure <140/90 mmHg, 3) absence of any prescription or over-the-counter medication that could affect cardiovascular or metabolic function, 4) weight stability (±2 kg by medical history within the past 6 mo), and 5) no medical history of diabetes. No women were taking hormone replacement therapy, and all women were postmenopausal. All volunteers were nonsmokers and were not participating in a regular exercise program. The extent of racial admixture was assessed by a personal interview. African-American individuals were eligible if they identified that their parents and grandparents were of African ancestry. The procedures used were approved by the Institutional Review Boards of the University of Vermont and the University of Maryland at Baltimore. Written informed consent was obtained from each subject before the start of the investigation.
Timing of tests.
All tests were performed in the morning at 0700, at which time the
following measurements were made: 1)
RMR, 2) body composition by DEXA,
3) peak aerobic capacity
(O2 peak), and
4) leisure time physical activity.
Subjects were provided instructions on recording dietary intake.
Height was measured to the nearest 0.5 cm by using a stationary
inflexible measuring tape and head board, and body mass was measured to
the nearest 0.5 kg using a calibrated scale. Body composition, fat and
lean tissue weights (g), total body fat, and fat-free mass were
determined from a total body scan using a Lunar DPX-L densitometer
(Lunar Version 1.3z DPX-L extended analysis program for body
composition).
Measurement of total daily energy expenditure. Total daily energy expenditure was measured over 10 days under free-living conditions with the doubly labeled water technique. Briefly, a baseline urine sample was collected the evening before the testing for the determination of background isotope enrichments, and subjects were administered a mixed dose of doubly labeled water containing ~0.092 g H218O and 0.078 g 2H2O/kg body weight. A total of four timed urine samples were collected the morning after the dosing (n = 2) and ~10 days later on a return visit to the facility (n = 2). The first void of the morning was discarded to avoid isotopic concentration of the urine that takes place during the night after the dosing. At the time of dosing, a weighed 1:400 dilution of the dose and sample of the water used for dilution were prepared and analyzed for each subject. Samples were analyzed in triplicate for 2H2O and H218O using the off-line zinc reduction method and a modification of the CO2 equilibration technique. In the modified CO2 equilibration technique 1.0-ml aliquots were equilibrated with 0.5 ml of 99.9% pure CO2 gas. The samples were allowed to equilibrate overnight at room temperature by shaking. The CO2 gas was analyzed by injecting the gas into a packed gas chromatography column before introduction to the isotope ratio mass spectrometer. Isotope turnover rates and zero-time enrichments were calculated as previously described (10) and converted to energy expenditure (34).
RMR. RMR was established for each subject by indirect calorimetry for 45 min using a Deltatrac metabolic cart (Sensormedics, Yorba Linda, CA) (21). Subjects were asked to refrain from all vigorous activity before these measurements. Subjects were measured after an overnight fast in a darkened, temperature-controlled room. After being allowed to void, the subjects returned to their beds, were placed in a supine position, and had a clear plastic hood placed over their head. Constant O2 consumption and CO2 production were monitored as room air was drawn through the hood. Air flow rate was measured using a pneumotachograph. O2 and CO2 gas volumes were converted to energy equivalents (kcal/day) using the equation of Weir (34). Test-test conditions of six older female and six older male volunteers yielded an intraclass correlation of 0.94 and a coefficient of variation of 4.1%.
Physical activity energy expenditure.
Physical activity energy expenditure was estimated from measurements of
daily energy expenditure and RMR and by an estimation of the thermic
response to feeding (10% of daily energy expenditure) (22). Thus
physical activity energy expenditure was derived as follows: physical
activity energy expenditure (kcal/day) = (0.9 × daily energy
expenditure) RMR.
Measurement of
O2 peak and leisure
time physical activity.
O2 peak was
measured by a progressive and continuous treadmill test to exhaustion,
as previously described (21, 32). Volunteers self-selected their
running speed, and the incline was increased by 2.5 degrees every 2 min
from an initial level of 0 degrees until volitional fatigue. A
structured interview evaluating the energy cost of leisure time
activities (LTA) during the previous 12-mo period was carried out using
the Minnesota Leisure Time Physical Activity Questionnaire (8,
31).
Energy intake and socioeconomic status. Self-reported energy intake was obtained from a 3-day, self-recorded food diary, which included two weekdays and one weekend day. Subjects were strongly encouraged not to change their normal dietary habits and were instructed by research assistants to measure food intake. Dietary scales, measuring cups, and spoons were provided to each subject to help quantify the portion consumed. Food records were checked for completeness by a research assistant at the time of their return by the subjects. The records were analyzed for energy content by using the Nutritionist 4.1 software package for Windows (First DataBank Computing, San Bruno, CA). Socioeconomic status was defined by determination of education level, income status, and living status. Volunteers were classified on percentages of having completed high school, having a household income exceeding $20,000 per year, and whether they lived alone or with a spouse or partner.
Statistics.
Values are presented as mean values ± SD. Race and gender effects
were assessed by a 2 × 2 factorial analysis of variance and
covariance. In analysis of covariance, we used fat-free mass as the
covariate because stepwise regression analysis identified this variable
as the best predictor of variation
(r2) in total
daily energy expenditure, physical activity energy expenditure, and RMR
(r2 values 50%). Moreover, analysis of covariance allows for the removal of the
linear effect of fat-free mass on the dependent variables. This
approach takes into account that the correlation between the dependent
variables and fat-free mass is not r = 1.0 and that the mathematical relationship between the two variables has an intercept different from zero (23, 33). We also examined race
and gender differences in energy expenditure after controlling for the
effects of fat mass, educational level, annual income, and living
status. Control for these variables did not change the results. Thus
adjusted values for total daily energy expenditure and its components
by use of fat-free mass as the single covariate are presented in the
text.
2 Statistics were used to
analyze categorical variables. An
-level less than
P < 0.05 was considered
statistically significant.
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RESULTS |
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Table 1 shows physical characteristics for African-Americans and Caucasians. The results are presented by race and gender, which is consistent with grouping factors (i.e., main effects) derived from the analysis of variance.
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Race effects.
African-Americans were slightly younger
(P < 0.001) than Caucasians. No race
effect was found for standing height. Body mass index was higher
(P < 0.001) in African-Americans
compared with Caucasians and highest in African-American women
(P < 0.01) (i.e., interaction term)
compared with the other groups. African-Americans weighed more
(P < 0.001) than Caucasians because
of a greater fat mass and fat-free mass (both at
P < 0.001). Fat mass was highest in
African-American women compared with the other groups
(P < 0.01; interaction term).
African-Americans had a greater waist circumference
(P < 0.001) compared with
Caucasians. LTA was lower in African-Americans compared with Caucasians
(P < 0.05). No difference in
O2 peak was found
between the races. African-Americans reported consuming more calories
than Caucasians (P < 0.05). Fewer
African-Americans had an education above the high school level compared
with Caucasians (P < 0.001). Fewer
African-Americans earned over $20,000 per year than Caucasians
(P < 0.001), and a greater
percentage lived alone compared with Caucasians
(P < 0.05).
Gender effects.
As expected, men were taller (P < 0.001) than women and weighed more (P < 0.01). Men had lower amounts of fat mass
(P < 0.001) but a greater quantity
of fat-free mass than women (P < 0.001). Men had a larger waist circumference compared with women
(P < 0.001). Reported LTA was lower
in women (P < 0.01) compared with men. O2 peak was lower
(P < 0.001) in women compared with
men. Men reported consuming more calories than women did
(P < 0.001). No gender effects were
noted for education, household income, and living status.
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DISCUSSION |
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We examined differences in total daily energy expenditure, RMR, and physical activity energy expenditure in a relatively large sample of free-living African-Americans and Caucasians that were carefully characterized for total daily energy expenditure, body composition, and socioeconomic status. This is the first study to report that free-living African-Americans have lower levels of total daily energy expenditure than Caucasians that are independent of differences in fat-free mass. African-Americans, particularly African-American women, may have a greater predisposition for obesity because of their low energy expenditure.
Total daily energy expenditure.
Our results show a lower total daily energy expenditure in
African-Americans (225 kcal/day) compared with Caucasians after controlling for differences in fat-free mass. The low level of total
daily energy expenditure was unmasked in African-Americans after
controlling for fat-free mass. Variation in energy expenditure among
individuals is primarily determined by differences in fat-free mass
(28), and African-Americans had more fat-free mass than Caucasians
(Table 1). Thus adjustment for fat-free mass permits a meaningful
comparison of the rate of energy expenditure independent of the
confounding influence of differences in body size and composition. The
most striking finding was the diminished levels of total daily energy
expenditure in African-American women. They showed a lower total daily
energy expenditure when compared with Caucasian women (12%),
African-American men (18%), and Caucasian men (24%) (Fig. 1). Thus
our results suggest that African-Americans, particularly African-American women, require fewer calories for their body size to
maintain body weight. Low rates of total daily energy expenditure may
be an important metabolic factor predisposing African-American women to
obesity and its metabolic consequences.
Physical activity energy expenditure and RMR.
Differences in total daily energy expenditure are a function
of its components (RMR and the energy expenditure of physical activity). Accordingly, we found lower physical activity energy expenditure (122 kcal/day) and a lower RMR (
81 kcal/day) in African-Americans compared with Caucasians, with differences being most
pronounced in African-American women.
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ACKNOWLEDGEMENTS |
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We thank the volunteers who participated in this study.
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FOOTNOTES |
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This study was supported by grants from the National Institutes of Health (AG-07857, AG-05564, and DK-52752 to E. T. Poehlman), the General Clinical Research Center at the University of Vermont (RR-109), The American Federation of Aging Research, and the Geriatric Research Education and Clinical Center at the University of Maryland.
Address for reprint requests: E. T. Poehlman, Dept. of Medicine, Univ. of Vermont, Burlington, VT 05405.
Received 16 May 1997; accepted in final form 24 September 1997.
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