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Original Research Communication |
1 From the Division of Pediatric Gastroenterology and Nutrition, New England Medical Center, Boston (LGB and AM); the Clinical Research Center, Massachusetts Institute of Technology, Cambridge, MA (LGB, AM, and JLS); the Department of Family Medicine and Community Health, Tufts University School of Medicine, Boston (AM); the School of Nutrition Science and Policy, Tufts University, Medford, MA (JLS); and the Division of Physical Activity and Nutrition, Centers for Disease Control and Prevention, Atlanta (WHD).
2 Supported by NIH grants MOI-RR-00088, DK-HD50537, and 5P30 DK46200.
3 Address reprint requests to LG Bandini, E18Room 447, Clinical Research Center, 77 Massachusetts Avenue, Cambridge, MA 02139. E-mail: lgbandin{at}mit.edu.
| ABSTRACT |
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Objective: The objective was to determine whether differences in energy expenditure among premenarcheal girls are related to the pubertal stage and the race-ethnicity of the girls or to the weight status of their parents.
Design: We measured the body composition and the energy expenditure of 196 nonobese girls enrolled in a longitudinal study. Total body water was measured by the isotopic dilution of 18O water. We measured resting metabolic rate with the use of indirect calorimetry and daily energy expenditure by the doubly labeled water method. We used established criteria to determine sexual maturation. Parental weight status was based on body mass index.
Results: Resting metabolic rate was higher among girls with
1 overweight parent than among girls with 2 normal-weight parents. Total energy expenditure was also higher among girls with
1 overweight parent, but these results were of borderline significance. We found no effect of pubertal stage on resting metabolic rate. Nonresting energy expenditure was significantly lower among pubertal girls than among prepubertal girls. After adjustments for age and body composition, we noted that resting metabolic rate, nonresting energy expenditure, and total energy expenditure were all significantly lower among black girls than among white girls.
Conclusions: Differences in resting metabolic rate and total energy expenditure among premenarcheal girls were associated with parental weight status and the girls race-ethnicity, whereas differences in nonresting energy expenditure were associated with pubertal stage and race-ethnicity. Whether the observed differences in energy expenditure persist after puberty and predict weight gain during puberty awaits the results of longitudinal analyses.
| INTRODUCTION |
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Genetic and lifestyle factors may influence the rate of and differences in growth and development from preadolescence through adolescence. Several studies have examined the effects of parental obesity on energy expenditure among children, but the findings have been equivocal (46). The data from which to assess the effect of pubertal stage on energy expenditure are limited (79), and most of the reported studies that examined differences in energy expenditure and race-ethnicity have examined only resting metabolic rate (RMR) (5, 7, 913). No prior studies comprehensively examined the effects of maturation, race-ethnicity, and parental weight status on all the major components of energy expenditure.
In the 1980s, many cross-sectional studies were conducted to determine whether obesity was associated with a reduction in total energy expenditure (TEE) or in two of its components, RMR and the thermic effect of food. In several studies of nonobese and obese children and adolescents (8, 1416), obese children were not found to have lower RMR or TEE. Similar findings have been reported for adults (17, 18). Although no reductions in energy expenditure were reported in the obese state, a reduction in energy expenditure in the preobese state could be a risk factor for the subsequent development of obesity (19).
Other factors contribute to the variability in energy expenditure. Fat-free mass (FFM) is the major determinant of RMR (14, 16, 17, 2022), and fat mass appears to exert an independent effect (8, 23). Age and sex also contribute to variability in RMR (21, 24, 25). In one study, familial components of energy expenditure accounted for 11% of the variability in RMR in Native Americans from the southwestern United States (21).
To determine whether low energy expenditure among premenarcheal girls is a risk factor for greater body weight and fat gain during puberty, we initiated in 1990 a longitudinal study of the relation of energy expenditure to body composition, parental weight status, stage of sexual maturation, race-ethnicity, and premenarcheal age. After accounting for differences in age and body composition in this cross-sectional analysis, we determined whether maturational stage, race-ethnicity, or parental weight status had further effects on TEE, RMR, or nonresting energy expenditure (NREE).
| SUBJECTS AND METHODS |
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Study design
The girls were admitted to the Clinical Research Center (CRC) at MIT for an overnight visit. At the time of their initial visit, a physician obtained a medical history and examined each girl to ensure that she was in good health. On the evening of admission, study participants consumed no food or beverages after 1800. At 2000, the staff obtained a baseline urine sample, and each participant was given 0.25 g 18O water and 0.10.12 g 2H2 O/kg estimated total body water (TBW). After administering the isotopes, the study staff collected all urine voided until 0600 the next morning to determine the loss of isotope in the urine. The second void of the morning was collected at approximately 0800 to measure 18O and 2H enrichment above baseline values. This same sample was used to determine TBW and the initial time point of the energy-expenditure period. Participants were asked to collect a timed urine sample at home on days 1, 2, 4, 7, and 10. Participants returned to the CRC 2 wk after admission. At that time, the staff collected the second void of the day to end the energy-expenditure period.
Resting metabolic rate
We used an indirect calorimeter with customized software and fitted with a ventilated hood to measure RMR (14, 27) on 2 occasions: the morning after the participants were admitted to the Clinical Research Center and 2 wk later when they returned to the center at the end of the energy-expenditure period. We measured RMR for 30 min after an overnight fast; this measurement was immediately preceded by a 30-min rest period. During the measurement, the participants were allowed to read to minimize fidgeting. Their book was placed on top of the hood. A research assistant turned the pages for the participant as she cued with her eyes that she needed to have the pages turned. In a previous study (28), we found no differences in energy expenditure among subjects who were sitting, reading, or watching television. The intraclass correlation coefficient for RMR in a group of adolescents studied previously with this system was 0.98 (14). In the current cohort, the correlation coefficient for RMR measured in the same subject on 2 separate days was 0.96, which indicated that the measurements were highly reproducible.
The sample sizes for RMR, NREE, and TEE varied slightly because of missing or invalid data. RMR data from 5 girls were excluded because of excessive movement. If a girls second RMR determination was
10% different from the first, she was asked to return for a third measurement, and we used the mean of the 2 closest results in the analysis. Such excessive variation occurred in 23 girls. For 2 participants who returned for a third measurement and for 1 participant who could not return, the measurements from these visits agreed to within 12%. We included these data in the analyses. We excluded the RMR data for one participant whose 2 measurements differed by 18%. We also excluded energy expenditure data for 12 participants who became ill during the 2-wk study period or who were on vacation, which meant that the energy-expenditure period did not represent their usual activity pattern.
Other variables
Either a study physician or a female co-investigator used Tanners criteria (29) of breast development to determine sexual maturation. We classified the girls as either prepubertal (Tanner stage 1) or pubertal (Tanner stage 2 or 3).
Early in the study, we measured the height and weight of each participants biological parents (who were dressed but were not wearing shoes). We obtained the height and weight of 180 mothers and 139 fathers. Parental overweight was defined as a body mass index (BMI; in kg/m2)
25. Participants were classified as having 2 normal-weight biological parents or
1 overweight biological parent. A total of 42 participants could not be classified by parental weight status. These included participants who were adopted or for whom there were data from one lean biological parent only. We also included 20 sister pairs and 2 sets of 3 sisters. For the analyses of parental weight status, we randomly selected one sister to be included in the analysis. The participants were asked to indicate their race-ethnicity (white, black, Hispanic, Asian, or other) on the activity questionnaires.
Mass spectroscopy analysis
Isotopic analyses for assessment of body composition and energy expenditure were conducted at the US Department of Agriculture Human Nutrition Research Center at Tufts University (Boston) on 2 isotope ratio mass spectrometers (Hydra Gas; PDZ Europa Ltd, Northwich, United Kingdom, and Sira 10; Micromass, Altrincham, United Kingdom). The laboratory modified a technique of Prosser et al (30, 31) to measure isotopic enrichments of deuterium on the Europa instrument, and we used that instrument for the oxygen analyses in all but 4 participants. Approximately one-third of the hydrogen samples were analyzed on the SIRA 10 instrument, and the other two-thirds were analyzed on the Europa instrument. In a subset of 14 subjects, we compared the mean hydrogen elimination rate (kh) for the 2-wk period on the 2 machines and found no significant differences (0.0833 on the SIRA and 0.0835 on the Europa, P = 0.42). We based the criteria for acceptable values on the SE of replicate measures: 0.35 for oxygen and 1.5 for deuterium.
Body composition
We used TBW to estimate body composition. Oxygen dilution space was calculated according to the method of Halliday and Miller (32) with the assumption that the 18O dilution space was 1% higher than TBW and that the deuterium dilution space was 3% higher than the 18O dilution space (33). FFM was further assumed to be 73% water (34). We calculated percentage body fat by subtracting FFM from body weight and then dividing the difference (fat) by weight (x 100%).
Energy expenditure
We used a modification of the equation of Lifson and McClintock (35) to calculate the mean daily rate of carbon dioxide production (rCO2; in mol CO2/d):
![]() | (1) |
![]() | (2) |
Initially, we designed the study to use the multipoint method of determining the rates of elimination of the 2 isotopes, but the participants compliance in collecting urine samples at home was inconsistent. Thus, we used the 2-point method to calculate energy expenditure. We used for our calculations only the timed urine samples that were obtained at the CRC.
We used Weirs equation (37) to calculate TEE. We determined rCO2 by the doubly labeled water method and calculated oxygen consumption from rCO2 and the food quotient. Using the 7-d food records that were kept by the participants during the second week of the energy-expenditure study, we calculated the proportions of dietary fat, carbohydrate, and protein (38) to determine the food quotient. For girls who did not keep a food diary (n = 9), the mean food quotient for the cohort was imputed. We calculated NREE by subtracting RMR from TEE.
The theoretical precision of the doubly labeled water method for measuring rCO2 is 3% (39). Validation studies conducted in 4 laboratories using respiratory-exchange measurements found the precision of the doubly labeled water method to be 5% (40).
Statistical analysis
We used SAS software, version 8 (SAS Institute, Inc, Cary, NC) to conduct all analyses. We examined descriptive statistics and graphic displays to identify outliers and to ensure that assumptions of normality for linear regression modeling were met. All the variables except NREE were analyzed after the data were log transformed to achieve normality. Pearsons correlation coefficients were used to identify the body-composition variables that should be evaluated in subsequent multivariate linear models. We conducted chi-square tests to test for homogeneity of proportions for categorical variables (parental weight status, pubertal status, and race-ethnicity). Our strategy was first to determine which body-composition variables (or weight) were significantly related to the energy-expenditure variables. After adjustment for body composition or weight, we estimated separate general linear models to assess the influence of pubertal stage, race-ethnicity, and parental weight status on RMR, NREE, and TEE. Age was retained in all models, regardless of its statistical significance, to provide comparability with other studies. We estimated least-squares means from these models separately for the key categorical variables: parental overweight (yes or no), prepubertal or pubertal status, and white, black, or other race-ethnicity. We set the
level for all statistical tests at 0.05. We conducted stratified analyses when 2-way interactions between the key categorical variables were identified.
| RESULTS |
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1 overweight parent. On the basis of the categories for race-ethnicity, 141 participants classified themselves as white, 28 as black, 13 as Hispanic, and 10 as Asian; 5 participants categorized themselves as "other." As we expected, age, weight, and height were all significantly higher among the pubertal participants than among the prepubertal participants (P < 0.001). FFM and fat mass also were significantly greater among the girls in the pubertal group, even after adjustment for height. Mean percentage body fat was 22.8 ± 4.7% for prepubertal girls and 24.1 ± 6.0% for pubertal girls. RMR, NREE, and TEE were all significantly greater among the girls in the pubertal group than among those in the pubertal group (Table 1
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We observed no significant differences between the racial-ethnic distribution of the girls who had
1 overweight parent and the girls with 2 normal-weight parents or between those of the girls in the prepubertal and the pubertal groups. We did note that girls in the pubertal groups were more likely than were girls in the prepubertal group to have
1 overweight parent (chi-square test, P < 0.04).
Multivariate analysis
The final models that describe the relations of RMR, NREE, TEE, and body composition are shown in Table 2
. We found that FFM, fat mass, and age were all significantly related to RMR, FFM and fat mass were significantly related to NREE, and only FFM was significantly related to TEE. For both NREE and TEE, the models that included a term for FFM explained more of the variance than did the models that included weight (NREE: R2 = 0.37 compared with 0.24; TEE: R2 = 0.68 compared with 0.57). After identifying the best set of body-composition values for each energy-expenditure variable, we added maturational stage, race-ethnicity, and parental overweight separately to determine whether any had a significant effect on energy expenditure. Separate analyses were performed because of the complexity of the analysis and the relatively large sample size.
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1 overweight parent than among those with 2 normal-weight parents (Table 3
1 overweight parent, TEE was slightly higher, but the results were of only borderline significance (P < 0.08). These results did not change when we compared girls with 2 normal-weight parents and those with 2 overweight parents. Because of the significant difference in the number of prepubertal and pubertal participants with 2 normal-weight parents, we compared the RMR of the girls with 2 normal-weight parents with the RMR of those with
1 overweight parent, stratified by pubertal status. In both the prepubertal and pubertal groups, we found that RMR appeared to be higher among the girls with
1 overweight parent. The differences were significant in the pubertal girls (P < 0.05) and of borderline significance in the prepubertal girls (P < 0.08). We observed no significant differences in NREE according to parental weight status in either the prepubertal or the pubertal group. However, in the pubertal group, girls with
1 overweight parent had a significantly greater TEE than did girls with 2 normal-weight parents. In the prepubertal group, we found no significant difference between TEE among girls with 2 normal-weight parents or that among those with
1 overweight parent (Table 3
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| DISCUSSION |
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1 overweight parent are in keeping with those of Wurmser et al. Our data provide no support for the hypothesis that children with overweight parents may have a lower metabolic rate and thus are at elevated risk for developing obesity.
When we stratified our sample by pubertal status, we found higher RMR and TEE among pubertal girls with
1 overweight parent than among those with 2 normal-weight parents. The significance of these findings remains unclear. Because NREE was not higher among girls with overweight parents, the increase in TEE appears to reflect the increase in RMR. Although parental obesity was not associated with NREE in the pubertal period, an environmental influence of parental overweight on NREE may become evident later in adolescence.
The lack of differences in RMR between prepubertal and pubertal girls that we observed is consistent with the other cross-sectional analyses of pubertal status and energy expenditure (8, 9). In contrast with our findings, Sun et al (7) reported that RMR decreased among children from Tanner stage 1 to Tanner stage 3. The design of their study, although it is longitudinal, includes several limitations. Distinguishing prepubertal girls from pubertal girls is not difficult, but categorizing girls between Tanner stages 2 and 5 presents greater problems. In our analysis, we categorized girls as prepubertal (Tanner 1) or pubertal but premenarcheal (Tanner 2 or 3) to minimize misclassification. Several other limitations apply to the study by Sun et al: menarche was not accounted for, boys and girls were analyzed together, and the ages of the prepubertal children in the study ranged from 5 to 11 y.
Morrison et al (9) reported a lower RMR among postmenarcheal girls than among premenarcheal girls. This finding suggests that the decline in RMR with age may be related to the timing of menarche. The lower NREE that we observed among pubertal girls suggests that the energy spent on activity may decline as the girls enter puberty.
We found significantly lower RMR, NREE, and TEE among black girls than among white girls; NREE was significantly lower among black girls in the total sample, but not when stratified by pubertal status. This finding may reflect inadequate power. Our findings are consistent with other studies that found lower RMRs among black children than among white prepubertal children (5, 9, 10, 13), adolescents (12), and adults (44, 45). Two studies examined the effect of race-ethnicity on NREE and TEE; one study (5) found a lower TEE among black girls than among white girls, but the investigators saw no differences in the energy expended on activity. The other study reported no differences in any component of energy expenditure among black girls or white girls (11).
Morrison et al (9) found differences in RMR between prepubertal white girls and black girls but no such differences between pubertal white girls and black girls. In our sample, both RMR and TEE differed significantly between black girls and white girls of both maturational stages. Differences between the findings of Morrison et al and ours may be due in part to their study cohort, many members of which were overweight, whereas none of our subjects were overweight.
Although our study represents one of the largest single studies with comprehensive energy-expenditure measurements, several limitations of our study are noteworthy. We did not have sufficient numbers of Hispanic and Asian participants for the analyses to be stratified by race-ethnicity. Furthermore, we did not have adequate power to stratify our analysis of parental obesity by both pubertal status and race-ethnicity. However, when we limited our analysis to white girls, our findings regarding the effect of parental obesity on energy expenditure by pubertal status were essentially unchanged. Finally, our determination of FFM was based on the assumption of a hydration factor of 0.73 for FFM. Recent studies using a multicompartmental model in which the components of FFM, protein, water, and minerals were measured independently suggest that the hydration factor may vary among persons by age, sex, maturation, and race-ethnicity (46, 47). However, it is not clear whether the between-group variability exceeds the within-group variation.
Longitudinal studies are needed to determine whether reductions in energy expenditure during the premenarcheal period are associated with increased weight gain throughout adolescence. Such studies will also help to clarify whether reductions in energy expenditure in preobese black children contribute to the higher prevalence of obesity seen among black girls than among white girls (2, 48) and whether decreases in NREE and TEE with maturation represent risk factors for the development of obesity during adolescence. Our findings suggest that these relations are complex and that they vary among the energy-expenditure components. Thus, parental obesity, maturational stage, and race-ethnicity should be carefully considered in an analysis designed to examine energy expenditure and its changes over time.
| ACKNOWLEDGMENTS |
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| REFERENCES |
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