Determinantes del gasto energético en reposo en niños y adolescentes obesos y no obesos

ORIGINAL ARTICLES

Determinants of resting energy expenditure in young black girls and young white girls John A. Morrison, PhD, Mary Pat Alfaro, RD, Philip Khoury, MS, Brian B. Thornton, MS, a n d Stephen R. Daniels, MD, PhD From the Divisions of Cardiology and Gastroenterology and Nutrition, the Department of Pediatrics, University of Cincinnati College of Medicine and Children's Hospital Medical Center, Cincinnati, Ohio

Objective: To identify determinants of resting energy expenditure (REE) in black girls and white girls and to evaluate racial differences in REE. Study design: Cross-sectional study of 98 girls (47 black and 51 white girls), ages 6 to 16 years. Methods: Determinations of lean body mass, fat mass, and bone mass were made by dual-energy x-ray absorptiometry. Measurements of REE were made with the DeltaTrac metabolic monitor. Subjects fasted at least 3 hours before testing, had rested 30 minutes before the test, and had not engaged in strenuous activity for the previous 12 hours. Pubertal maturation was assessed with a three-stage scoring method: (I) prepubertal, (2) pubertal, but premenarcheal, and (3) postmenarcheal. Results: There were no significant differences in height, weight, lean body mass, or fat mass between the black and white subjects. Racial differences in total REE were also not significant, but REEstandardized by weight was significantly greater in white girls (40.3 kcal/day) compared with black girls (35.5 kcal/day) (p = 0.00 I). Resting energy expenditure was positively and significantly correlated with all measures of body composition. Multiple regression analysis identified lean body mass, sexual maturation, and race as significant main effects. After controlling for lean body mass and maturation, black girls had significantly lower REE.The racematuration interaction was of borderline significance (p= 0.09); prepubertal black girls had significantly lower REE(I 156 kcal/day) than prepubertal white girls (1399 kcal/day), but racial differences in stages 2 and 3 were not statistically significant. Conclusion: Lean body mass, maturation, and race are significant determinants of REE. Resting energy expenditure is significantly lower in black than white girls n the prepubertal stage. The cause of thls racial difference in REE is not known; it is not explained by differences in anthropometric variables. Racial differences in REE could explain in part the earlier onset of puberty in black girls compared with white girls and could be a factor in the difference in obesity in black and white women. (J Pediatr 1996; 129:637-42)

Supported in part by grant Nos. HL48941 and HL34698 from the National Heart, Lung, and Blood Institute, the National Institutes of Health and U.S. Public Health Service grant No. RR-08084 from the National Center for Research Resources, General Clinical Research Center Program, National Institutes of Health. Submitted for publication Jan. 24, 1996; accepted May 15, 1996.

Reprint requests: John A. Morrison, PhD, Division of Cardiology. Children's Hospital Medical Center, 3333 Bttrnet Ave., Cincinnati, OH 45229. Copyright © 1996 by Mosby-Year Book, Inc. 0022-3476/96/$5.00 + 0 9/21/75370

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Obesity is related to numerous major health problems in the United States, including hypertension, non-insulin-dependent diabetes mellitus, and cardiovascular disease.1 A better understanding of obesity, its origins, prevention, and treatment is needed, but obesity is a multifactorial problem that has eluded explication. Adult black women have twice the prevalence of obesity than white women 1 and significantly more non-insulin-dependent diabetes mellitus, hypertension. and cardiovascular disease. 1 Before the onset of pubertal maturation, racial differences in obesity are not significant. 2 However, black girls begin sexual maturation at a younger age than white girls, 3, 4 and differences in obesity appear with and continue after the onset of puberty. 5' 6 Factors linked to the development of obesity and the racial diver-

See related articles, pp. 621 and 643.

BMI Body mass index DEXA Dual-energyx-ray absorptiometry REE Restingenergy expenditure gence in obesity for girls during adolescence may include differences in basal metabolic rates and differences in caloric intake and energy expenditure of activity. Few studies have focused on the determinants of resting energy expenditure and the potential for racial differences in REE to explain racial differences in the development of obesity. The purpose of this study is to assess the determinants of REE in black and white girls, including age, body composition, and stage of pubertal maturation. Our goal was to determine whether there is a significant independent association between race and REE after controlling for other effects.

METHODS Study population. Subjects included 47 black and 51 white girls, ages 6 to 16 years, recruited from ongoing studies at the Children's Hospital Medical Center. Signed informed consent was obtained from parents or guardians before participation in the studies. The protocol Was approved by the Children's Hospital Institutional Review Board for Research on Human Subjects. Procedures. Measurements of body composition, including height, weight, and determinations of lean body mass, fat mass, and bone mass by dual-energy x-ray absorptiometry were obtained as previously described] Body mass index was calculated as weight in kilograms divided by height in centimeters squared. Body composition analysis by DEXA was determined with a Hologic model R-100 quantitative digital radiography unit and total body scan software version 5.47 (Hologic, Inc., Waltham, Mass.). This method of body composition analysis is rapid, safe, highly accurate, and not

The Journal of Pediatrics November 1996

dependent on race or sex-related information for its determination. Dual-energy x-ray absorptiometry uses a multicomponent model and provides measurements of bone mass, fat-free mass, and fat mass. In this model, lean body mass is equal to fat-free mass minus bone mass. Measurements of fat-free mass by DEXA have been shown to be comparable to those made by hydrodensitometry, s Further validation is provided by Ellis et al., 9 who compared measures of bone mineral content, nonbone lean tissue, and fat from DEXA and total carcass chemical analysis in 16 pigs with a weight range of 5 to 35 kg to determine the accuracy of DEXA measures in children. 9 The measures of fat-free mass and bone had R 2 values of 0.998 and 0.987, respectively. Reproducibility improved with size, being 1.0% and 0.6% at and above 15.7 kg, which was the lower limit of weights for the subjects in this study population. Resting energy expenditure was measured by indirect calorimetry with the DeltaTrac metabolic monitor (Sensormedics, Yorba Linda, Calif.) according to a protocol similar to that of Firouzbakhsh et al. 1° Subjects were seen in the morning at least 3 hours after eating a meal, and they had rested in the supine position quietly for at least 30 minutes before the test. Subjects had not participated in strenuous activity for the previous 12 hours. The measurement phase took a minimum of 15 minutes with at least 5 consecutive minutes of stable readings that varied less than 5%. The stable readings were averaged and the mean value was used as the REE for that subject. 1° This indirect calorimetric method has been shown to be very reliable with intraclass correlation coefficients of approximately 0.90. l°, 11 Pubertal maturation was determined with the GarnFalkner staging method, as previously described. 12 This method was developed as a modification of the Tanner staging for girls by Garn and Falkner to focus on areolar instead of breast development, making the staging more suitable for the study of obesity. Subject plates were made for both black and white girls to minimize differences in staging criteria by race. A comparison of this method with traditional Tanner staging showed them to be comparable. 13 For our analysis, a three-stage maturational staging method was used: stage 1 was prepubertal, stage 2 was pubertal but premenarcheal, and stage 3 was postmenarcheal. Statistical methods. All analyses were performed with SAS version 6.04 software. 14 Variables are presented as mean -+ standard deviation for continuous variables or as counts for categoric variables, Student t tests were used to explore racial differences in the following variables: lean body mass (in kilograms), fat-free mass (in kilograms), height (in centimeters), weight (in kilograms), BMI (in kilograms per meter squared), bone density (in gramS per centimeter squared), fat mass (in kilograms), bone mass (in kilograms), percent body fat (percent), sum of skin folds

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Table I. Age and body composition variables by race Black girls (n = 47)

White girls (n = 5 I)

Variable

Mean

SD

Mean

SD

p

Age (yr) Height (cm) Weight (kg) BMI (kg/m2) Sum of skin folds (mm) Measurements by DEXA Fat-free mass (kg) Bone density (gm/cm2) Bone mass (kg) Lean body mass (kg) Fat mass (kg) Percent body fat

11.4 149.8 52.0 22.5 53.3

2.4 12.7 20.6 6.5 33.7

11.9 150.2 47.2 20.3 47.0

2.4 13.5 16.3 4.5 25.1

NS NS NS NS (p = 0.06) NS

36.0 0.9 1.7 34.4 15.5 27.1

10.6 0.1 0.7 10.0 11.8 9.7

34.4 0.9 1.5 32.9 12.3 25.l

10.8 0.1 0.6 10.2 6.7 6.3

NS NS NS NS NS NS

DEXA, Dual-energyx-ray absorptiometry;NS, not significant.

Table III. Correlations of age and anthropometric

Table II. Maturation stage by race Maturation stage

Black girls

White girls

Prepuberty Puberty but premenarcheal Poslanenarcheal TOTAL

9 19 19 47

19 14 18 51

Chi-square value= 4.2. p >0.10. (triceps + subscapular + suprailiac) (in millimeters), and age (in years). A chi-square test was performed to test for i n d e pendence between race and maturation stage. Bivariate associations between REE and the body composition were evaluated with correlation analysis. The determinants of REE were evaluated by multiple regression with the stepwise procedure to identify significant main effects. It was specified that the regression coefficients for the independent variables all must be significantly different from zero to remain in the model. This analysis allows the assessment of the effect of race on REE, independent of body composition. A general linear models procedure was then used to test the significance of potential interaction terms. An alpha level of 0.05 was used to determine significance; a value of 0.05 to 0.10 was indicative of borderline statistical significance. RESULTS Descriptive statistics and results of the t tests for racial group differences in age and body composition variables are presented in Table I. Differences in anthropometric variables between the study cohorts were observed, including weight, BMI, fat-free mass, and fat mass, but the differences did not reach statistical significance. There were no differences in maturation staging by race (Table 11). There was also no significant difference in total REE between the two study cohorts (1284 kcal/day [white girls] vs 1207 kcal/day [black

variables with REE Anthropometric variable

Correlation with REE*

Age (yr) Height (cm) Weight (kg) BMI (kg/m2) Fat-free mass (kg) Lean body mass (kg) Fat mass (kg) Bone mass (kg) Bone density (gm/cm2) Percent body fat (%)

0.61 0.73 0.76 0.64 0.81 0.82 0.58 0.74 0.68 0.36

*Allp