Maximal aerobic capacity in African-American and Caucasian prepubertal children

Christina A. Trowbridge, Barbara A. Gower, Tim R. Nagy, Gary R. Hunter, Margarita S. Treuth, and Michael I. Goran

Division of Physiology and Metabolism, Department of Nutrition Sciences, University of Alabama at Birmingham, Birmingham, Alabama 35294-3360

    ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
References

The purpose of this study was to examine differences in resting, submaximal, and maximal (VO2 max) oxygen consumption (VO2) in African-American (n = 44) and Caucasian (n = 31) prepubertal children aged 5-10 yr. Resting VO2 was measured via indirect calorimetry in the fasted state. Submaximal VO2 and VO2 max were determined during an all out, progressive treadmill exercise test appropriate for children. Dual-energy X-ray absorptiometry was used to determine total fat mass (FM), soft lean tissue mass (LTM), and leg soft LTM. Doubly labeled water was used to determine total energy expenditure (TEE) and activity energy expenditure (AEE). A significant effect of ethnicity (P < 0.01) was found for VO2 max but not resting or submaximal VO2, with African-American children having absolute VO2 max ~15% lower than Caucasian children (1.21 ± 0.032 vs. 1.43 ± 0.031 l/min, respectively). The lower VO2 max persisted in African-American children after adjustment for soft LTM (1.23 ± 0.025 vs. 1.39 ± 0.031 l/min; P < 0.01), leg soft LTM (1.20 ± 0.031 vs. 1.43 ± 0.042 l/min; P < 0.01), and soft LTM and FM (1.23 ± 0.025 vs. 1.39 ± 0.031 l/min; P < 0.01). The lower VO2 max persisted also after adjustment for TEE (1.20 ± 0.02 vs. 1.38 ± 0.0028 l/min P < 0.001) and AEE (1.20 ± 0.024 vs. 1.38 ± 0.028 l/min; P < 0.001). In conclusion, our data indicate that African-American and Caucasian children have similar rates of VO2 at rest and during submaximal exercise, but VO2 max is ~15% lower in African-American children, independent of soft LTM, FM, leg LTM, TEE, and AEE.

oxygen consumption; fitness; energy expenditure; ethnicity

    INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References

IT IS ESTIMATED that 22-27% of all children in the United States are obese (25), and in certain ethnic populations this value may be greater (30). Cornelius (10) reported that African-American children in the United States were three times as likely to be overweight, compared with Caucasian children of the same age.

Most studies that have measured maximal aerobic capacity (VO2 max) have examined only Caucasian adolescents (20, 23, 24, 29), and only a few have examined adolescents of different ethnic groups (15, 28). Pivarnik et al. (28) reported that a group of adolescent African-American females (mean age = 13.4 yr) had VO2 max values that were 14% lower than values reported for Caucasian adolescents of the same age in several other countries (2, 18, 19) and were ~14% lower (43.4 vs. 37.3 ml · kg-1 · min-1) than those of female Caucasian adolescents (mean age = 13.6 yr) in five prior US studies. Whereas some studies have found lower aerobic capacity in African-Americans (14, 27, 28), the data were not adjusted for soft lean tissue mass or leg soft lean tissue mass. It is also unclear whether the lower VO2 max that has previously been shown in African-Americans can be explained by lower resting oxygen consumption (VO2), or by the "net" VO2 at maximal effort during exercise. It is also unknown how habitual physical activity energy expenditure in African-American and Caucasian children affects aerobic capacity. By using the doubly labeled water technique, we were able to measure total energy expenditure (TEE) and calculate activity energy expenditure (AEE) to examine whether the energy cost of daily physical activity might contribute to ethnic differences in aerobic capacity.

The objective of this study was therefore to examine in African-American and Caucasian male and female children (aged 5-10 yr) resting, submaximal, and maximal VO2. By using dual-energy X-ray absorptiometry (DEXA) to assess body composition and doubly labeled water to measure free-living energy expenditure, we were able to examine whether observed differences in VO2 were explained by differences in soft lean tissue mass, leg soft lean tissue mass, total fat mass, TEE, or AEE.

    METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
References

Protocol. Children were admitted to the General Clinical Research Center (GCRC) in the late afternoon for an overnight visit. On arrival, anthropometric measurements were obtained, and dinner was served at ~1700. An evening snack was allowed as long as it was consumed before 2000. After 2000, only water and noncaloric, noncaffeinated beverages were allowed until after the morning testing was completed. The following morning, resting energy expenditure (REE) was measured for 30 min, starting between 0600 and 0730. After testing was completed, the children were fed breakfast and allowed to leave. Two weeks later, the children arrived at the Energy Metabolism Research Unit at 0700 in the fasted state. Submaximal and maximal VO2 were determined by an all out, progressive treadmill test using a protocol appropriate for children. Body composition was determined by DEXA.

Subjects. A total of 94 healthy African-American (n = 60) and Caucasian (n = 34) children 5-10 yr of age completed the protocol. Children were recruited from the Birmingham, Alabama area by use of radio advertisements, flyers, and word of mouth. Children who were taking medications known to affect body composition or physical activity were excluded from the study, as were children diagnosed with Cushing's syndrome, Down's syndrome, type I diabetes, or hypothyroidism. The study was approved by the University of Alabama at Birmingham (UAB) Institutional Review Board for human use, and informed consent was obtained from all subjects before testing.

REE. REE was measured in all subjects in the early morning in the fasted state after subjects had spent the night at the GCRC. A Deltatrac Metabolic Monitor (Sensormedics), which was calibrated before each test against standard gases, was used for each REE measurement. During testing, all subjects were instructed to lie as still as possible. An adult-size canopy hood was used to collect the expired air for 20 min after a 10-min equilibrium period, and VO2 and carbon dioxide production (VCO2) were measured continuously during this time. Energy expenditure was calculated using the equation of de Weir (11).

Exercise testing. Subjects reported to the Energy Metabolism Research Unit at 0700 in the fasted state. After becoming familiar with the testing equipment, such as the mouthpiece and headgear, the children were allowed to practice walking on the motorized treadmill until they were able to walk without holding on to the railings. Subjects followed an all out, progressive, continuous treadmill protocol appropriate for children (3). The children walked for 4 min at 0% grade and 4 km/h, after which the treadmill grade was raised to 10%. Each ensuing work level lasted 2 min, during which the grade was increased by 2.5%. The speed remained constant until a 22.5% grade was reached, at which time the speed was increased by 0.6 km/h until the subject reached exhaustion.

VO2 and VCO2 were measured continuously via open circuit spirometry and analyzed with the use of a Sensormedics metabolic cart (model no. 2900, Yorba Linda, CA). Before each test session, the gas analyzers were calibrated with certified gases of known standard concentrations. During the treadmill test, heart rate was monitored by a Polar Vantage XL heart rate monitor (model no. 61204). Submaximal VO2 as well as submaximal heart rate were measured during the first 4-min phase of the treadmill test. Three criteria were used to determine whether a successful maximal test had been performed: 1) a leveling or plateauing of VO2 (defined as an increase of VO2 <2 ml · kg-1 · min-1), 2) heart rate >195 beats/min, and 3) respiratory exchange ratio >1.0. VO2 max was defined by attainment of two of the three criteria. VO2 max was attained in 75 of the 94 children. Of the 19 children not successfully attaining VO2 max, four were Caucasian (2 males, 2 females) and 15 were African-American (7 males, 8 females). These children were excluded from the data analysis.

Measurement of body composition. DEXA was used to measure total and regional body composition (Lunar DPX-L densitometer, Lunar Radiation, Madison, WI). The total dose of radiation for a scan is less than several hours of background exposure (0.02 mrem). The following information was obtained from the DEXA scan: fat, lean, and bone mineral mass (in grams). Soft lean tissue mass is defined as fat-free mass plus essential lipids. Scans were analyzed using the pediatric medium or large mode (n = 73) (Pediatric Software, Version 1.5e) or the adult fast mode (n = 2) (DPX-L Version 1.3z), depending on the weight of the child. Limb soft lean tissue mass was used as an index of appendicular skeletal muscle mass (16).

Measurement of total and physical activity-related energy expenditure. TEE was measured over 14 days under free-living conditions with the doubly labeled water technique, using a protocol that has a theoretical precision of <5% for assessment of VCO2, as previously described (13). Briefly, four timed urine samples were collected after oral dosing with doubly labeled water, two the morning after dosing, and two in the morning 14 days later with a loading dose of 0.15 g H218O and 0.12 g of 2H2O/kg body mass. Samples were analyzed in triplicate for H218O and 2H2O by isotope ratio mass spectrometry at The Energy Metabolism Research Unit in the Department of Nutrition Sciences at the UAB. The facility at UAB consists of a Fisons Optima isotope ratio mass spectrometer, and samples are prepared and analyzed in a similar fashion to that previously described (13), except that carbon dioxide is analyzed for oxygen-18 content by continuous flow isotope ratio mass spectrometry. The intra-assay standard deviation for triplicate analysis of samples at the laboratory is ~4 per thousand  and 0.20 per thousand  for deuterium and oxygen-18, respectively. Complete doubly labeled water was obtained for 62 of the 75 children.

Physical activity-related energy expenditure was estimated from the difference between TEE and REE. A correction for the thermic effect of food was necessary, since REE was measured in the fasted state. AEE was derived from the following equation: AEE = 0.9 × TEE - REE. The aforementioned equation makes an assumption of the thermic effect of food accounting for ~10% of TEE.

Statistical analysis. All statistical analyses were performed using SAS (SAS Institute, Cary, NC; SAS for Microsoft Windows; Release 6.10). A two-way analysis of covariance (ANCOVA) design was used to test for the main effects of gender and ethnicity, as well as for the interactive effect of ethnicity by gender. Because gender did not affect the major outcome variables, all subsequent analyses were combined into two groups (African-American and Caucasian). The main outcome variables were resting VO2, submaximal VO2, and VO2 max, with soft lean tissue mass as the covariate. After adjustment for soft lean tissue mass, fat mass and leg soft lean tissue mass were entered into the model to determine if either total body fat or regional soft lean tissue mass distribution explained differences in VO2 between the two ethnic groups (African-American and Caucasian).

To determine if either TEE or AEE contributed to the difference in aerobic capacity, each was entered separately into the model after adjustment for total soft lean tissue mass and fat mass.

    RESULTS
Top
Abstract
Introduction
Methods
Results
Discussion
References

Descriptive statistics for the four subgroups (African-American and Caucasian males and females) are shown in Table 1. All groups were similar in age. A significant gender difference was found for both total body weight (P < 0.05) and total fat mass (P < 0.001), with females being heavier and having greater fat mass than males. However, soft lean tissue mass was similar among all four subgroups. Data for VO2 in absolute terms are summarized in Table 2. No significant influence of gender or ethnicity was observed for resting VO2 or submaximal VO2, but a significant effect of ethnicity was found for VO2 max (P < 0.01). African-American children had absolute peak VO2 values that were 15% lower than those of the Caucasian children. The lower VO2 max was seen in both males and females. There were no significant effects of gender or ethnicity on maximum respiratory exchange ratio or maximum heart rate (Table 2). Submaximal heart rate was significantly higher (P < 0.05) in females, and submaximal respiratory exchange ratio was significantly greater (P < 0.05) in African-Americans (Table 2). Caucasian males had significantly longer treadmill times than their African-American counterparts (P < 0.01), and the same was true for females (P < 0.01; Table 2).

                              
View this table:
[in this window]
[in a new window]
 
Table 1.   Descriptive statistics

                              
View this table:
[in this window]
[in a new window]
 
Table 2.   Resting, submaximal, and maximal VO2 of African-American and Caucasian children

Resting VO2, submaximal VO2, and VO2 max were related to soft lean tissue mass in both African-American and Caucasian children. Resting VO2, submaximal VO2, and VO2 max are plotted as a function of soft lean tissue mass in Fig. 1. For resting VO2 adjusted for soft lean tissue mass, there were no significant differences in either slopes or intercepts between African-American and Caucasian subjects (Fig. 1A). For submaximal VO2 adjusted for soft lean tissue mass, there were also no significant differences in either slopes or intercepts between African-Americans and Caucasians (Fig. 1B). For VO2 max, the regression slopes adjusted for soft lean tissue were not significantly different, but the intercepts were significantly different (P < 0.05), with the African-American children demonstrating lower adjusted VO2 max (Fig. 1C). Similarly, the lower VO2 max persisted in African-American children after data were adjusted for leg soft lean tissue mass (Fig. 2). The lower VO2 max in African-American children could not be explained by differences in total body fatness. When total fat mass was entered into the model, in addition to soft lean tissue mass, the lower adjusted VO2 max values persisted in the African-American group (1.23 ± 0.025 vs. 1.39 ± 0.031 l/min; P < 0.01; Table 3).


View larger version (16K):
[in this window]
[in a new window]
 
Fig. 1.   A: relationship between resting oxygen consumption (VO2) and soft lean tissue mass (LTM) in African-American (dashed line; r2 = 0.61) and Caucasian (solid line; r2 = 0.65) children. Slopes (0.0053 ± 0.00064 vs. 0.0048 ± 0.00065 l · min-1 · kg soft LTM-1) and intercepts (0.065 ± 0.014 vs. 0.073 ± 0.015 l/min) were similar in both groups. B: relationship between submaximal VO2 and soft LTM in African-American (r2 = 0.52) and Caucasian (r2 = 0.65) children. Slopes (0.025 ± 0.0037 vs. 0.027 ± 0.0038 l · min-1 · kg soft LTM-1) and intercepts (-0.051 ± 0.083 vs. -0.069 ± 0.089 l/min) were similar in both groups. C: relationship between maximal VO2 (VO2 max) and soft LTM in African-American (r2 = 0.65) and Caucasian (r2 = 0.83) children. Slopes (0.056 ± 0.0063 vs. 0.061 ± 0.0052 l · min-1 · kg soft LTM-1) were similar, but intercept was significantly lower in African-American children (-0.024 ± 0.14 l/min vs. 0.025 ± 0.12 l/min; P < 0.05).


View larger version (20K):
[in this window]
[in a new window]
 
Fig. 2.   Relationship between VO2 max and leg soft LTM in African-American (dashed line; r2 = 0.64) and Caucasian (solid line; r2 = 0.84) children. Slopes were similar between groups (0.15 ± 0.12 vs. 0.13 ± 0.15 l · min-1 · kg leg soft LTM-1), but intercept was significantly lower in African-American children (0.15 ± 0.12 l/min vs. 0.25 ± 0.10 l/min).

                              
View this table:
[in this window]
[in a new window]
 
Table 3.   Adjusted means of resting, submaximal, and maximal VO2 in African-American and Caucasian children

Although we were confident that we reached a true VO2 max in the children who achieved two of the three criteria, it could be argued that children who reached three criteria may be more motivated in determining VO2 max. Therefore, a subset of subjects (n = 39) who achieved all three of the physiological criteria for VO2 max were analyzed. When the relationship between leg soft lean tissue mass and VO2 max in this subset was plotted, the lower VO2 max in the African-American children persisted (Fig. 3). When the values were adjusted for leg soft lean tissue mass, the resulting VO2 max values of the African-American subjects remained significantly lower than those of the Caucasian children (1.54 ± 0.040 vs. 1.27 ± 0.044 l/min, respectively, P < 0.01).


View larger version (19K):
[in this window]
[in a new window]
 
Fig. 3.   Relationship between VO2 max and leg soft LTM in a subset (n = 39) of African-American (dashed line; r2 = 0.54) and Caucasian (solid line; r2 = 0.79) children who achieved all 3 of physiological criteria for a successful VO2 max test (see Exercise testing for description of criteria). Slopes were similar between groups (0.13 ± 0.30 vs. 0.15 ± 0.17 l · min-1 · kg soft LTM-1), but intercept was significantly lower in African-American children (0.20 ± 0.27 vs. 0.31 ± 0.14 l/min; P < 0.05).

Adjusted means of TEE, REE, and AEE are shown in Table 4. There were no significant differences for TEE, REE, or AEE between the four groups. No significant effect of ethnicity was found for any of the dependent variables. To determine if TEE or AEE could explain the observed difference in VO2 max between the two groups of children, both variables were entered into the ANCOVA model separately in addition to soft lean tissue mass and fat mass. When TEE was entered into the model, the lower VO2 max persisted in the African-American children (1.20 ± 0.024 vs. 1.38 ± 0.028 l/min; P < 0.001). When AEE was entered into the model in addition to soft lean tissue mass and fat mass, the lower VO2 max also persisted in the African-American children (1.20 ± 0.024 vs. 1.38 ± 0.028 l/min, P < 0.001, in African-American and Caucasian children, respectively).

                              
View this table:
[in this window]
[in a new window]
 
Table 4.   Energy expenditure components adjusted for soft LTM and FM

    DISCUSSION
Top
Abstract
Introduction
Methods
Results
Discussion
References

The main conclusion from this study is that VO2 max was 15% lower in the African-American vs. Caucasian prepubertal children. The lower aerobic capacity was also associated with decreased exercise endurance in the African-American children, as measured by treadmill time to exhaustion. Another indication of the lower fitness in general in the African-American children was that 25% of tested children, compared with 12% of Caucasian children, did not meet the criteria for a successful VO2 max test, possibly reflecting a reduced motivation for maximal exercise effort.

The significantly lower VO2 in African-American children was observed only at maximal exercise effort and was independent of soft lean tissue mass, total fat mass, and leg soft lean tissue mass. In other words, none of the independent variables we examined explained the ethnic difference in VO2 max, and, in additional regression analysis, ethnicity remained a significant and independent predictor of VO2 max. The difference in aerobic capacity also was independent of physiological criteria for reaching VO2 max (heart rate >195 beats/min, respiratory exchange ratio >1.0 and/or plateauing of VO2), indicating that the ethnic difference was not due to differences in motivation during the treadmill test. Moreover, the ethnic difference in VO2 max was independent of habitual free-living physical activity-related energy expenditure.

Other body composition variables may play a role in the observed differences in aerobic capacity seen between the two groups. Previous studies have shown that the contribution of bone mass to fat-free mass may be greater in African-American than in Caucasian adults (26). One could speculate that the lower VO2 max found in the African-American children in our sample was due to the fat-free mass of the African-Americans containing more bone mass and less skeletal muscle compared with that of the Caucasian children. However, our analyses were done using soft lean tissue mass (bone excluded). Furthermore, the African-American and Caucasian children had similar amounts of skeletal muscle. Thus differences in bone mineral content of the African-American children and the Caucasian children were not a factor in the observed lower VO2 max in the African-American children.

Habitual physical activity and exercise patterns may have a significant influence on aerobic capacity. Although there are many social and behavioral factors that determine physical activity habits, some studies have implicated ethnicity as a determinant of exercise patterns, with African-American and other ethnic minorities being less active than Caucasians (7, 8, 12). According to the 1990 Youth Risk Behavior Survey, female African-American students (grades 9-12) were the least likely to be vigorously active three or more times per week (9). We do not have any descriptive data regarding the physical activity patterns of the children, but we do have the daily AEE for the children derived from the doubly labeled water data. It is important to note that the AEE value represents only the average daily AEE and does not give any information about the intensity or duration of the activities performed by the children. There were no significant differences between the groups with regard to TEE or AEE. However, in additional analysis, we found that the relationship between VO2 max and AEE was significant in the Caucasian children (r2 = 0.18; P < 0.05) but was not significant (r2 = 0.066; P = 0.143) in African-American children. In addition, African-American and Caucasian children had similar AEE values, but VO2 max was lower in African- American children. Although we are not able to make a conclusive statement regarding this finding, these data suggest that the Caucasian children participated in activities at higher intensities. One limitation of the doubly labeled water technique is that, although AEE can be calculated, it gives no information regarding the type or intensity level of the activities, and thus further studies using more qualitative assessment of physical activity pattern are warranted.

There are several factors that we did not examine that could possibly explain the lower VO2 max in the African-American children. Although the lower aerobic capacity in African-American children could not be explained by leg soft lean tissue mass (Fig. 2), it could have been due in part to differences in muscle fiber type in these two groups. African-American adult males have been found to have a greater percentage of type II, anaerobic fibers, and lower percentage of type I, aerobic fibers, compared with Caucasian males (1). Because fiber type and peak VO2 have been shown to be significantly correlated in adults (4, 17), it is possible the lower proportion of type I fibers in African-Americans may limit the ability to perform continuous, endurance-type activities that require a steady rate of aerobic energy transfer (21). If these differences in fiber type also occurred in this sample of young children, they may have been responsible for all or part of the difference in reduced VO2 max and treadmill time in the African-American children. This hypothesis could not be examined because muscle tissue from these children was not available.

Another factor that might explain our findings is ethnic differences in hemoglobin concentrations ([Hb]). When [Hb] levels are low, there is a decrease in the blood's oxygen carrying capacity and, consequently, a corresponding decrease in ability to perform even mild aerobic exercise (21). Pivarnik et al. (27) found that, in a group of African-American and Caucasian adolescent females (age = 13.5 yr), the African-Americans had [Hb] levels that were significantly lower than those of the Caucasian girls (13.0 ± 1.1 vs. 13.8 ± 0.9 g/dl; P < 0.01). Whereas the values were within normal physiological limits, it is unknown whether the lower [Hb] concentration contributed to a lower O2 extraction during exercise (27).

The implications and clinical significance of the difference in the VO2 max between African-American and Caucasian children cannot fully be defined, and it is difficult to generalize our findings to the general population. However, current epidemiological data indicate that low fitness is a powerful precursor of mortality in adults. Moderate levels of physical fitness exhibit a protective effect against the influence of such mortality predictors as smoking, hypertension, and hypercholesterolemia (5). It is unknown whether aerobic capacity or physical activity patterns in children would affect long-term adult health outcomes. However, it has been postulated that physical activity and/or fitness during childhood serves as the foundation for a lifetime of regular physical activity (6). Further research is warranted both to determine habitual physical activity patterns of children of different ethnic and cultural groups and to find appropriate ways to educate and motivate children to adopt regular physical activity patterns. The long-term relationship between aerobic fitness and risk of obesity and other chronic diseases has yet to be determined in different ethnic groups.

In conclusion, VO2 max was significantly lower in African-American compared with Caucasian prepubertal boys and girls. This difference could not be explained by differences in body composition, including fat mass, total soft lean tissue mass, leg soft lean tissue mass, TEE, or AEE. The difference in VO2 was not apparent during rest or submaximal exercise and was only observed at maximal effort of exercise.

    ACKNOWLEDGEMENTS

We thank all of the children who participated in the study as well as Tena Hilario for the wonderful job of recruiting subjects.

    FOOTNOTES

This study was supported by grants to M. I. Goran (United States Department of Agriculture Grant 95-37200-1643 and the National Institute of Child Health and Human Development Grants R29-32668 and RO1 HD/HL-33064) and in part by a grant from the General Clinical Research Centers at the University of Alabama at Birmingham (M01-RR-00032).

Address for reprint requests: M. I. Goran, Division of Physiology and Metabolism, Dept. of Nutrition Sciences, Univ. of Alabama at Birmingham, Birmingham, AL 35294-3360.

Received 28 April 1997; accepted in final form 3 July 1997.

    REFERENCES
Top
Abstract
Introduction
Methods
Results
Discussion
References

1.   Ama, P. F. M., J. A. Simoneau, M. R. Boulay, O. Serresse, G. Theriault, and C. Bouchard. Skeletal muscle characteristics in sedentary Black and Caucasian Americans. J. Appl. Physiol. 61: 1758-1761, 1986[Abstract/Free Full Text].

2.   Armstrong, N., J. Balding, P. Gentle, J. Williams, and B. Kirby. Peak oxygen uptake and physical activity in 11-to-16-year olds. Pediatr. Exerc. Sci. 2: 349-358, 1990.

3.   Astrand, P. O. Exercise Studies of the Physical Walking Capacity in Relation to Sex and Age. Copenhagen: Enjar Munksgaard, 1952.

4.   Barstow, T. J., A. M. Jones, P. H. Nguyen, and R. Casaburi. Influence of muscle fiber type and pedal frequency on oxygen uptake kinetics of heavy exercise. J. Appl. Physiol. 81: 1642-1650, 1996[Abstract/Free Full Text].

5.   Blair, S. N., J. B. Kampert, H. W. Kohl III, C. E. Barlow, C. A. Macera, R. S. Paffenbarger, Jr., and L. W. Gibbons. Influences of cardiorespiratory fitness and other precursors on cardiovascular disease and all-cause mortality in men and women. JAMA 276: 205-210, 1996[Abstract].

6.   Blair, S. N., H. W. Kohl III, R. S. Paffenbarger, Jr., D. G. Clark, K. H. Cooper, and L. W. Gibbons. Physical fitness and all-cause mortality: a prospective study of healthy men and women. JAMA 262: 2395-2401, 1989[Abstract].

7.   Caspersen, C. J., G. M. Christensen, and R. A. Pollard. The status of the 1990 Physical Fitness Objectives---evidence from NHIS 1985. Public Health Rep. 101: 587-592, 1986[Medline].

8.   Caspersen, C. J., and R. K. Merritt. Trends in physical activity patterns among older adults: the behavioral risk factor survey system, 1986-1990 (Abstract). Med. Sci. Sports Exerc. 24: S26, 1992.

9.   Centers for Disease Control and Prevention. Physical activity among high school students---United States, 1990. Morb. Mortal. Wkly. Rep. 41: 33-35, 1992[Medline].

10.   Cornelius, L. J. Health habits of school-age children. J. Health Care 2: 374-395, 1991.

11.   De Weir, J. B. New methods for calculating metabolic rate with special reference to protein metabolism. J. Physiol. (Lond.) 109: 1-9, 1949.

12.   Dipietro, L., and C. J. Caspersen. National estimates of physical activity among white and black Americans. (Abstract). Med. Sci. Sports Exerc. 23: S105, 1991.

13.   Goran, M. I., W. H. Carpenter, A. McGloin, R. Johnson, J. M. Hardin, and R. L. Weinsier. Energy expenditure in children of lean and obese parents. Am. J. Physiol. 268 (Endocrinol. Metab. 31): E917-E924, 1995[Abstract/Free Full Text].

14.   Gutin, B., S. Islam, T. Manos, N. Cucuzzo, C. Smith, and M. E. Stachura. Relation of percentage of body fat and maximal aerobic capacity to risk factors for atherosclerosis and diabetes in black and white seven- to eleven-year-old children. J. Pediatr. 125: 847-852, 1994[Medline].

15.   Gutin, B., A. Trinidad, C. Norton, E. Giles, A. Giles, and K. Stewart. Morphological and physiological factors related to endurance performance of 11- to 12-year-old girls. Res. Q. Exerc. Sport 49: 44-52, 1978.

16.   Heymsfield, S. B., R. Smith, M. Aulet, B. Bensen, S. Lichtman, J. Wang, and R. N. Pierson, Jr. Appendicular skeletal muscle mass: measurement by dual-photon absorptiometry. Am. J. Clin. Nutr. 52: 214-218, 1990[Abstract].

17.   Ivy, J. L., D. L. Costill, and B. D. Maxwell. Skeletal muscle determinants of maximum aerobic power in men. Eur. J. Appl. Physiol. and Occup. Physiol. 44: 1-8, 1980[Medline].

18.   Kemper, H. C. G., and R. Verschuur. Maximal aerobic power in 13-14 year-old teenagers in relation to biologic age. Int. J. Sports Med. 2: 97-100, 1981[Medline].

19.   MacDougall, J. D., P. D. Roche, O. Bar-Or, and J. R. Moroz. Maximal aerobic capacity of Canadian school-children: prediction based on age-related oxygen of running. Int. J. Sports Med. 4: 194-198, 1983[Medline].

20.   Maffeis, C., F. Schena, M. Zaffanello, L. Zoccante, Y. Schutz, and L. Pinelli. Maximal aerobic power during running and cycling in obese and non-obese children. Acta Paediatr. 83: 113-116, 1994[Medline].

21.   McArdle, W. D., F. I. Katch, and V. I. Katch. Energy transfer in exercise. In: Exercise Physiology: Exercise, Nutrition and Human Performance. Philadelphia, PA: Lea & Febiger, 1991, p. 123-144.

22.   McArdle, W. D., F. I. Katch, and V. I. Katch. Gas exchange and transport. In: Exercise Physiology: Exercise, Nutrition and Human Performance. Philadelphia, PA: Lea & Febiger, 1991, p. 254-269.

23.   McCormack, W. P., K. J. Cureton, T. A. Bullock, and P. G. Weyand. Metabolic determinants of 1-mile run/walk performance in children. Med. Sci. Sports Exerc. 23: 611-617, 1991[Medline].

24.   Nagle, F. J., J. Hagberg, and S. Kamei. Maximal O2 uptake of boys and girls ages 14-17. Eur. J. Appl. Physiol. Occup. Physiol. 36: 75-80, 1977.[Medline]

25.   National Health and Nutrition Examination Survey. Prevalence of overweight among adolescents---United States, 1988-1991. Morb. Mortal. Wkly. Rep. 43: 818-821, 1994[Medline].

26.   Ortiz, O., M. Russell, T. L. Daley, R. N. Baumgartner, M. Waki, S. Lichtman, J. Wang, R. N. Pierson, Jr., and S. B. Heymsfield. Differences in skeletal muscle and bone mineral mass between black and white females and their relevance to estimates of body composition. Am. J. Clin. Nutr. 55: 8-13, 1992[Abstract].

27.   Pivarnik, J. M., M. S. Bray, A. C. Hergenroeder, R. B. Hill, and W. W. Wong. Ethnicity affects aerobic fitness in U. S. adolescent girls. Med. Sci. Sports Exerc. 27: 1635-1638, 1995[Medline].

28.   Pivarnik, J. M., J. E. Fulton, W. C. Taylor, and S. A. Snider. Aerobic capacity in black adolescent girls. Res. Q. Exerc. Sport 64: 202-207, 1993[Medline].

29.   Walker, J. L., T. D. Murray, C. M. Johnson, D. L. Rainey, and W. G. Squires, Jr. The oxygen cost of a 20-minute steady-state jog for high school boys and girls. Pediatr. Exerc. Sci. 2: 272-280, 1990.

30.  Webber, L. S., W. A. Wattigney, S. R. Srinivasan and G. S. Berenson. Obesity studies in Bogalusa. Am. J. Med. Sci. 310, Suppl. 1: S53-S61, 1995.


AJP Endocrinol Metab 273(4):E809-E814
0193-1849/97 $5.00 Copyright © 1997 the American Physiological Society