Tracking of Physical and Physiological Risk Variables among Ethnic Subgroups from Third to Eighth Grade: The Child and Adolescent Trial for Cardiovascular Health Cohort Study

Preventive Medicine 34, 324 –333 (2002) doi:10.1006/pmed.2001.0990, available online at http://www.idealibrary.com on

Tracking of Physical and Physiological Risk Variables among Ethnic Subgroups from Third to Eighth Grade: The Child and Adolescent Trial for Cardiovascular Health Cohort Study 1 Steven H. Kelder, M.P.H., Ph.D.,* ,2 Stavroula K. Osganian, M.D., M.P.H.,‡ Henry A. Feldman, Ph.D.,‡ Larry S. Webber, Ph.D.,§ Guy S. Parcel, Ph.D.,* Russell V. Leupker, M.D., M.S., ¶ Margaret C. Wu, Ph.D.,㛳 and Philip R. Nader, M.D.** *School of Public Health, Center for Health Promotion and Prevention Research, University of Texas Health Science Center at Houston, Houston, Texas 77030; ‡Clinical Research Program, Children’s Hospital, Harvard University, Boston, Massachusetts 02114; §Department of Biostatistics, School of Public Health and Tropical Medicine, Tulane University, New Orleans, Louisiana 70118; ¶Division of Epidemiology, School of Public Health, University of Minnesota, Minneapolis, Minnesota 55455; 㛳Division of Epidemiology and Clinical Applications, National Heart, Lung, and Blood Institute, Bethesda, Maryland 20892; and **Division of Community Pediatrics, University of California at San Diego, La Jolla, California 92093

Background. The Child and Adolescent Trial for Cardiovascular Health (CATCH), a multisite field trial, tested the effectiveness of multiple interventions for cardiovascular disease risk behaviors in children in third through fifth grades. This paper reports the tracking of physiologic variables through eighth grade. Methods. The cohort began with 5,106 third grade students from diverse ethnic backgrounds: 69% Caucasian, 14% Hispanic, 13% African American, and 4% other. Seventy-two percent of students were remeasured. Measures described are serum lipids, blood pressure, and body anthropometrics. Tracking was examined across three time points (third, fifth, and eighth grades) with a scaled Kendall concordance coefficient and percentage retention within quintiles across time. Results. For the overall sample, tacking was strongest for body mass index (BMI) (Kendall coefficient ⴝ 0.86) and weight (0.86), followed by skinfold thicknesses (0.72– 0.78), serum lipids (0.67– 0.72), and blood pressure (0.45– 0.51). For BMI, 96% of students stayed within ⴞ1 quintile from third to fifth grades; 90% stayed within this range from third to eighth grades. Conclusions. There were small but noticeable gender and ethnic differences: tracking was stronger among boys and African American students. These re1

Funding for this project was provided by the National Heart Lung and Blood Institute. 2 To whom correspondence and reprint requests should be addressed at Center for Health Promotion and Prevention Research, University of Texas Health Science Center at Houston, School of Public Health, 7000 Fannin, Suite 2600, Houston, TX 77030. E-mail: [email protected]. 0091-7435/02 $35.00 © 2002 American Health Foundation and Elsevier Science (USA) All rights reserved.

sults demonstrate that the children’s relative level of cardiovascular risk remained stable over a 6-year period. © 2002 American Health Foundation and Elsevier Science (USA) Key Words: youth; adolescent; CATCH; lipids; blood pressure; BMI; tracking. INTRODUCTION

Several longitudinal studies have shown that measures of certain CHD 3 risk factors taken during childhood and adolescence predict adult values [1–11]. This phenomenon is called tracking, and it is signaled by the maintenance of relative rank on any variable over time. Existing studies have demonstrated tracking for variables such as total cholesterol, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, blood pressure, and body mass index (BMI). The Bogalusa Heart Study [9, 12], for example, examined tracking over a 20-year period in children whose baseline ages ranged from less than 1 year to 14 years. The study revealed considerable maintenance within original percentiles (i.e., tracking) for obesity, serum lipids, lipoproteins, blood pressure, weight, and height. Excess weight in adolescence tracked most strongly into young adulthood and had a strong adverse effect on levels of blood pressure, total cholesterol, and LDL cholesterol in adulthood. These variables are CHD risk factors for adults and to a lesser

3

Abbreviations: apo-B, apolipoprotein B; BMI, body mass index; CATCH, Child and Adolescent Trial for Cardiovascular Health; CHD, coronary heart disease; HDL, high-density lipoprotein; LDL, low-density lipoprotein.

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extent for children, and they predict future morbidity and mortality [13–16]. While these longitudinal studies are informative, they included either relatively small or racially/ ethnically homogeneous samples of children. As a result, they did not thoroughly explore racial/ethnic differences in longitudinal patterns of CHD risk factors. This is important because of the established differences in CHD morbidity and mortality by race/ethnic groups [17]. Only the Bogalusa study examined tracking in an adequate sample of African American males and females [9]. By contrast, sample sizes in the Child and Adolescent Trial for Cardiovascular Health (CATCH), a multisite field trial in the United States [18], were large enough to allow the examination of longitudinal changes in risk factors among ethnic subgroups. In the CATCH sample, at baseline, 69% were Caucasian, 14% Hispanic, 13% African American, and 4% other (Asian, Native American, unknown). Although it was shorter in length compared to other tracking studies (6 years vs 20 or more), the CATCH study provides one of the few reports on racial/ethnic differences over time. This report used the CATCH data to describe such changes in physical and physiologic risk factors among male and female Caucasian, Hispanic, and African American youth from 8 to 13 years of age. MATERIALS AND METHODS

The CATCH cohort was established in 1991 when 5,106 students were recruited from 96 public elementary schools in the four states of California, Louisiana, Minnesota, and Texas. The study hypotheses, design, and methods were reviewed and approved by each field site and the coordinating centers’ human subject institutional review board. For each participating child, a signed consent form was obtained from a parent or guardian. Students were initially assessed in the fall of 1991 (third grade) and were then reassessed in the spring of 1994 (fifth grade) and the spring of 1997 (eighth grade). At the fifth grade assessment 4,019 (78.7%) students were reexamined and at eighth grade 3,659 (72%) students were reexamined. By eighth grade 285 students (6%) refused to participate; 90 (2%) gave consent but were not examined because of absence from school or other logistic difficulties; and 1,072 students (21%) did not respond, were living beyond a 100-mile radius, or were lost to follow-up. There were minor but statistically significant differences in participation rates according to site, age, BMI, and skinfold measurement. For all measures, higher participation was recorded in Louisiana and Minnesota than in California and Texas. Nonparticipants were slightly older at baseline screening (age 8.81 vs 8.74 years, P ⬍ 0.001) and had greater mean age-adjusted BMI (17.81 vs 17.52 kb/m 2, P ⫽ 0.002) and skinfold

325

thickness (12.6 vs 12.3 mm for triceps, P ⫽ 0.03; 8.7 vs 8.1 mm for subscapular, P ⫽ 0.007). No significant difference in participation was found with respect to age-adjusted baseline levels of serum cholesterol and blood pressure. Measures Venous blood for serum lipids and measures of height, weight, thickness of triceps and subscapular skinfold thicknesses, and blood pressure were taken by trained staff using the same protocol each time. Detailed descriptions of CATCH physical and physiologic measures and methods have been published elsewhere [13, 19 –21]; the measures are described briefly below. Anthropometry. Height was measured to the nearest 0.1 cm using a portable stadiometer, weight to the nearest 0.1 kg, and skinfolds to the nearest millimeter. A Detecto-Medic scale (Detecto Scales, Inc., Brooklyn, NY) was used for weight measurements in the third and fifth grades, and a Seca Integra 815 portable scale (Seca, Rumilly, France) was used in the eighth grade. Scales were calibrated each day they were used. For each measurement, the student removed shoes, belt, sweater, and jacket and then stood motionless in the center of the scale. BMI was calculated as weight (kg)/ height (m) 2. Three consecutive replicate measurements of triceps and subscapular skinfolds were conducted using Lange calipers. The average of the three measurements was used for analysis. Blood pressure. Circumference and length of the upper arm were used to select the appropriate cuff size for measuring blood pressure [22]. Each subject sat in a quiet room for 5 minutes and then had five recordings of systolic and diastolic blood pressure. Measurements were taken at 1-minute intervals using a Dinamap Automated Device model 8100 XT (Critikron, Inc., Tampa, FL). The average of the last three readings was used for analyses. Interrater reliability was assessed by repeating a set of anthropometric and measurements on two randomly selected children on each day of measurement. Intraclass correlation coefficients were 0.998 for both height and weight scales; 0.98 for triceps; and 0.98 for subscapular skinfolds. Five repeated measures of blood pressure were taken on all students at 1-minute intervals. The intraclass correaltions for the third to fifth measurements were 0.72 for systolic and 0.64 for diastolic blood pressure. Serum lipids. Nonfasting venipuncture samples for lipid determinations were drawn, and serum samples were sent to a central laboratory (Miriam Hospital, Providence, RI). Total cholesterol was determined by enzymatic methods on a Beckman CX4 autoanalyzer (Beckman Instruments, Inc., Fullerton, CA). For a ran-

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domly selected 30% subsample, HDL cholesterol was determined following precipitation with heparin and manganese chloride (Lipid Research Clinic Protocol) and apolipoprotein B (apo-B) cholesterol was assayed by nephelometry (Behring Diagnostics, Inc., Westwood, MA). The laboratory participates in the Centers for Disease Control and Prevention Lipid Standardization Program. Reliability was assessed by taking a 10% random blind duplicate sample of subjects. Intraclass correlation coefficients from serum total cholesterol, HDL, and apo-B were high: 0.996, 0.977, and 0.993, respectively. Statistical Analysis To quantitate the strength of tracking, we used Kendall’s scaled index of concordance (W*), a nonparametric index describing the correlation among two or more repeated measures [23, 24]. The Kendall score has the advantage over other tracking indices of being based on individual rank scores rather than on an arbitrary grouping such as quartiles or quintiles. Perfect tracking is indicated by W* ⫽ 1 and absence of tracking by W* ⫽ 0. With two time points, W* is mathematically equivalent to the Spearman correlation coefficient. In the present case, W* is equal to the average of the three pairwise Spearman correlations between risk– factor measures at grades 3, 5, and 8. W* was tested for significant deviation from the null value of zero by an asymptotic ␹ 2 test, ␹ 2 ⫽ (n ⫺ 1)(1 ⫹ W*(T ⫺ 1)), where T is the number of time points and n is the sample size [24]. An asymptotic standard error for W* is given by SE(W*) ⫽ W* ⫼ ( ␹ 2 ) 1/ 2 . Strength of tracking was compared between subgroups by an asymptotic z test using the standard errors; for example, the test between control (C) and intervention students (I) was z ⫽ (W *C ⫺ W *I) ⫼ (SE 2 (W *C) ⫹ SE 2 (W *I)) 1/2. To illustrate tracking patterns, the data were divided into quintiles according to baseline value (Grade 3). Percentages of students remaining in the baseline quintile or an adjacent quintile were tabulated. The risk factor means for each baseline quintile at grades 3, 5, and 8 were depicted graphically by spline-fitted lines. The division into quintiles is for illustration only and has no impact on the Kendall coefficient or on statistical inferences based upon it. Although CATCH was originally designed to assess the effects of a multiple component intervention, subjects in the control and intervention conditions were merged into one group for the analyses described here. Comparisons among the original control and intervention groups showed no significant differences for any of tracking measures presented here [25, 26; Table 3], so merging the samples and including a treatment condition co variable was justified and increased power.

TABLE 1 Students’ Characteristics at Baseline (Grade 3) and Reevaluation (Grades 5 and 8) in Four States (California, Louisiana, Minnesota, Texas), the Child and Adolescent Trial for Cardiovascular Health (CATCH), 1991–1997

Overall, N (%) Age in years, mean (sd) Sex, N (%) Male Female Ethnicity, N (%) Caucasian African American Hispanic Other Site, N (%) California Louisiana Minnesota Texas

Grade 3, 1991–1992

Grade 5, 1994

Grade 8, 1997

5,106 (100) 8.8 (0.5)

4,019 (79) 11.1 (0.5)

3,659 (72) 14.1 (0.5)

2,645 (52) 2,461 (48)

2,062 (51) 1,957 (49)

1,881 (51) 1,778 (49)

3,529 (69) 675 (13) 708 (14) 194 (4)

2,809 (70) 507 (13) 565 (14) 138 (3)

2,595 (71) 467 (13) 469 (13) 128 (3)

1,379 (27) 1,299 (25) 1,237 (24) 1,191 (23)

1,008 (25) 1,009 (25) 1,003 (25) 999 (25)

919 (25) 1,023 (28) 940 (26) 777 (21)

RESULTS

Table 1 presents characteristics of the sample at the third, fifth, and eighth grades. The ethnic composition of the sample varied substantially; in California the breakdown was 70% Caucasian, 7% African-American, 16% Hispanic, and 7% other. For the other sites the ethnic breakdown was, respectively, Louisiana (69, 28, 2, and 1%), Minnesota (90, 3, 1, and 5%), and Texas (46, 15, 38, and 2%). For each dependent variable, the sample size, overall mean value at baseline, and the mean values within each quintile are presented in Table 2. With the exception of lipids, mean values were in the clinically normal range, indicating a healthy population. However, the average value for total serum cholesterol was 170.3 and approximately 50% of the sample had values higher than the mean in the LRC Prevalence Study [27]. Figure 1 presents scaled Kendall scores for three variables, illustrated by the use of quintile tracks. The overall Kendall score for BMI was 0.86, the highest for any variable, followed by scores for serum cholesterol and diastolic blood pressure, the lowest. The higher the Kendall scores, the clearer the tracking separation and for each quintile, the more defined the eighth grade distribution. The first of our two tracking indicators, the Kendall coefficient of concordance (W*), was calculated among the baseline and two follow-up evaluations. The overall W* is the average of the three pairwise Spearman correlations among grades 3 to 5, 5 to 8, and 3 to 8 and thus represents an intraclass correlation using ranks of the variables. In Table 3, for nearly all variables, the

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TABLE 2 Distribution among Quintiles of Students’ Baseline (Grade 3) Values for Physical and Physiological Measures, the Child and Adolescent Trial for Cardiovascular Health (CATCH), Spring 1991 Mean value within quintile

Anthropometrics Body mass index (kg/m 2) Height (cm) Weight (kg) Triceps skinfolds (mm) Subscap. skinfolds (mm) Serum lipids (mg/dl) Total cholesterol HDL cholesterol Apo-B cholesterol Blood pressure Systolic (mm Hg) Diastolic (mm Hg)

N

Overall

1

2

3

4

5

5,083 5,097 5,098 5,104 5,100

17.6 132.7 31.2 12.4 8.3

14.6 124.3 23.7 6.8 4.0

15.8 129.3 27.0 9.3 5.2

16.8 132.5 29.6 11.2 6.4

18.3 135.7 33.0 14.0 8.6

22.5 141.4 42.6 20.8 17.4

5,106 2,332 2,337

170.3 51.7 89.6

134.1 37.9 64.8

154.8 45.6 78.9

168.3 51.0 88.5

183.4 56.5 98.6

209.7 68.4 117.2

5,066 5,067

105.0 53.4

94.0 44.3

100.4 49.9

104.6 53.3

109.1 56.7

117.0 62.8

correlation coefficients between third to fifth grade were higher than compared to fifth to eighth grade and third to eighth grades. Generally similar levels of risk–variable tracking among ethnic subgroups were observed (Table 3). There were, however, significant differences among subgroups for height, triceps skinfold thickness, and diastolic blood pressure. African American students’ Kendall values for triceps thickness and diastolic blood pressure were significantly higher than those of Caucasian and Hispanic students, and Hispanic students’ values for diastolic blood pressure were significantly lower than those of both African American and Caucasian students. On 8 of the 10 variables, African American students had the highest Kendall values, or the greatest tracking. On 7 of the 10 variables, Hispanic students had the lowest values, or the least tracking. Numerical differences between highest and lowest subgroup coefficients were relatively small for 7 variables (difference of 0.03– 0.05 for body mass index, weight, subscapular skinfold thickness, total cholesterol, HDL cholesterol, apo-B cholesterol, and systolic blood pressure), moderate for 1 variable (0.07 for diastolic blood pressure), and larger for 2 variables (0.10 or 0.11 for triceps and height skinfold thickness). The Kendall coefficient also revealed generally similar tracking among the two sexes, for all variables (Table 3). Males had significantly higher Kendall coefficients than females for height, systolic blood pressure, and diastolic blood pressure; females had significantly higher values for apo-B cholesterol. Males had the higher coefficients on 5 of the 10 variables (BMI, height, weight, and systolic and diastolic blood pressure), females had the higher values on 3 variables (triceps skinfold thickness, total cholesterol, and apo-B cholesterol), and males and females had identical values on the 2 remaining variables (subscapular skinfold thickness and HDL cholesterol). Numerical differences

between coefficients for the two sexes were low for 8 variables (difference of 0.00 – 0.04 for BMI, weight, triceps skinfold thickness, subscapular skinfold thick-

FIG. 1. Spline fit quintile tracks and scaled Kendall concordance (W*) over three time points (third to fifth, fifth to eighth grade), the Child and Adolescent Trial for Cardiovascular Health (CATCH), 1991–1997.

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TABLE 3 Tracking Indicators (Scaled Kendall Coefficient of Concordance) among Three Evaluation Periods (Grades 3, 5, and 8) for Students’ Physical and Physiological Measures, the Child and Adolescent Trial for Cardiovascular Health (CATCH), 1991–1997 Physical and physiological measures a Anthropometric Variable

Subgroup

Overall W* Spearman CC Third to fifth Fifth to eighth Third to eighth Race Caucasian African Amer. Hispanic Sex Male Female Condition Control Treatment Site California Louisiana Minnesota Texas

Serum lipid

Blood pressure

Body mass Triceps Subscapular Total HDL Apo-B Systolic blood Diastolic blood index Height Weight skinfold skinfold cholesterol cholesterol cholesterol pressure pressure 0.86

0.80

0.86

0.72

0.78

0.69

0.67

0.72

0.51

0.45

0.90 0.88 0.80 0.85 0.88 0.87 0.87 0.86 0.86 0.86 0.84 0.87 0.83 0.89

0.92 0.71 0.77 0.83 0.76 C 0.72 C 0.88 0.79 m 0.79 0.81 0.82 T 0.81 T 0.83 T 0.75

0.91 0.86 0.82 0.87 0.86 0.84 0.88 0.86 0.87 0.86 0.85 0.88 0.86 0.86

0.79 0.70 0.67 0.71 0.80 C 0.70 A 0.72 0.73 0.71 0.73 0.70 T 0.74 M 0.68 0.77 M

0.82 0.80 0.72 0.77 0.80 0.76 0.77 0.77 0.78 0.78 0.76 T 0.79 M 0.74 T 0.82

0.73 0.70 0.66 0.69 0.72 0.69 0.68 0.71 0.69 0.69 0.70 0.69 0.70 0.69

0.73 0.65 0.62 0.66 0.70 0.65 0.68 0.68 0.66 0.67 0.67 0.67 0.67 0.65

0.75 0.72 0.68 0.72 0.75 0.72 0.69 0.75 m 0.74 0.71 0.71 0.71 0.72 0.73

0.57 0.51 0.46 0.50 0.54 0.51 0.53 0.50 m 0.53 0.51 0.48 MT 0.50 T 0.51 T 0.55

0.46 0.47 0.41 0.45 0.49 C 0.42 C,A 0.47 0.44 m 0.46 0.45 0.42 LM 0.49 T 0.45 L 0.43

a Superscripts denote significant difference from reference groups indicated by these codes: A, African American; C, Caucasian; m, male; L, Louisiana; M, Minnesota; T, Texas. The critical P value was selected to compensate for multiple comparisons, so that each column had an approximately 90% probability of containing no Type I errors.

ness, total cholesterol, HDL cholesterol, and systolic and diastolic blood pressure) and higher for 2 variables (0.06 – 0.09 for height and apo-B cholesterol). The three types of physical and physiological variables had somewhat different overall strengths of tracking. The anthropometric variables had the highest overall Kendall coefficients (range 0.72 to 0.86), the serum lipid variables had somewhat lower overall coefficients (range 0.67 to 0.72), and the blood pressure variables had the lowest overall coefficients (range 0.45 and 0.51). There were no significant differences in Kendall coefficients between students who were in the original CATCH control group and the treatment groups, and subgroup coefficients generally resembled the overall levels, with the differences across variables ranging from 0.0 to .04. Among the four sites, moderate to large differences were observed on 6 of the 10 variables (0.06 – 0.11 BMI, height, triceps and subscapular skinfolds, and systolic and diastolic blood pressure), although no discernible pattern emerged by site. Our second tracking indicator, the percentage of students remaining in or near their baseline quintile (within 1), confirmed the occurrence of strong tracking for many variables (Fig. 2) and showed how level of tracking differed when assessed at the fifth grade (Table 4a) and the eighth grade (Table 4b) follow-up periods. Figure 2 illustrates that for each of the 10 variables the majority (67 to 91%) of students at eighth grade remained in the original baseline quintile or within its adjacent quintile (71 to 97% at fifth grade).

A stringent definition of tracking strength is remaining within the same quintile at baseline and follow-up. By this definition, a sizeable number of students whose

FIG. 2. Proportion of students who remained within ⫾1 quintile over two time intervals (third to fifth grade, third to eighth grade), the Child and Adolescent Trial for Cardiovascular Health (CATCH), 1991–1997.

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TABLE 4a Third to Fifth Grade: Percentages of Students Remaining in or near the Same Baseline Quintile (Grade 3) and Reevaluation (Grade 5) in Four States (California, Louisiana, Minnesota, Texas), the Child and Adolescent Trial for Cardiovascular Health (CATCH), 1991–1997 Physical and physiological measures (percentages) a Quintile

Anthropometric

Serum lipid

Blood pressure

Third Fifth Body mass Triceps Subscapular Total HDL Apo-B Systolic blood Diastolic blood grade grade index Height Weight skinfold skinfold cholesterol cholesterol cholesterol pressure pressure Overall

1 1 5 5 Plus or minus 1 quintile (third to fifth)

1 1 or 2 4 or 5 5

74 94 98 81 96

82 97 98 77 97

78 95 99 83 97

61 86 92 67 88

61 87 97 75 91

59 84 85 59 84

60 85 86 63 85

63 86 88 62 85

49 73 78 51 75

43 69 68 45 71

Caucasian

1 1 5 5 Plus or minus 1 quintile (third to fifth)

1 1 or 2 4 or 5 5

75 93 99 81 95

84 98 99 78 97

79 96 99 82 97

62 86 91 66 88

55 87 96 75 91

59 83 85 58 83

61 82 89 65 86

62 86 89 63 86

50 74 77 53 75

44 69 69 47 71

African American

1 1 5 5 Plus or minus 1 quintile (third to fifth)

1 1 or 2 4 or 5 5

74 95 98 83 96

75 95 100 76 97

73 96 98 86 98

79 94 96 72 93

64 89 100 82 93

66 88 85 54 86

61 82 89 60 86

73 89 85 51 83

42 68 74 51 74

42 68 76 45 72

Hispanic

1 1 5 5 Plus or minus 1 quintile (third to fifth)

1 1 or 2 4 or 5 5

73 94 96 83 96

80 95 96 71 96

77 94 97 81 98

63 83 92 60 90

65 87 97 64 91

52 78 87 64 83

65 88 82 67 85

63 84 89 62 85

52 80 78 51 76

48 73 64 41 70

Boys

1 1 5 5 Plus or minus 1 quintile (third to fifth)

1 1 or 2 4 or 5 5

74 93 99 80 96

88 99 100 83 99

78 96 99 83 97

60 86 93 66 90

62 83 96 74 88

58 84 85 58 83

61 85 86 66 86

61 84 89 60 86

49 75 77 50 75

45 70 69 46 71

Girls

1 1 or 2 4 or 5 5

74 95 99 80 96

79 96 97 77 96

77 96 99 84 97

63 85 93 66 88

67 87 97 75 90

60 85 87 63 85

62 85 88 60 85

66 89 87 65 86

50 72 78 51 75

43 70 67 44 70

1 1 5 5 Plus or minus 1 quintile (third to fifth) a

The table is interpreted as the proportion of students who remain in their baseline quintile from third to fifth grade BMI, for example, for boys, 74% of students who started in quintile 1 were still in quintile 1 at fifth grade. Similarly, 94% started in quintile 1 and were either in quintiles 1 or 2 at fifth grade.

third grade baseline values were very low or very high (i.e., in baseline quintile 1 or quintile 5) had strong tracking at both the fifth grade and the eighth grade evaluations (Tables 4a and 4b). For example, in the overall analyses, for anthropometric variables percentages of students with values in quintile 1 at both the third grade baseline and the eighth grade follow-up evaluations ranged from 56 to 66%, while percentages with baseline and eighth grade values in quintile 5 ranged from 59 to 72%. For serum lipid measures, analogous percentages were over 50% (range 53– 60% for quintile 1, 54 –55% for quintile 5). For blood pressure variables, percentages were 39% or greater (range 40 – 42% for quintile 1, 39 – 47% for quintile 5).

A less stringent definition of tracking strength is a follow-up value that falls either in the same quintile as the baseline value or in the quintile adjacent to the baseline quintile. With this definition, and considering only the two extreme quintiles (i.e., quintiles 1 and 5 at baseline and 1 or 2 and 4 or 5 at follow-up), tracking occurred in 69 –99% of students between third grade baseline and fifth grade follow-up (Table 4a) and in 64 –94% of students between third grade baseline and eighth grade follow-up (Table 4b). Tracking was weaker at the eighth grade follow-up evaluation than at the fifth grade evaluation, but even at eighth grade, all variables had notably strong tracking. As with the Kendall coefficients, values for percent-

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TABLE 4b Third to Eighth Grade: Percentages of Students Remaining in or near the Same Quintile at Baseline (Grade 3) and Reevaluation (Grade 8) in Four States (California, Louisiana, Minnesota, Texas), the Child and Adolescent Trial for Cardiovascular Health (CATCH), 1991–1997 Physical and physiological measures (percentages) a Quintile

Anthropometric

Serum lipid

Blood pressure

Third Eighth Body mass Triceps Subscapular Total HDL Apo-B Systolic blood Diastolic blood grade grade index Height Weight skinfold skinfold cholesterol cholesterol cholesterol pressure pressure Overall

1 1 5 5 Plus or minus 1 quintile (third to eighth)

1 1 or 2 4 or 5 5

63 88 93 72 90

64 90 87 63 87

66 91 94 72 91

56 80 83 59 79

59 84 89 64 84

53 78 80 55 79

53 78 79 55 79

60 82 84 54 82

42 65 70 47 69

40 66 64 39 67

Caucasian

1 1 5 5 Plus or minus 1 quintile (third to eighth)

1 1 or 2 4 or 5 5

63 87 91 69 88

68 91 90 65 89

65 91 94 72 91

57 79 83 58 79

56 80 88 63 82

51 75 80 55 79

50 73 76 52 78

56 77 83 58 81

43 66 69 45 69

41 67 64 40 67

African American

1 1 5 5 Plus or minus 1 quintile (third to eighth)

1 1 or 2 4 or 5 5

60 89 96 78 91

64 90 85 61 85

69 91 90 74 90

69 95 90 69 86

64 88 91 67 85

67 87 80 51 82

60 83 89 67 84

77 91 86 52 87

40 66 72 46 70

38 66 71 51 70

Hispanic

1 1 5 5 Plus or minus 1 quintile (third to eighth)

1 1 or 2 4 or 5 5

63 88 91 73 90

57 85 77 59 80

67 89 89 71 88

54 77 81 60 80

53 81 85 61 82

51 77 81 52 79

56 74 80 43 75

60 93 80 43 84

43 63 69 49 69

41 67 58 38 67

Boys

1 1 5 5 Plus or minus 1 quintile (third to eighth)

1 1 or 2 4 or 5 5

64 88 92 71 90

68 91 93 70 90

63 88 95 75 91

53 76 81 58 78

44 82 88 63 84

55 77 81 54 78

50 78 78 50 80

54 81 82 52 81

44 65 72 45 70

41 69 62 39 68

Girls

1 1 or 2 4 or 5 5

66 87 94 70 89

61 90 84 61 86

66 90 95 71 90

54 81 82 61 81

54 80 89 67 82

55 79 82 55 80

54 82 87 56 81

66 87 86 58 82

40 64 65 45 68

38 65 67 40 68

1 1 5 5 Plus or minus 1 quintile (third to eighth) a

The table is interpreted as the proportion of students who remain in their baseline quintile from third to eighth grade BMI, for example, for boys, 63% of students who started in quintile 1 were still in quintile 1 at eighth grade. Similarly, 88% started in quintile 1 and were either in quintiles 1 or 2 at eighth grade.

age remaining within plus or minus the baseline quintile differed somewhat among the three types of physical and physiological variables: Although the ranges overlapped for the three types, percentages of students at follow-up who remained plus or minus the baseline quintile were generally highest for anthropometric variables and lowest for blood pressure for both time intervals. For all quintile combinations shown in Tables 4a and 4b, percentages for the anthropometric variables within 1 quintile of original value ranged from 88 to 99% for the fifth grade interval and from 78 to 91% for the eighth grade interval, percentages for the serum lipid measures ranged from 83 to 86% for the fifth grade interval and from 78 to 87% for the

eighth grade interval, and percentages for the blood pressure variables ranged from 70 to 76% for the fifth grade interval and from 67 to 70% for the eighth grade interval. Few meaningful differences were observed comparing across Caucasian, African American, and Hispanic samples. DISCUSSION

The physical and physiological data presented here demonstrate that significant tracking (i.e., persistence over time) of CHD risk variables occurs from the third grade through the eighth grade in each of three ethnic subgroups of children—Caucasian, African American,

TRACKING CHD RISK FACTORS IN YOUTHS

and Hispanic—and in both females and males. The data confirm previous evidence that CHD risk factor measures track in children, as well as findings that many youngsters maintain moderate-risk or high-risk blood lipid profiles over a long period. The existence of childhood tracking and the fact that significant numbers of children maintain moderate- or high-risk lipid profiles have important implications for the nature and timing of prevention efforts. Our study results are consistent with findings from other tracking studies of physiologic variables, and they extend our knowledge about youth risk factors to ethnic subgroups not previously well studied. Anthropometric variables displayed the strongest tracking, suggesting that it is difficult to alter the pattern of overweight that exists as early as age 8 or 9 years. Blood lipid variables also tracked strongly, with the highest quintile averaging well above the desirable level for children (total cholesterol ⬎170 ml/dl) [27]. Blood pressure variables displayed significant but lower levels of tracking, which was probably due to greater day-to-day and within-day variation or difficulties due to measurement error such as machine calibration [19, 20]. Given the known measurement errors associated with blood pressure measurement, it would be very unlikely to observe strong levels of tracking. We found small differences in tracking by race/ ethnicity and gender. Where significant differences were observed, African American students displayed the strongest associations which suggests that these children’s risk profiles were somewhat less likely to change over time. The results for Hispanic children are of note because CATCH is the one of the first major studies to report on this population. In all but two Kendall concordance comparisons, Hispanic children displayed the lowest tracking values, and in most comparisons Caucasian children fell between the African American and the Hispanic children. Although speculative, possible biologic explanations for these findings may be found in differential rates of sexual maturation. Although not measured in the CATCH study, differences in the onset of sexual maturation are well known between African American and Caucasian children. Herman-Giddens reported that by age 8, 48.3% of African American versus 14.7% of Caucasian girls showed breast and/or pubic hair development [28]. At every age and for every maturation characteristic, African American girls were more advanced than Caucasian girls. Very little is known about the timing of sexual maturation among Hispanics, although a recent report using NHANES data among boys showed significant ethnic differences with the earliest maturation among African Americans, followed by Caucasian and Hispanics [29]. Because sexual maturation has also been linked to CVD risk factors [30] and increased body weight [31] it is possible that the small but consistently

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observed racial differences reported here are due to differences in maturation. By gender, significant differences in W* were observed on height (0.09), apo-b cholesterol (0.06), blood pressure (0.03), and, with the exception of apo-b cholesterol, boys displaying modestly stronger tracking. Even where significant, with the exception of height, the differences are moderate to small, suggesting that the influence of differential maturation, with later onset for boys, is well documented [32, 33]. The CATCH data indicate that substantial numbers of children who are at risk for CHD remain at elevated risk over a 6-year period. Our findings support the implementation of interventions to alter CHD risk profiles in children as early as third grade. Of particular concern is the stability of measures for anthropometric variables, and especially for BMI, where overall 93% of children starting in quintile 5 remained in either quintile 4 or quintile 5 after 6 years: 96% for African American and 91% for both Caucasian and Hispanic children. The increasing prevalence of obesity among children in the United States [34, 35] and results from the CATCH study provide compelling evidence of the need for early intervention to prevent and reduce obesity, starting by grade 3 or earlier. An important unanswered question is what intensity of intervention is required for students to “jump track” toward a healthier profile. The CATCH intervention efficacy study demonstrated favorable changes in children’s diet and physical activity outcomes that persisted over time [25, 26]. Although the CATCH intervention produced behavioral changes, it failed to produce statistically significant results for any of the physiologic variables described in this paper. When CATCH was designed, it was hypothesized that improvements in children’s health behaviors would produce favorable changes in physiology and particularly in blood lipids. This assumption was based on research with adults, most notably that of Keys and colleagues [36 –38]. It may be that the link is less pronounced during childhood because of normal growth and development and limited time for sufficient dose of intervention in public health school-based research. If so, then establishing more favorable risk behavior patterns in childhood might not translate into physiological changes until children pass through their growth spurt. The CATCH results are in contrast to those obtained by the DISC study, where an intensive clinically based behavioral intervention demonstrated favorable outcomes on dietary fat intake and serum cholesterol among children with elevated cholesterol levels [39]. Together, the results suggest that intensive interventions are necessary to produce physiologic effects among high-risk youth and that more intensive school-based strategies should be explored. In conclusion, the CATCH findings demonstrate that

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in a large, diverse sample children’s relative level of physiologic risk remains stable over a 6-year period. In addition, these results indicate that large numbers of children are at risk and that strategies targeting the entire population, as well as the environmental conditions associated with increased risk, are therefore warranted. ACKNOWLEDGMENTS The authors acknowledge the CATCH investigative team who were instrumental to the success of the project: Kathy Bachman, Sandra Kammerman, Ann Clesi, Peter Cribb, Kathleen Cook, Sharon Cummings, John Elder, Marguerite Evans, Todd Galati, Deanna Hoelscher, Carolyn Johnson, Leslie Lytle, Thom McKenzie, Sonja McKinlay, Sheryl Pederson, Cheryl Perry, Patty Strikmiller, Elaine Stone, and Jerri Ward. The authors also acknowledge the editorial assistance of Pat Salis.

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