Abstract
Background
Childhood obesity is a major risk factor for adult obesity, and obese children tend to be taller than their normal weight peers.
Purpose
The aim of the study is to evaluate whether childhood height influences the probability that normal or overweight children become overweight young adults.
Methods
A multicenter prospective cohort study of subjects assessed in both 3rd grade and 12th grade, n=2,802. Main exposures were CDC childhood BMI categories and height quartiles from 3rd grade measurements. Main outcome measure was CDC adult BMI categories from 12th grade measurements. Associations between childhood height quartiles, childhood BMI categories and adult BMI categories were assessed using chi-square tests and logistic regression models.
Results
Overall, 79% of overweight children remained overweight as young adults. Among children who were overweight or obese, the probability of becoming an overweight or obese young adult was 85% for children in the top quartile of height and 67% for children in the bottom quartile of height (p = 0.007). Among children who were normal weight, the probability of becoming an overweight or obese young adult was 25% for children in the top height quartile versus 17% for children in the bottom height quartile (p = 0.003).
Conclusions
When clinicians classify children by BMI categories and counsel about the risk for future obesity, they should recognize that greater height may be a marker for increased risk of adult overweight and obesity.
INTRODUCTION
For the prevention and treatment of obesity, clinicians are instructed to categorize children according to BMI percentiles, and then counsel.1-3 BMI was derived as an index of adiposity that adjusts a subject’s weight for his/her height so there is minimal association between BMI and height. While this has generally been true in adults, in children several studies have noted a positive association between BMI and height.4-8
Previous analysis of this study’s cohort examined the interrelationships between child height and BMI as continuous variables, and found a statistical interaction in the prediction of young adult BMI.9 In children with a BMI below the 80th percentile the association of child and adult BMI did not differ among children of different heights. However, among children with a BMI above the 80th percentile, those who were taller had significantly higher young adult BMI compared with those who were shorter.
There is longitudinal tracking of overweight categories from childhood into adulthood.10 The aim of this study is to evaluate whether childhood height influences the probability that normal or overweight children become overweight young adults..
METHODS
Study population
Participants are from the Child and Adolescent Trial for Cardiovascular Health (CATCH), a multicenter, cluster-randomized field trial designed to evaluate the effectiveness of school and home-based interventions to reduce cardiovascular risk factors. In Fall 1991, 5,106 3rd-graders were assessed. As 12th-graders (Spring 2001), 2,909 were reassessed, of which 2,802 had complete data and were used for this analysis. Details of the original study11 and this longitudinal cohort are described elsewhere.9
The intervention and control groups did not differ with respect to weight, height, or BMI at baseline or follow-up. They were therefore merged and analyzed as a single cohort. Furthermore, group assignment was controlled statistically as a fixed effect.
Anthropometry
Weight and height were measured in 3rd and 12th grades by trained and certified study examiners.12 The mean of two measurements were used for analysis. BMI was calculated as weight (kg)/height (m)2.
Exact percentiles for weight, height and BMI for age and gender were calculated relative to the CDC-2000 reference growth charts.13,14 Third-graders were referred to as “children,” with normal weight defined as BMI < 85th percentile, overweight defined as BMI ≥ the 85th percentile and < the 95th percentile, and obesity defined as BMI ≥ 95th percentile.3 Twelfth-graders were referred to as “young adults,” with normal weight defined as BMI < 25, overweight defined as BMI ≥ 25 kg/m2 and < 30 kg/m2, and obesity defined as BMI ≥ 30 kg/m2.
Statistical Analyses
Cross-tabulations of childhood height (quartiles by CDC norms) with weight status (normal, overweight or obese) were constructed, and tested for association by chi-square tests. The predictive values of childhood overweight status for adult overweight status were assessed via logistic regression adjusted for age, gender, race/ethnicity, geographic region and intervention condition. Individual school was included as a random effect. Differences in positive and negative predictive values between strata of childhood height were tested by large-sample t-tests. The analyses were conducted in 2008-2009 using SAS version 9.1.
RESULTS
Child height was positively associated with both child overweight status (r = 0.29, p < 0.0001) and adult overweight status (r = 0.21, p < 0.0001). In young adults, overweight status and height were uncorrelated (r = −0.005). Table 1 shows descriptive data.
Table 1.
Descriptive characteristics of the CATCH cohort with measurements in both 3rd and 12th grades, n=2802
| Gender (female %) | 51.2 | |
| Race/ethnicity, n (%) | ||
| Non-Hispanic white | 2,070 (74%) | |
| Non-Hispanic black | 335 (12%) | |
| Hispanic | 290 (10%) | |
| Other | 107 (4%) | |
| Children | Young adults | |
| Age in years, M (SD) | 8.7 (0.45) | 18.3 (0.5) |
| Date of measurement | Fall 1991 | Spring 2001 |
| Weight (kg), M (SD) | 30.9 (6.9) | 74.1 (17.5) |
| Weight percentilea: M (SD) Median [25th, 75th percentile] |
59.6 (28.1) 62.3 [37.6, 84.9] |
64.1 (28.5) 70.7 [42.5, 89.4] |
| Height (cm), M (SD) | 132.5 (6.1) | 170.5 (9.4) |
| Height percentilea: M (SD) Median [25th, 75th percentile] |
53.8 (27.5) 55.8 [31.3, 77.1] |
53.7 (28.6) 54.5 [29.2, 79.0] |
| BMI, M (SD) | 17.5 (2.9) | 24.5 (5.3) |
| BMI percentilea: M (SD) Median [25th, 75th percentile] |
59.9 (28.1) 63.3 [37.9, 85.3] |
61.9 (29.4) 67.2 [38.7, 88.5] |
| Overweight and Obese,* n (%) | 712 (25.4%) | 998 (35.6%) |
| Obese only,b n (%) | 295 (10.5%) | 367 (13.1%) |
Overweight for children is a BMI ≥ 85th percentile and for adults is a BMI ≥ 25
Obese for children is a BMI ≥ 95th percentile and for adults is a BMI ≥ 30
As seen in Table 2, there was a strong positive association between childhood overweight and adult overweight. Among the 712 children who were overweight or obese in 3rd grade, 566 (79%) remained overweight or obese as young adults. Among the 2090 children who were normal weight in 3rd grade, 1659 (coincidentally, also 79%) remained normal weight as young adults.
Table 2.
Predictive values for the longitudinal tracking of overweight status from childhood into young adulthood, stratified by childhood height.
| Total | Childhood height quartile #1 (shortest) |
Childhood height quartiles #2 and 3 |
Childhood height quartile #4 (tallest) |
|||||
|---|---|---|---|---|---|---|---|---|
| Adult normal weighta (n=1805) |
Adult overweight or obeseb (n=997) |
Adult normal weighta (n=426) |
Adult overweight or obeseb (n=120) |
Adult normal weighta (n=995) |
Adult overweight or obeseb (n=491) |
Adult normal weighta (n=384) |
Adult overweight or obeseb (n=386) |
|
|
Childhood Weight, Normalc (n=2090) |
1659 TN |
431 FN |
408 TN |
83 FN |
917 TN |
239 FN |
334 TN |
109 FN |
|
Childhood Weight, Overweight or Obesed (n=712) |
146 FP |
566 TP |
18 FP |
37 TP |
78 FP |
252 TP |
50 FP |
277 TP |
|
Positive predictive value (%) |
79 | 67 | 76 | 85* | ||||
|
Negative predictive value (%) |
79 | 83 | 79 | 75** | ||||
Adult normal weight defined as BMI < 25
Adult overweight or obese defined as BMI ≥ 25.
Childhood normal weight defined as BMI < 85th percentile
Childhood overweight or obese defined as BMI ≥85th percentile.
Test of difference in PPV between top and bottom quartiles has p=0.007.
Test of difference in NPV between top and bottom quartiles has p = 0.003.
TP, true positives; FP, false positives; FN, false negatives; TN, true negatives; PPV, positive predictive value; NPV, negative predictive value.
Positive predictive value = TP/(TP + FP)
Negative predictive value = TN/(FN +TN)
Within similar childhood BMI categories, greater childhood height was significantly associated with adult overweight or obesity. The two middle quartiles of child height were merged. Among children who were overweight or obese, those who were also tall had an 85% probability of becoming an overweight or obese young adult. Among those children who were short and either overweight or obese, the probability of becoming an overweight or obese adult was significantly lower at 67% (p = 0.007). Assessing children who were normal weight, those who were tall had a 75% probability of remaining normal weight as a young adult. Among children who were normal weight and short, 83% remained normal weight as a young adult (p = 0.003). Adjustment for race/ethnicity, gender, site, school, and location did not materially alter the findings.
DISCUSSION
Consistent with previous analysis,9 this investigation found a positive association between child height and young adult overweight status among children who had an elevated BMI. Overweight children who were in the tallest quartile had an 18% higher probability of remaining overweight or obese as young adults compared with overweight children who were in the shortest quartile (85% vs 67%, respectively).
The current study has novel findings in children who had normal BMIs. Among children who were normal weight, taller child height was associated with a higher risk of young adult overweight or obesity. Specifically, among those children who were normal weight, 25% who were in the tallest quartile became young adults who were overweight or obese. Only 17% of those who were normal weight and in the shortest quartile became overweight or obese adults. These differences in risk are comparable to differences seen between those with and without accepted obesity-related risk factors, such as ethnicity and geographic location.15,16 Possible explanations for this positive finding in children with normal BMI percentiles in contrast to the null finding in the previous study could be due to different outcome variables (i.e., categoric, BMI ≥ 25, in the current study v. continuous BMI in the previous study) or due to an interaction occurring between child height and BMI in those with a BMI between the 80th and 85th percentiles compared with those with a BMI < 80th percentile.
There are many aspects of the current study which make it relevant to clinical and research issues in preventive medicine. We have not found any published study of a larger longitudinal U.S. cohort with measured heights and weights from both childhood and adulthood. We are unaware of any previous analysis evaluating the effect modification of child height on the relationship of child overweight status and the prediction of young adult overweight status.
Our study had limitations. Only 55% of the original cohort was available for the longitudinal analyses. Analysis of the longitudinal cohort suggests that they were generally similar to the original cohort but with a tendency to be female (51% vs 45%) and non-Hispanic (90% vs 82%).
Overweight children are at high risk for becoming overweight adults.10 When assessing a child who is overweight and also tall, one may hope the child’s weight growth will slow, his/her height growth pattern will continue, and his/her BMI will normalize. From this perspective, tallness may be interpreted as protective against future obesity. The current results suggest the opposite. Compared with shorter children; overweight or obese children who were tall were more likely to remain overweight or obese as young adults. In addition, normal weight children who were tall were more likely to become overweight or obese young adults. For the prevention of young adult overweight and obesity, health professionals should recognize that greater childhood height may be a marker for increased risk.
Acknowledgement
This work was supported by a career research training grant (for SDS) from the NIH/NCRR (5K12-RR023247-04), funds from the NCI Centers for Transdisciplinary Research on Energetics and Cancer (U54CA116849), and the University of Minnesota Obesity Prevention Center. The opinions or assertions contained herein are the private ones of the authors and are not considered as reflecting the views of the NIH.
The authors would like to acknowledge William L. Baker, B.S. and Gabriela Vazquez, PhD for their data management and preliminary analyses, respectively. The authors thank the CATCH coordinating center, the four CATCH operating sites, and all the children and families who participated in the study.
Footnotes
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