MCHB Training Module -- Adolescent Physical Development: Uses and Limitations of Growth Charts

5. Case #2: Use of Growth Charts to Screen for Excessive Weight Gain During Adolescence

Discussion

José's case illustrates three key points in the application of growth charts to adolescents who are gaining weight rapidly:

1. José's growth charts demonstrate a steep slope in weight gain, with no increase in stature over two years. The rate of weight gain is concerning because, if it continues at this rate, he will be at increased risk for various health related problems associated with obesity.

His failure to gain any stature over the past two years could be due to three factors:

a. the stature or weight recorded at 13 years of age could have been wrong
b. he may have already reached his adult stature due to early pubertal development, and/or
c. medications related to the treatment of his asthma could have stunted his growth in stature

Now that he is in a school-based health clinic, there is an opportunity to monitor his growth in weight and stature, using the CDC charts, and to interpret these findings in ways that might motivate him to change some of his lifestyle habits.

2. Assuming that all of the measurements on the growth charts were accurate, José has had a rapid increase in BMI over the past two years. The new CDC growth charts allow plotting of BMI-for-age, similar to plotting the curves for stature-for-age and weight-for-age. BMI values are positively correlated with total body fat, fat as a proportion of body weight (% body fat), as well as the total of all lean tissue (fat-free body mass).

When BMI-for-age is used to identify adolescents who have an excess amount of body fat (obese), there are more false negatives (individuals who are truly obese, but not identified as such by BMI-for-age) than false positives (individuals who are not obese, but who are identified as such by BMI-for-age).

There are other means of approximating body fat, such as triceps skinfold thickness, but this method is prone to large errors, depending on the skill and experience of the individual performing the measure. BMI-for-age is preferable as a clinical tool, since stature and weight are easier to measure reliably and the CDC growth charts now plot population reference standards.

Bright Futures recommends using the 85th percentile of BMI-for-age as the cutoff for "overweight" and the 95th percentile as the cutoff for "obese." Adolescents who are at risk of overweight should be screened for family history, blood pressure, total cholesterol, large change in BMI-for-age over time, and concerns about weight. If any of these factors are positive, an in-depth medical assessment is indicated. An in-depth medical assessment is also indicated for adolescents whose BMI-for-age is > 95th percentile.

There are substantial data to support these recommendations. High BMI-for-age values in children and adolescents are related to high blood pressure, cholesterol and low-density lipoprotein cholesterol and triglycerides (Freedman, 1999). In addition, high BMI-for-age values in childhood are related to increased morbidity and mortality rates in adulthood (Must, 1992). Thus, having a healthy BMI-for-age is a health issue, not merely a cosmetic one.

Since José's BMI has increased at least three-times the normal rate (from 18.5 to 25 over 2 years), he deserves a more in-depth medical assessment. In addition to the screening factors noted above, this should include evaluation of exogenous causes of obesity, the complications of obesity, as well as a diet and activity history.

3. Medications used to treat asthma, especially corticosteroids, may increase appetite and retard growth in stature. In addition, breathing difficulties associated with asthma may make exercising difficult. Thus, José may be experiencing a troublesome combination of increased energy intake and reduced physical activity, similar to that reported in the general population of adolescents over the last 20 years. If at all possible, medications that might increase his appetite or decrease his growth in stature should be avoided, and every effort should be made to improve his exercise-induced symptoms, with the goal of facilitating increased physical activity.