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

6. Case #3: Use of Growth Charts to Monitor Weight Fluctuation During Adolescence

Discussion

Don's case illustrates three key points in the application of growth charts to adolescents whose weight fluctuates during adolescence:

1. Don's goal of weighing 155 lbs at 71 inches would result in a BMI of 21.6. This would be a "healthier" BMI than he had at 13 years of age (170 lbs and 66 inches: BMI 27.5). However, the way in which an obese individual reduces his/her high BMI-for-age must be examined closely, and every effort should be made to maintain the lower BMI-for-age once it is attained.

There is evidence in adults that having a BMI fluctuate between high and lower values ("yo-yo") may be associated with greater morbidity and mortality than maintaining a moderately elevated BMI. It appears as if Don's approach to "getting in shape" uses healthy means of reducing energy intake and increasing activity. However, attempting to reach an artificial goal established by a competitive sport could be a problem, especially if his athletic performance becomes compromised by a too-low weight.

2. Because the formula for BMI (weight in kg / [stature in m]²) does not consider body composition, athletes who have a higher than average amount of lean body mass (muscle) may have a higher than expected BMI, without being obese. This could result in a "false positive" label of obesity when using BMI-for-age cutoffs.

However, it is usually possible to identify, on physical examination, adolescents whose high BMI is due to extreme muscularity rather than excessive fat tissue. The use of triceps skinfold thickness in this situation can help confirm the clinical impression of normal body fat stores (Himes, 1989).

This underscores the importance of applying the CDC growth charts to individuals in the context of their history and physical examination. It is even possible for adolescent athletes to have a high BMI-for-age and a low body fat, especially if they are using anabolic steroids.

3. Previous growth charts ended at 18 years of age. The inclusion of population standards up to age 20, including BMI-for-age charts, is a major advantage of the new CDC charts. However, it is important to keep in mind that the new BMI-for-age charts are still based on population-level data, and the pattern of an individual youth's BMI time may not follow the smooth percentile curves depicted on the charts.

For example, between 18 and 20 years of age, there is a range of more than 55 lbs between the 10th percentile and the 90th percentile in weight-for-age. This amount of variability is normal within a group of adolescents, but is not normal for an individual adolescent over a brief period of time. Thus, in Don's case, the primary value of the CDC growth charts is to plot his trend toward improved health based on his decreased BMI-for-age. If his BMI-for-age were to continue to drop, however, the issue of excessive weight loss would need to be addressed with him. Since he appears to have completed his growth in stature, one would not expect any further increase in height. Some adolescent males who experience a late growth spurt, however, can continue to grow into their early twenties.