3. Normal Changes in Body Mass Index (BMI) During Adolescence

Tracking of BMI

BMI decreases during early childhood, reaches a nadir (the so-called rebound point) between 4 and 7 years of age, and then increases to 20 years of age. These changes in BMI reference values with advancing age reflect normal changes in body composition during puberty.

Fat-free body mass increases in both sexes, but its accumulation is more marked in boys than in girls after 13 years of age. Body fat continually increases in girls during most of the second decade, while boys tend to decrease fat after age 14. The sum total of changes in fat-free and fat body mass result in the numerator (weight), while the sum total of the changes in stature result in the denominator (stature)² in the equation for BMI. These normal changes must be considered when interpreting data for individual adolescents, as well as for groups of adolescents.

The increase in BMI is nearly linear in boys during the second decade of life in all but the highest percentiles (in which the line becomes slightly convex upward). The BMI-for-age percentiles for adolescent girls, on the other hand, are all slightly convex upward. Just as weight-for-age and stature-for-age tend to follow a percentile "channel," so does BMI-for-age. This "tracking" of BMI-for-age can be used to identify changes in growth that may indicate an underlying problem or call for some kind of intervention.