3. Normal Changes in Body Mass Index (BMI)
Tracking of 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