3. Normal Changes in Body Mass Index (BMI)
Lower Ranges of BMI-for-age
In the lower ranges, BMI-for-age tends to increase more slowly in both
adolescent boys and girls. At the 5th percentile, BMI-for-age increases
at the rate of about 0.5 unit/year during most of the second decade.
Higher Ranges of BMI-for-age
In the higher ranges, BMI-for-age tends to increase more rapidly in both
boys and girls. At the 97th percentile, for example, BMI-for-age can increase
more than 1 unit/year. An increase in BMI-for-age of > 1 unit/year
puts an adolescent at risk of overweight and obesity. The new BMI-for-age
charts are useful in monitoring adolescents for an excessive rate of weight
gain relative to increase in stature.
At any given age between 10 and 14 years old, girls at the 50th percentile
have a slightly higher BMI than do boys. Below the 50th percentile, girls
have a lower absolute BMI value than boys at any given percentile. Above
the 50th percentile, girls have a higher absolute BMI value than boys
at any given percentile.
For example, at 14 years of age: the 5th percentile BMI-for-age for girls
and boys is 15.8 and 16.0, respectively; the 95th percentile BMI-for-age
for girls and boys is 27.2 and 26.0, respectively.