The Average Adolescent Bmi

The average body mass index, or BMI, of adolescents has increased significantly during the past 30 years, according to surveys taken by the Centers for Disease Control and Prevention that included measuring the teens' height and weight. This trend is worrisome because obese adolescents are far more likely than their thinner peers to be obese adults, and obesity is a major risk factor for many diseases and early death, according to data reported by Dianne Hales in her book, "An Invitation to Health."

Identification

The BMI is "a reliable indicator of body fatness for most children and teens," according to the CDC report, "About BMI for Children and Teens." Adolescents' and adults' BMIs are calculated identically, but the interpretations are different. Adolescents are at a healthy weight if their BMI is between the 5th and 85th percentile of their age and gender, underweight when it's below the 5th percentile, overweight if it's between the 85th and 95th percentiles and obese if it's above the 95th percentile.

Healthy BMIs

Healthy-weight men and women of all ages have a BMI between 18.5 and 24.9. Healthy-weight boys and girls have different BMIs that will change if their peers' BMIs change. Adolescents' healthy BMIs also change monthly. Currently, boys' healthy BMIs are 15.5 to 21.8 if they're 13 years old and zero months, 17 to 24.2 if they're 16 years old and zero months and 18.6 to 26.4 if they're 19 years old and zero months. Girls' healthy BMIs are 15.3 to 22.5 if they're exactly 13, 16.7 to 24.6 if they're exactly 16 and 17.8 to 26.1 if they're exactly 19.

Boys' Averages

The average BMI of male adolescents increased significantly in every age group, according to the CDC's "Body Mass Index, United States, 1960-2002" report. The biggest increase was among 17-year-olds, whose average BMI increased to 24.5 in the 2002 study from 21.8 in the 1980 study. The other average BMIs in 2002 were 20.7 for 13-year-olds, 22.3 for 14-year-olds, 22.5 for 15-year-olds, 24.1 for 16-year-olds, 24.2 for 18-year-olds and 24.9 for 19-year-olds. The average BMI increased 1.2 to 2.7 for the seven age groups.

Girls' Averages

The average BMI of female adolescents also increased significantly in every age group, according to the "Body Mass Index" report. The largest increase was among 19-year-olds. Their average BMI increased to 25.5 in the 2002 study from 22.4 in the 1980 study. The other average BMIs in 2002 were 22.6 for 13-year-olds, 22.9 for 14-year-olds, 23.2 for 15-year-olds, 24.0 for 16-year-olds, 23.1 for 17-year-olds and 24.4 for 18-year-olds. The average BMI increased 0.8 to 3.3 for the seven age groups.

Significance

"Obesity in American children has increased 100 percent in the last 20 years," according to Hales's "An Invitation to Health," which reports that 14 percent of adolescents are overweight. Obese 15- to 17-year-olds without obese parents have a 54 percent chance of becoming obese adults, while their healthy-weight peers have only a 5 percent chance of becoming obese adults. Obese adults have a "higher risk of dying from all causes," including heart disease, according to Hales.