BMI, or body mass index, is a single number used to assess whether a person's weight is appropriate in relation to his or her height. In the past, BMI was used only for adults.
About 25% of children and teens, ages 6 to 17, are overweight or obese. Childhood obesity increases the risk of adult obesity. Children who are obese at age 6 have a 50% or greater chance of becoming obese adults.
The new charts that include BMI percentile can help identify children who run the risk of becoming overweight or obese in the future. BMI percentile serves as an early warning signal.
Most people are familiar with the growth charts developed in 1977 by the National Center for Health Statistics (NCHS), a part of the Centers for Disease Control and Prevention (CDC). The growth charts are a series of curves that show the ranges of growth for children in the United States. These charts are used to plot children's height and weight to make sure they are on the path to normal growth. Adding BMI percentile has made these growth charts a much more useful and accurate tool.
The English formula for body mass index is: BMI = weight in pounds divided by height in inches, divided by height in inches, multiplied by 703. Example: A 35-pound child is 36 inches tall. Thirty-five pounds divided by 36 inches, divided by 36 inches, multiplied by 703 = 18.9 BMI.
The metric formula for body mass index is: BMI = weight in kilograms (kg) divided by height in centimeters (cm), divided by height in centimeters, multiplied by 10,000. Example: A 17-kg child is 106 cm tall. Seventeen kg divided by 106 cm, divided by 106 cm, multiplied by 10,000 = 15.1 BMI.
Once the BMI is calculated, this number and the child's sex and age are used to plot a percentile on the BMI percentile-for-age growth chart. There are separate charts for girls and boys because they differ in amount of body fat as they mature.
The BMI percentile determines whether the child is within a normal weight range, overweight, at risk of becoming overweight, or underweight.
Growth charts and BMI percentile calculators can be found at the CDC website at www.cdc.gov/nccdphp/dnpa/bmi/index.htm.
BMI-for-age percentiles that cause some concern in those ages 2 to 20 are: less than the 5th percentile -- underweightat or above the 85th percentile - "at risk of overweight" according to the CDC, but "overweight" according to the American Obesity Associationat or above the 95th percentile - "overweight" according to the CDC, but "obese" according to the American Obesity Association
Besides being a tool for determining appropriate weight, the BMI percentile can help determine health risk. Sixty percent of children and teens with a BMI percentile-for-age above the 95th percentile have at least one risk factor for cardiovascular disease. Twenty percent have two or more risk factors for the disease.
When used properly, these charts can explain patterns of growth and identify goals for positive change. They can be used to evaluate and support changes in weight and growth over time. The general growth pattern over time is more important than a single measurement plotted at any one visit. Growth charts using BMI percentile are a very helpful assessment tool, but should not be the only means of evaluating a child's development.
Using BMI percentile as part of growth charts is a good way to find a child's risk for becoming obese or overweight. This and other growth problems can be detected early. That means steps can be taken to better prevent excess weight gain in children. A healthcare professional can measure children to see where they stand on the new growth and BMI percentile charts.