Definitions of Obesity

Part 1 – Measuring and Defining Childhood Obesity

The terms “overweight” and “obesity” refer to body weight that’s greater than what is considered healthy for a certain height. The most useful measure developed thus far of overweight and obesity is body mass index (BMI). BMI is calculated from your height and weight.

The Center for Disease Control and Prevention provides the following definitions:

        • underweight: BMI below the 5th percentile
        • normal weight: BMI at the 5th and less than the 85th percentile
        • overweight: BMI at the 85th and below 95th percentiles
        • obese: BMI at or above 95th percentile

What is BMI?

Body Mass Index (BMI) is a number calculated from a child’s weight and height. BMI is a reliable indicator of body fatness for most children and teens.  It does not measure body fat directly, but research has shown that it correlates to direct measures of body fat. Additionally, BMI is an inexpensive and easy-to-perform method of screening for weight categories that may lead to health problems.

What is a BMI percentile?

After BMI is calculated for children and teens, the BMI number is plotted on the CDC BMI-for-age growth charts (for either girls or boys) to obtain a percentile ranking. Percentiles are the most commonly used indicator to assess the size and growth patterns of individual children in the United States. The percentile indicates the relative position of the child’s BMI number among children of the same sex and age. The growth charts show the weight status categories used with children and teens (underweight, healthy weight, overweight, and obese).

BMI-for-age weight status categories and the corresponding percentiles are shown in the following table.

Weight Status Category Percentile Range
Underweight < the 5th percentile
Healthy Weight 5th percentile to < 85th percentile
Overweight 85th percentile to <95th percentile
Obese = to or > 95th percentile

How is BMI used with children and teens?

BMI is used as a screening tool to identify possible weight problems for children. CDC and the American Academy of Pediatrics (AAP) recommend the use of BMI to screen for overweight and obesity in children beginning at 2 years old.

How is BMI calculated and interpreted for children and teens?

1. Before calculating BMI, obtain accurate height and weight measurements.

2. Calculate the BMI and percentile using the Child and Teen BMI Calculator at http://apps.nccd.cdc.gov/dnpabmi/.

3. The BMI-for-age percentile is used to interpret the BMI number because BMI is both age-and sex-specific for children and teens. These criteria are different from those used to interpret BMI for adults — which do not take into account age or sex. Age and sex are considered for children and teens for two reasons:

  • The amount of body fat changes with age. (BMI for children and teens is often referred to as BMI-for-age.)
  • The amount of body fat differs between girls and boys.

The CDC BMI-for-age growth charts for girls and boys take into account these differences and allow translation of a BMI number into a percentile for a child’s or teen’s sex and age.

4. Find the weight status category for the calculated BMI-for-age percentile as shown in the following table. These categories are based on expert committee recommendations.

Weight Status Category Percentile Range
Underweight < the 5th percentile
Healthy Weight 5th percentile to < 85th percentile
Overweight 85th percentile to < 95th percentile
Obese = to or > 95th percentile

See the following example of how some sample BMI numbers would be interpreted for a 10-year-old boy.

clip_image002_BMI

This concludes the discussion on defining obesity.  In the next section we will discuss the rates of overweight and obese children in the United States.