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Defining Overweight and Obesity: What is Body Mass Index (BMI)?

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Defining Overweight and Obesity: What is Body Mass Index (BMI)?

The first step in assessing weight status is to determine the Body Mass Index (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. BMI can be considered an alternative for direct measures of body fat. BMI is an inexpensive and easy-to-perform method of screening for healthcare professionals when evaluating weight categories that may lead to health problems.

For children and teens, BMI is age and sex-specific and is often referred to as BMI-for-age. 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, at risk of overweight, and overweight).

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

Weight status category Percentile range
Underweight Less than the 5th percentile
Healthy weight 5th percentile to less than the 85th percentile
At risk of overweight 85th to less than the 95th percentile
Overweight Equal to or greater than the 95th percentile

Source: Center For Disease Control – BMI information

CDC and the American Academy of Pediatrics (AAP) recommend the use of BMI to screen for overweight in children beginning at 2 years old. Parents may go to the BMI calculator, and calculate their child’s BMI and plot it on the age-appropriate growth chart to obtain an estimate as to whether their child’s weight is at risk or not. After this initial screening, it is recommended that the family seek further assessment and medical nutrition therapy advice by seeking the assistance of a Registered Dietitian Nutrition. You can also find helpful tips on meal planning, parenting food and fitness on our site!


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About the Author

Picture of Lucille Beseler, MS, RD, LD/N, CDE

Lucille Beseler, MS, RD, LD/N, CDE

Lucille is a Registered and Licensed Dietitian Nutritionist and a Certified Diabetes Educator in South Florida. She founded the Family Nutrition Center of South Florida in 1991 with a vision to improve the nutritional health of families and children. As an author, professional speaker, licensed nutritionist and registered dietitian she has reached many.

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