Tuesday, December 12, 2017

Understanding the Body Mass Index (BMI) in Children

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by Eric Ball, MD
June 2013

Obesity has become an epidemic in the United States, with over one third of children classified as overweight or obese prior to entering Kindergarten. These children are more likely to become obese adults and have dramatically higher rates of heart disease, diabetes, and hypertension. Childhood is the best time to tackle the obesity epidemic. Our goal as pediatricians is to identify children at risk and implement lifestyle changes before these children become obese adults. One of the tools we use to identify children at risk for becoming overweight is the body mass index, or BMI.

BMI is a number calculated from a child’s weight and height and gives us an idea of whether a child weighs an appropriate amount for their given height. Although BMI does not directly measure body fat density, it is an inexpensive and easy-to-perform method that gives an accurate approximation of body fat. In adults, BMI is a set number: a healthy weight BMI is 18.5-25; less than 18.5 is considered underweight; 25-29.9 is considered overweight, and 30 and higher is considered obese. With children, the BMI is measured as a percentile, the same way that height and weight are measured. In our practice, we report BMI percentiles for all children older than two years old. Below is a copy of the BMI chart, with scenarios mapped out for various ten-year-old boys:

Please click to view larger image. Image from CDC.gov

Please click to view larger image. Image from CDC.gov

Using the BMI-for-age growth charts, BMI is expressed as a percentile. The percentile indicates the relative position of a child’s BMI number compared to other children of the same age and sex. Underweight is under the 5th percentile; healthy weight is between the 5th and 85th percentiles; overweight is between the 85th and 95th percentiles; obese is equal to or greater than the 95th percentiles.

Note that the curve changes for each age. This is because body fat composition changes quite dramatically in childhood as a child grows. There is also no ‘healthy weight’ range, as a healthy weight depends on a child’s height and sex. For example, I weigh 195 pounds, but I am a healthy weight because I am 6 feet 4 inches tall. If my 5 foot 7 inch tall wife weighed 195 pounds, she would be considered obese.

While reviewing your child’s BMI, keep in mind that body composition changes with age. You may have two children with the same BMI numbers but very different percentiles. This is because the interpretation of BMI-for-age varies by age and sex. In children who are not exactly the same age or sex, the BMI numbers have different meanings. Body mass index tends to dip in the early childhood years. Most children have a significant amount of ‘baby fat’ initially. Once they become mobile toddlers, they begin to thin out. This continues for the next several years, with the BMI chart hitting a dip at around age 6 or 7. The average 6 year old should be quite thin, many with visible ribs and skinny little arms. Although this is concerning to many parents, it is normal for that age (the average BMI for a 6 year old is 15, which is considered severely underweight for an adult, although normal for the 6 year old). After age 6 or 7, children begin putting on more fat again, and many parents notice that these children start to eat more. The BMI then begins to slowly rise throughout puberty with children gradually gaining more fat and muscle mass. By the time puberty is over, we start using adult BMI values to assess healthy weight. Understanding the normal ranges of BMI and how BMI changes throughout childhood will help all of us as parents and doctors keep our children healthy and thriving!

Adopted from the Centers for Disease Control and Prevention “About BMI for Children and Teens”

The contents of this web site are provided as an informational tool. This is not intended to replace medical advice or care administered by a healthcare professional. Common sense should always be used when referencing this site. If, at any time, you feel your child is experiencing a medical emergency, call 911 immediately.

Dr. Ball has been with SOCPA since 2006.  He is currently providing care in our Ladera Ranch office. Click here to learn more about Dr. Ball.

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