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Body-Mass Index Formula (BMI) in Children

 

​​By: Gary Kirkilas, DO, FAAP

​You may hear the term body mass index or “BMI" during your child's checku​ps, or even at your own health visits. But what does it  mean, and why is it important to your child's health? Read on for information about how BMI is calculated, what ranges pediatricians like to see in children, and why BMI--while an important screening measurement of health--can have limitations.

Wh​at is Body Mass In​​dex (BMI)?

In simple terms, a person's Body Mass Index or BMI is a calculated measurement of someone's weight in relation to their height. This number serves as a quick estimate of body fat. It is puts people in weight categories such as underweight, overweight, and obesity.

We know that children are constantly growing, and do so at different rates. In addition, male and female children have differences in body fat distribution, within a range of normal body shapes and sizes. Instead of just using the calculated BMI number as we do with adults, BMI percentiles consider children's growth and development to figure out if they are within their healthy weight range.

How is a child's BMI calculat​​ed?

Your pediatrician will measure your child's height and weight with their shoes and heavy clothes off, and then calculate BMI with this formula:

Body-Mass Index Formula

Method

Example

1. Multiply their weight (in pounds) by 703

184 × 703 = (A) 129,352

2. Multiply their height (in inches) by itself

69 × 69 = (B) 4,761

3. Divide (A) by (B)

129,352 ÷ 4,761 = 27.2 (BMI Score)

There are also BMI calculators avail​​able, such as this one from the U.S. Centers for Disease Control and Prevention:​​​​​




​​​​This number is then compared to other U.S. children of the same age and sex to determine what's called a BMI percentile. For example, a BMI-for-age percentile of 65 means that the child's weight is greater than that of 65% of other children of the same age and sex. Pediatricians plot this number on a standardized growth chart for a visual comparison, and to help track growth trends over time. 

The best way to know your child's BMI is to have your child's pediatrician measure and discuss the results with you. Your pediatrician can advise you on ways to strategies that focus on developing and supporting healthy habits at home.

What are BMI percentile categories?

The terms underweight, healthy weight, overweight, and obese can be another source of confusion for many parents. For pediatricians, these are useful categories that can show whether a child is gaining weight too quickly or not quickly enough.

What does your child's BMI mean?

To find out which category a child is in, pediatricians will use both the BMI number and its corresponding percentile. For example, a 5-year-old boy with a BMI in the 88th percentile means this child's BMI is higher than 88% of other 5-year-old boys and would be considered in the overweight category. The BMI percentile-ranges and weight status categories:

BMI percentile range​​

Category

Less than 5th percentile​

Underweight

5th to 84th percentile

Healthy weight​​

85th to 95th percentile

Overweight

Above 95% percentile

Obese

BMI: understanding the numbers

Ideally, children should fall in the target ranges between the 5th and 85th percentiles. Children below 5th percentile could have a nutritional shortfall—either not taking in enough calories or burning up more calories than they are getting, or both. Likewise, children above the 85th percentile are likely, but not always, getting too many calories in their diet, not burning up enough calories through physical activity, or both. Although less common, some medical conditions can cause children to gain or lose weight more easily.

Obesity is a serious health concern that can put children at risk for complications that can affect their health in the short- and long-term. It can raise the risk for problems such as diabetes, hypertension, chronic joint pain, sleep apnea, and psychosocial stress such as bullying and low self-esteem. Scientists have also found​​ obesity to be a risk factor for severe illness with COVID-19 infection.

We also know that children with obesity tend to be more likely to have obesity later in life as adults. However, it is never too late to make healthy and positive changes for your family!

If your child falls outside of the 5th and 85th percentiles, talk with your pediatrician about determining the best treatment options tailored to their individual needs.

​Just one piece of the health puzzle

A child's BMI is a valuable screening tool, but it's only one piece of the puzzle to finding out if a child is at a healthy weight. First, it is important to know that BMI is not a perfect measurement. For example, shorter children with a muscular build may have a high BMI but little body fat. In general, BMI percentiles higher than 95% are a reliable sign that a child has excess body fat and is at risk for health complications.

Second, it is especially important to not focus solely on specific numbers but rather, using the BMI along with making healthy choices. It is entirely possible to have a BMI in the healthy range but routinely make poor health decisions (eating junk food​ and not exercising). Likewise, it is also possible to have a BMI in the overweight or obese category while making regular healthy choices.

Remember

Preventing obesity and keeping weight in the healthy range is critical for children's overall health and well-being, as well as continued health as they grow and become adults. Remember to talk with your pediatrician if you have any concerns about your child's weight. Every family should aim to incorporate a balanced and nutritious diet in addition to daily exercise into their routine.

Healthy choices lead to healthier children!

More Information

About Dr. Kirkilas

Gary Kirkilas, DO, FAAP, is a general pediatrician at Phoenix Children's Hospital with a unique practice. His office is a 40-foot mobile medical unit that travels to various homeless shelters in Phoenix providing free medical care to families. He and his lovely wife, Mary (a pediatric emergency doctor), have three wonderful (most of the time) children and two dachshunds.



Last Updated
11/16/2020
Source
American Academy of Pediatrics (Copyright © 2020)
The information contained on this Web site should not be used as a substitute for the medical care and advice of your pediatrician. There may be variations in treatment that your pediatrician may recommend based on individual facts and circumstances.
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