BMI: Is it Just a Number?
A friend called me the other day and said “we got a letter from school [about the kids] and she asked, her voice filled with panic, “Is my child too fat (or too skinny)”? Your pediatrician’s office and most school districts routinely measure your child’s height and weight and calculate their BMI (body mass index) , and may inform you that your perfect package of wonderful has a BMI that is too high or too low, or in my friend’s case, both.
You calculate BMI by weight in lbs. times 703 divided by height in inches squared. A percentile is then given when the BMI calculated is compared with the average for children with the same age and gender.
A child is considered to be:
• Overweight if they have a BMI over the 95th percentile for their age
• At risk of becoming overweight if they have a BMI between the 85th and 95th percentile for their age
• Underweight if they have a BMI under the 5th percentile for their age
A child has a “healthy” BMI if it is between the 5th and 85th percentile.
Before you panic, BMI does not take into consideration differences in bone mass, muscle, or fat. It does not consider growth spurts or delays, ethnic differences, or medical conditions that may affect any of the above. Most children who are “large boned” can be classified as overweight or even obese, the same as a lean muscular child can be labeled as underweight.
As a nutrition professional, I think BMI is often overused, oversimplified, and misunderstood to categorize children. I have seen parents handed the BMI for their child with tips for weight loss or how to put on weight, with no real solution or explanation. It is important to not classify or label any child as obese, anorexic or any other indicator that can affect their self esteem. If you become one of the parents who gets “the letter” first hug your child. . Consider all of the above factors that may be affecting this “number” and develop a plan to make it change if you are truly concerned. Consult a registered dietitian and/or your pediatrician who can help with a defined plan. If slowed or stunted growth may be a problem, the pediatrician can run studies to look further into that possibility. Athletes may have abnormally higher or lower BMIs. Don’t focus on the BMI as an end point, but as a starting point for your child’s health.
Laura Zurita is a registered dietitian and Montgomery County mom of a once-classified-as-underweight 6 year old. Her and her family like to read, hike, and watch Philadelphia Phillies baseball.