Body mass index or BMI: What parents should know

By Dr. Angela Dangvu, a CHOC Children’s pediatrician

 As a pediatrician, I spend much of my day doing well child visits for my patients. A big part of those appointments is looking at a child’s growth. Parents are usually interested in knowing their child’s weight and height percentiles, but they rarely ask directly about body mass index or BMI.

What is body mass index?

BMI is a measure of a person’s weight in relation to their height. It varies based on the age and gender of the person and is used to estimate the amount of body fat.

BMI is calculated starting at 2 years of age.  At this age it tends to be higher and will generally get lower until a child is 5 or 6 years old. Then, it will increase with age as they grow into early adulthood. Often, parents of children ages 3 to 4 ask me if their child is too skinny. They’ve begun to notice that their once chubby-looking toddler has thinned out. This is completely normal but can cause parents to overfeed their child because they think they are underweight.

Kids come in many different shapes, so they shouldn’t all have the same BMI. There is a wide range of BMIs that are considered normal. On the lower end, the normal range for BMI starts at the 5^th percentile — meaning that 5% of children of the same age and gender will have a BMI lower than them. On the higher end of normal is a BMI in the 85^th percentile, which means that 85% of kids of the same age and gender will have a lower BMI.

A BMI lower than the 5^th percentile is considered underweight, while a BMI between the 85^th and 95^th percentile is in the overweight category. A BMI of more than the 95^th percentile is considered obese.

When a child’s BMI indicates that they are underweight, it can be a sign of a medical condition that is preventing them from gaining weight. When a child’s BMI puts them in the overweight or obese range, we as pediatricians worry that they could be gaining too much weight, putting them at risk for diseases like diabetes or high blood pressure when they are older.

What body mass index is not

BMI is not a perfect measurement of a child’s body fat, but rather it is just one tool that we use in evaluating a patient’s overall growth and nutrition.

For example, a child being underweight doesn’t always mean they have a medical condition or are not getting enough calories. The child could be underweight because of genetic factors; perhaps both parents were thin as kids, and their child is taking after them.

How your pediatrician can help you improve your child’s BMI

As physicians, we look at multiple sources of information to determine why a patient is underweight. We look at the growth chart, medical history and family history. We might order lab tests and refer our patients to a specialist depending on the findings. If the child appears to simply need to consume more calories, I usually encourage the family to incorporate some more calorie-dense foods into the child’s diet, rather than getting into a power struggle with their child about the amount of food they are eating.

If your child’s BMI is in the overweight or obese category, your physician should work with your family to determine possible causes, and solutions as well. Many parents already know that their child’s weight is a concern, but others may not notice that it has become an issue.  With my patients, I try to identify factors in their diet and activity levels to find potential areas for change. One thing to remember with children is that they will continue to grow into their teens, meaning that the goal doesn’t usually need to be weight loss. If they can slow their weight gain or even go for a period of time without gaining weight as they grow in height, their BMI will improve.

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