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There’s a lot to say about obesity in children. It’s unquestionably important. For many, it’s also a really emotional and sensitive subject. But we do have to talk about it. Obesity impacts children’s health. Defining and understanding how we measure obesity in kids is a necessary first step.
Today, I’ll talk about body mass index (BMI), why and how it’s used to evaluate obesity in children. I’ll go over what it can and cannot tell us about someone’s health. I’ll also review a few things parents can think about in order to prepare for a preventive visit with their child’s doctor.

What is BMI in children?

BMI is a math equation. It’s based only on someone’s height and weight. If someone has a high weight and is very short, the BMI will be high. If someone is tall and has a lower weight, their BMI will be lower.

There are online calculators: (one example of a BMI calculator from the CDC). For adults the equation’s answer defines someone as underweight, healthy weight, overweight or obese.

For kids, there’s an additional step. It’s not the raw BMI numbers that matter so much as the percentile compared to other kids of the same age and sex. There are BMI growth charts, similar to the charts used to track height and weight. If a child’s BMI percentile is at the 75%, for example, it means their BMI is higher than 75% of kids their age.

According to the Centers for Disease Control and Prevention (CDC), if a child’s BMI is more than the 95% for their age, they are classified as having obesity.
Overweight is classified as between the 85%-95% for age. “Normal” weight is between 5%-85%.

But what does obesity actually mean?

What is the actual issue with obesity? Beyond defining numbers, obesity means excess body fat. This extra fat causes health problems. So, when we think about health problems related to obesity, we’re thinking of problems due to the extra fat, not just a number on a scale.

What BMI can and cannot tell us about a child’s health

At first, using BMI percentiles in kids seems problematic. It’s based only on height and weight. It’s not based on how much extra fat someone has. True body fat measurements are technically difficult to do. For example underwater weighing called densitometry is one way. Scanning a child in an MRI machine would be another. These accurate measurements are often done only for research purposes. Kids going to their pediatrician for a check up won’t be able (practically speaking) to have their true body fat measured.

This is frustrating, of course. Someone who is very muscular will have a higher weight (and therefore BMI) than someone with less muscle mass. Obviously, this does not necessarily mean they are obese with extra body fat.

BMI is useful at the highest range.

However, measuring actual body fat in research has proven helpful! Research shows BMI is useful, at a population level. Kids with BMIs over the 95% are very likely to also have excess body fat. In other words these highest BMIs in kids correlate with medically relevant actual obesity. Someone with a BMI above the 95% is likely to be truly “obese.”

Of course it’s not true for every individual. This is why talking it over with a child’s pediatrician helps. They can help interpret the numbers, examine your child, and take into account ethnicity and family history. Their recommendations will be individualized to your child, not just a BMI number.

So, even though BMI does not directly measure body fat, it’s a useful marker at the highest percentiles.

BMI in “overweight” children

On the other hand, the CDC defines a child as overweight if their percentile is between 85-95%. Research shows this is less accurate for measured body fat, especially for some populations. (In this study for example, non-Hispanic Black teen girls may have higher BMIs for the measured body fat. Mexican-American girls had the opposite: higher body fat percentages for their BMI).

This is where individual discussions are most important. In my experience, some of these “overweight” kids have excess fat and need some intervention. Others do not.

So, the BMI measurement is not perfect. But, in order to help children with too much extra fat, we have to start somewhere. We must have a way of alerting that they’re at risk for obesity. From a physician’s point of view, seeing the numbers in a patient’s chart is a helpful way to start an evaluation and conversation. The evaluation may include blood tests or discussion of lifestyle.

We also need information and data in order to be able to measure changes. BMI is one piece of that data. Just one piece. Weight itself, ethnicity, levels of physical activity, diet, stressors, and support system are some of the other pieces.

What parents should know before their child’s preventive care visit

In my opinion and experience, discussing a child’s BMI can all be done without any shame or blame. There is simply no place for it in these conversations. Rarely (never?) is there a single cause for a child’s obesity over which they alone have control. As a reminder, adolescents can usually talk privately with their doctor.

So, now that we know what BMI is and why it’s used, how can a parent prepare themselves and their child for a doctor’s visit where BMI may be discussed?

• First, realize it may be discussed. It’s common to at least mention it at well child visits for kids over age 3 years old. Some health insurance companies specifically require that a physician document and discuss BMI, along with diet and exercise recommendations.
• Second, please know there are doctors who are not judging you or your parenting if your child has obesity. They really just want to help. It may feel lonely in an exam room, but obesity is common. Many factors are at play.
• Third, if you’re the parent of a teen, please encourage them to chat with their physician privately for part of the visit.
• Lastly, understand what BMI does and does not tell us. It may be useful, but it’s also incomplete information. That’s what I’ve tried to cover here today.

Summary

In order to help someone with obesity we have to be able to identify them. Currently, BMI is the most common method. Despite its limits, it is useful, especially for the children at the highest percentiles.

I’ll discuss obesity more in future posts. We need to cover the actual causes of obesity. For now, I want to be clear that the onus of obesity is not solely on the child or teen. It amazes me that in these modern times, with all that we know about health, that someone’s character could be judged based on their BMI. That’s beyond today’s post, but it does happen. And, it’s both wrong and unhelpful.

As I’ve said to many teens, obesity is not really their fault. However, they can still do something about it. We’ll talk about that in future posts as well.