What body mass index can — and can’t — tell you about your child’s health

One of the best parts of my job as a pediatrician is meeting my patients as infants and watching them grow into teens. In doing this, I take my responsibility of counseling kids and parents on healthy ways to live seriously. 

When your children come in for their annual well visits, we check their physical and mental health. One way we measure physical well-being is through understanding their body mass index (BMI). What exactly is BMI and why is it important to your child’s health now and as they grow into adulthood? 

What is BMI? 

The easiest way to explain BMI is a calculated measurement of someone’s weight in relation to their height. The first part of your child’s annual well visit includes a stop to get your child’s height and weight without shoes or heavy clothing like a jacket. With a calculation of these numbers, we can get a quick estimate of a child’s weight in relation to their height. This number on their growth chart lets us know if your child is underweight, normal weight, overweight or in some cases obese. 

Every child grows differently, so as pediatricians we take this into account. More acutely, boys and girls have differences in body fat distribution with a range of normal body shapes and sizes. BMI percentiles factor in a child’s growth and age to determine if they are within a healthy weight range. 

Not the whole story

BMI doesn’t tell the whole story. It’s not a perfect measurement, but it does guide us. Shorter kids with an athletic build can have a higher BMI. Some kids can be within a healthy range for BMI but do not eat healthy and get little activity. Conversely, there are some kids who are active and eat healthy, and still have a high BMI. We take all of this into account when meeting with our patients and counsel them accordingly. 

BMI and race

One thing that has come up is the question of BMI and race. In 2009, a study of 1,300 young adults by the British Journal of Nutrition concluded BMI may over and under represent obesity in certain races. Since the BMI scale was largely derived from data using older white adults, it does not take into account the varying body fat percentage, muscle density or bone size of different ethnic groups. While that may be true for adults, it is not the case for children.

While adult BMI uses value ranges (less than 18.5 is considered underweight, over 30 is considered obese, etc…), for children we use percentiles, which allow for more flexibility and take into account a child’s growing body. Furthermore, these percentiles were developed using multiple surveys of thousands of children in the United States between 1963 and 1994, so they’re far more representative and inclusive.

While being aware of implicit bias is always important in health care, it’s also important to remember that BMI is not a diagnostic tool and is only one piece doctors consider — along with diet, physique, physical activity and family medical history — when assessing a child’s overall health and well-being.

Understanding your child’s BMI

I like to see when kids’ BMI falls between the 5th and 85th percentile. Kids above or below could have nutritional shortfalls, either not eating enough or eating too much. Low BMI can also be affected by needing more nourishing food to fuel activities, while a high BMI can indicate a need for increased activity and to incorporate healthier eating habits. Less commonly, medical conditions, illnesses and medications are just a few instances that can affect a person’s ability to maintain a healthy weight. 

Health risks

Children with a BMI above the 85th percentile are at risk for problems as they get older such as diabetes, high blood pressure, chronic joint pain, sleep apnea and psychosocial stress such as bullying and low self-esteem. Research shows that children who fall within the overweight and obese ranges tend to take that extra weight into adulthood. However, it is never too late to make healthy and positive changes for your family, which I talk about a little later in this blog. 

Tips for a BMI percentile in the healthy range

It can be hard to keep your BMI in a healthy range. Over the past year, a lot of families have done a good job staying home to reduce the risk of spreading COVID-19, but have increased some other unhealthy habits such as more frequent snacking and increased screen time. 

So to keep BMI in a healthy range, I recommend a few strategies for patients and their families. 

• Veggies and fruits: Make sure your family is getting at least five servings of vegetables and fruits each day. They are nutrient dense and fuel our bodies in a healthy way. I often hear from parents that their kids are picky eaters, so they stop serving these foods. I always encourage families to continue to serve fruits and veggies. When children see a food over and over, they’re more likely to try it eventually. Make sure vegetables and fruit are part of every meal and snack. Things like carrot or celery sticks, fresh berries and apple slices are easy grab-and-go snacks.
  
  
• Limit screen time: This is a tough one given that many kids attend school virtually right now. It’s recommended to limit screen time to less than two hours each day (TV, video games, YouTube, phone, etc.), not counting the time kids are on screens for school. If your kids are attending school online, I encourage them to get some physical activity during breaks such as doing jumping jacks, sit ups, walking up and down stairs or running up and down the driveway. A little physical activity during the school day is good for getting all the wiggles out and helps them use their brain in a different way. 
  
  
• Physical activity: Speaking of physical activity (outside of school), everyone needs moderate activity every day. Moderate physical activity causes your heart rate to increase and your body to sweat. Try for at least an hour of physical activity each day. Many kids get this from participating in sports, gym or recess. During the pandemic, many of these activities now look different for families and it’s required everyone to get creative when it comes to being active. Don’t let winter stop your family from being active. Bundle up and take a walk, or build a snowman in your yard or at the park. Some of the creative ideas parents have shared on how they are keeping their families active include building obstacle courses, taking the family pet on walks each day and having family dance parties.
  
  
• Limit sugar: Finally, we want families to limit sugar-sweetened beverages and snacks. Drinks such as sports drinks, sodas and fruit juice can all have added sugar. I encourage families to stick to water, reduced-fat milk, and whole fruits and vegetables. Look at food labels for added sugars and do your best to avoid these. This is a small step that can reap measurable benefits. 

I offer these tips with caution. Don’t ever put your kids on a fad diet or incorporate an exercise program without talking to your pediatrician first. Children have unique nutritional needs for their healthy growth and development. Involve your kids in choosing healthy snacks and meal preparation if you can. There are a lot of cookbooks and websites dedicated to making healthy, kid-friendly snacks and meals. My go-to website for families is healthychildren.org (the American Academy of Pediatrics website for families). The section on nutrition includes healthy recipes and short articles about a variety of nutrition topics from strategies for tackling picky eating to discussing alternatives to cow’s milk. 

Your pediatrician also has many resources for helping patients and families live healthy lives. If you have any concerns about your child’s well-being or want some help incorporating healthier habits into your family’s routine, check with your pediatrician for more resources. It’s what we are here for!

Jessica De Valk, MD Pediatrician Franklin Pediatrics

The Children's Wisconsin Clinical Nutrition team believes that every child deserves the best and is dedicated to helping each child reach their potential through optimal and personalized nutrition. Our team of Registered Dietitians (RDs) works with children and their families throughout the hospital, both inpatient and outpatient, across multiple specialties to help each child thrive.