Determining health risks in childhood: How does BMI measure up?

                                                                       Emily Goldsmith
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​Watching this past summer’s Olympics, one of my favorite breakout stars was Team USA Rugby player Ilona Maher. Maher has gained attention for promoting body positivity for young girls; she often talks about how her Body Mass Index (BMI) has always qualified her as overweight. While she was insecure about her larger size as a child, she now emphasizes her size allowed her to have the strength and athleticism to become a champion rugby player. As a mom with two young kids, Maher’s comments about BMI and childhood body image made me think about how frequently I see BMI numbers for my kids and how little I understand about what they mean, which led me to explore the topic.  

For each of my children’s After Visit Summaries (AVS) following pediatric visits, BMI is listed at the top. For children, BMIs are given a percentile and it is this that is used to categorize a child’s weight. The Center for Disease control provides the following definitions:

•  Underweight: less than 5th percentile
•  Healthy Weight: 5th to less than 85th percentile
•  Overweight: 85th to less than 95th percentile
•  Obese: 95th percentile or greater
•  Severe Obesity: 120% of the 95th percentile or greater 

While pediatricians have reported BMI for my children starting from birth, the American Academy of Pediatrics states that there are no accepted definitions for underweight, overweight, or obesity for children under two years of age. It is at age two that universal BMI screening is recommended for annual physicals.

Both physicians and parents can agree that a focus on childhood obesity prevention is important, both to mitigate possible health consequences facing children and to decrease the likelihood of obesity in adulthood. In fact, numerous studies have shown that children with obesity are more likely to also have obesity as adults. Early weight management in kids seems crucial; for example, a 2018 study found that a critical window for rapid weight gain is between the ages of two and six.

If early weight management is important, how do we monitor it? There are criticisms that BMI does not measure body fat or distribution and does not identify the factors leading to obesity in a patient. To address this, the American Medical Association issued a new policy in 2023 urging physicians to use other measurements, including visceral fat, body adiposity, waist circumference, and genetic/metabolic factors, as these measurements better predict obesity. Lending support to these recommendations, a study published in 2024 found that using waist circumference-to-height ratio is a more accurate predictor of fat mass in children compared to BMI.

Other studies suggest that physical activity levels, not BMI, have a greater impact on mortality. One metanalysis found that cardiorespiratory fitness predicted mortality risk better than BMI, with “fit” individuals categorized as overweight/obese having similar mortality risks to “fit” individuals with normal weights. In addition, “unfit” individuals had twice the mortality risk of “fit” individuals, regardless of BMI.

​However, despite growing debate regarding its use, BMI is still the CDC’s recommended obesity screening metric for children. Given this, when it comes to that ever-present BMI number, my takeaway is that it I shouldn’t fixate on it or let my children. It is just one piece of information to help understand the full health picture. As we saw this past summer, Olympic athletes come in many shapes and sizes, which highlights the significance of encouraging kids to discover the forms of physical activity that best suit their interests and abilities.