First, BMI is a measure of body size, it is not a measure of health. There are many large-bodied children and adults who are healthy according to metabolic measures. In fact, a 2016 study found that half of those in the “overweight” and a quarter of those in the “obesity” BMI categories had healthy metabolic measures, while over 30% of those in the “normal” weight category had unhealthy metabolic measures. Using weight cutoffs and treating everyone with obesity as having a life-threatening disease regardless of their metabolic health inadvertently characterizes all large bodies as unhealthy.
Second, BMI measures weight, it doesn’t measure body fat. Obesity is defined as abnormal or excess adipose tissue that results in increased health risks. Yet BMI is a poor predictor of body fat, particularly for children under nine years. Importantly, it does not distinguish between different types of body fat or location–which matter significantly more to health risk than total fat mass. It also cannot differentiate between fat and muscle.
Finally, standards for BMI are based on Western ideals and European body types. A “normal” body size for a child is often thought of as slim or athletic—yet thinness is a Western ideal with racist roots. During the slave trade, Western scientists portrayed Black bodies as excessively sensous, reflected in their body sizes and shapes. Europeans on the other hand were viewed as disciplined and self-controlled, as evidenced by thin bodies. Developed 200 years ago, the BMI subtly encodes these biases about the ideal body size. And while the BMI has been updated since then, it is still based primarily on data from Euro-American body types and does not adjust for ethnicity or race, despite the fact that different populations tend to have different body compositions. What’s more, BMI-for-age growth charts are based on data sets that exclude extremes despite the wide range of body sizes among children, presenting a biased view of average or “normal.”
Other countries have started on this journey, and there is much we can learn from them. For example, in the UK, researchers have tried an alternative method for assessing children’s body fat which combines height, weight, and waist circumference to better understand body composition. In Japan, annual checkups include a battery of laboratory tests in addition to body size measures to assess risk of metabolic disease.
From the day they are born, we monitor our kids’ weight. We ask at their birth: “What did the baby weigh?” And we ask as they grow: “What percentile are they in?”
Once classified as overweight, children are often reduced to this single aspect of their health. We focus on weight to the exclusion of other conditions, prescribing diet and exercise and feeding into narratives about personal responsibility—blaming large bodies on a lack of self-control and bad parenting.