BMI and Kids: Why It’s Bullshit

Today the NY Post ran a story about a girl in Staten Island who was told by her school that she was fat.

One look at her and it’s obvious that she’s anything but. However, the notice from her school wasn’t based on looking at her, it was based on her BMI – body mass index.

BMI is calculated for adults using a person’s weight and height. Depending on the person this might not be an accurate measure of how fit a person is or if he’s overweight, because it doesn’t take into account muscle mass, which is heavier than fat. Many toned athletes would be considered obese based on BMI alone.

But for kids, the measurement is skewed even more, because a third factor is taken into account: age. Kids are compared to other kids of the same age. This causes a huge problem for kids like my son, who are taller than their peers.

When Jake was seven he got a notice from the school that he was in the obese category. I laughed it off, but my husband took it seriously and was worried. I had to find a children’s BMI chart and show him why the number was ridiculous:

Take a look at how that chart works: a BMI that is acceptable at one age is overweight or obese at another age. The chart isn’t showing what a healthy BMI is, it’s simply showing what the BMI is for other kids of the same age and gender.

At 7, Jake had a BMI of 19.5. That put him in the top 5% of BMI for kids his age. This didn’t surprise me at all, because he was very tall for his age – he was towering over the other kids in his class.

Look at the chart below: if Jake had been 10 years old instead of 7, with the exact same height and weight, his BMI of 19.5 would have been inside the normal range (green) instead of in the obese range (red). At 14 years old it would have put him in the 50th percentile – right in the middle. The issue wasn’t his weight, it was that he was doing his growing faster than the other kids that he was being compared to.

Given the vastly different rates at which children grow, I think it’s ridiculous that the DOE continues to use BMI as a factor in determining if a child is obese or overweight (not to even mention whether or not they should be reporting this information).

Incidentally, Jake’s BMI has stayed roughly the same over the years, but now that he’s older he’s firmly in the “normal” category. So what did the BMI chart tell us about him when he was 7? Absolutely nothing.