Errors in BMI letter

If you received a “BMI letter” in the mail from the school nurse, please note that there are some minor errors in the bottom portion where the Body Mass Index ranges are explained. It should read (corrections in ALL CAPS):

BMI less than 5th percentile – underweight
BMI betweeen 85th and 95th percentiles – at risk for overweight OVERWEIGHT
BMI equal or greater than 95th percentile – overweight OBESE

The category names were changed six years ago by the CDC, but apparently didn’t send a memo (the school is using an old letter template provided by the Commonwealth).

By the way, the letter doesn’t say what Body Mass Index is, so if you’re curious, it’s kg/[(meters)(meters)] … or you can cheat and use a web form.

The BMI percentile-for-age is a way to compare your child’s BMI with a sample of kids from 1977 (when parents were kids!). If you ever want to compute the BMI-for-age percentile yourself, this page will do the calculation for you. By the way, the CDC provides nice graphs of these percentile data. Here’s one for boys, with examples of 10 year-olds with different BMIs:

BMI for age and height for boys

Finally, this sentence (also from State-suggested verbiage) confuses many people:

“For example, some athletes and serious dancers may have a higher than expected BMI due to their increased muscle mass, which weighs more than fat mass”

It essentially means that a cup of muscle weighs more than a cup of fat — it’s why large bodybuilders have high BMIs but low amounts of fat.  High-for-age BMIs in children are not usually due to unusually high amounts of muscle (according to most sources), though it can happen.  (I have no idea what “serious dancers” might mean — ballerinas?)


About Colin Purrington

evolutionary biologist, photographer
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