In a recent TikTok, US rugby player Ilona Maher shared a comment she’d gotten on a previous video.

It read, “I bet that person has a 30% BMI” (it seems she was referencing a BMI of 30, which is the point at which a person is officially classed as “obese” by the index).

“Hi, thank you for this comment. I think you were trying to roast me, but this is actually a fact,” Ilona began her video in response to the remark.

“I do have a BMI of 30. Well, 29.3 to be even more exact. I’ve been considered ‘overweight’ my whole life,” the professional athlete explained.

After sharing that she had been classed as “overweight” as the result of a physical she’d completed in high school, the rugby star said, “I was so embarrassed.”

Since then, though, things have changed.

“I chatted with my dietician, because I go off of, you know, facts,” she explained, “and we talked about BMI. And we talked about how it really isn’t helpful for athletes,” she said.

That’s because muscle is denser than fat, meaning a square inch of muscle will be heavier than a square inch of fat; you can have a very low body fat percentage (the thing doctors tend to worry about) while maintaining a high weight, especially as a sportsperson.

“BMI doesn’t tell you much. It just tells you your height and weight and what that equals,” Ilona shared. “I’m 5′10″, 200 pounds ― and I have about, and this is an estimate, but about 170 pounds of lean muscle,” she added.

That puts her body fat percentage at 15% (that’s at the lower limit of the Royal College of Nursing’s recommended body fat percentage for women aged 20-40, which is 15% to 31%).

Maher added, “BMI doesn’t really tell you what I can do… So, I do have a BMI of 30. I am considered ‘overweight.’ But alas, I’m going to the Olympics, and you’re not.”

Not only is BMI not very useful for athletes, but it wasn’t even devised to measure people’s health.

Lambert Adolphe Jacques Quetelet came up with it in the 1830s as a part of his measure of the “average” man, which he saw as aspirational. (“Average” to Quetelet was, of course, exclusively Western European men.)

Researchers from the Perelman School of Medicine, University of Pennsylvania, have published an article in the journal Science which shared that BMI “is an inaccurate measure of body fat content and does not take into account muscle mass, bone density, overall body composition, and racial and sex differences.”

Nick Trefethen, Professor of Numerical Analysis at Oxford University’s Mathematical Institute, also told The Economist in a letter that the calculations of the index are off.

“We live in a three-dimensional world, yet the BMI is defined as weight divided by
height squared. It was invented in the 1840s, before calculators, when a formula had to be very simple to be usable.”

“As a consequence of this ill-founded definition, millions of short people think they are thinner than they are, and millions of tall people think they are fatter,” he wrote.

Take THAT, Wii Fit circa 2008…