Why BMI is bullshit

In 2019, I wrote an opinion piece for the New Zealand Medical Journal about why I don’t think BMI has a place in the health and wellbeing check for our four-year-olds. You can read the full thing here. It must have been a slow news day when it came out, because I had my two seconds of fame, with a bunch of media requests for interviews, etc. The thing is, I could hardly believe this was NEWS at all. It’s not like it hasn’t been known for yonks that BMI, on an individual level, is bullshit.

You’ve probably heard about the BMI categories. Basically, you take your weight and divide it by your height squared and this magically pops out what category you are: underweight, healthy weight, overweight or obese. This use of the BMI tool to essentially diagnose obesity is encouraged with children and adults of all ages when they visit their doc. And I’m not even gonna go into the issues that could arise from this labelling, I’m just gonna stick with the limitations of the actual BMI calculation. If you feel like a rant, or info to back up why you don’t care about your or your child’s BMI, read on.

Nick Trefethen, Professor of Numerical Analysis at the University of Oxford, summarised many of the limitations of BMI in an opinion piece he wrote, stating that “the body-mass index that you (and the National Health Service) count on to assess obesity is a bizarre measure. 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”. And that pretty much sums it up. I mean, even the World Health Organization admit that BMI is a “rough guide”.

Prof. Trefethen’s mathematical viewpoint of the limitations of BMI is backed up by scientific studies. A 2016 study compared blood pressure, lipids, glucose, insulin resistance and C-reactive protein with BMI categories in 40,420 adult participants from the National Health and Nutrition Examination Survey. The conclusion? Nearly half of ‘overweight’ and 29% of ‘obese’ individuals were metabolically healthy, and over 30% of ‘normal weight’ individuals were metabolically unhealthy. That’s a LOT. In the States alone that would be like 75 million adults misclassified.

But what about kids specifically? A 2015 systematic review and meta-analysis analysed data from 53,521 kids aged four to 18 years and concluded that the ability of BMI to identify those with higher levels of body fat was only 73%. Let’s flip that around. That means 27% of children with high fat levels were not correctly identified using BMI.

But that’s not all. The very concept of telling parents the BMI category of their child has been called into question by recent research.

It’s well established that parents tend to think their child is perfect and other children are too thin or too fat. So all this effort has gone into correcting their view, with the thinking that parents won’t do anything about it unless they can see the problem. Sounds logical, right? Well, the thing is, human psychology isn’t logical.

A 2016 study of 3,557 Australian children and their parents found that when parents perceive their child to be overweight, the child was actually more likely to gain more weight throughout childhood. This finding was independent of the actual weight of the child. So, they could have been a perfectly normal weight, but when the parent/s thought they were overweight, they became overweight over the next few years.

I can hear you going “But how does that work?” We need more research but one hypothesis is that it’s to do with parents restricting their child’s food intake, and so creating feelings of deprivation and food obsession in their child. This is backed up by smaller studies that show a link between parental concern about child overweight and restrictive feeding practices, and restrictive feeding and weight gain. If you’ve ever been on a restrictive diet yourself you’ll probably know this. When you’re told you’re not allowed certain foods, you tend to crave them more. And when you can’t take it any longer you tend to binge out on those foods and gain weight.

So, if the BMI sucks so much and telling parents their child/ren are overweight causes the child/ren to become overweight, what the hell are we doing? Forget the BMI. It’s only useful when you’re looking at population data, not an individual person.

I advocate for a universal approach. Every person, every family (regardless of size) could have a conversation about healthy living, instead of a conversation about weight. Let’s talk about adequate sleep, cooking at home, eating meals together, letting kids decide how much they eat (known as the division of responsibility), encouraging child-led active play, managing stress, etc. This approach disregards size and focuses on wellbeing. Sure, it’s not as simple as BMI, but it’s far more meaningful in everyday life.

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