BMI is used in every doctor’s office. But why? Dietitian Lucy Carey thinks BMI can do more harm than good.
“We’re not getting into this again,” he huffed at me, the buttons on this shirt straining as he shifted his weight in his chair. I had the distinct impression that he was very close to banging the table with this fist. “It may not be perfect, but BMI is the best we’ve got so we’re going to use it.”
I was seated across from him at the boardroom table, with only a couple years as a dietitian under my belt. I was developing firm opinions on a number of nutrition issues, but I was still very much a baby when it came to handling workplace politics.
In this case, I vehemently disagreed with measuring body mass index (BMI) in 4-year-olds, then telling the parents of the supposedly ‘overweight’ children that they needed to sort it out.
Nick Trefethen, Professor of Numerical Analysis at the University of Oxford, summarised many of the limitations of BMI when he wrote,
“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.”
His mathematical viewpoint is backed up by scientific studies comparing BMI with more accurate measures of body fatness and health, like blood pressure, lipids, glucose, insulin resistance and C-reactive protein. One study in over 40,000 adults found that nearly half of ‘overweight’ and 29% of ‘obese’ individuals were metabolically healthy. To add insult to injury, over 30% of ‘normal weight’ individuals were found to be metabolically unhealthy, too. In the USA alone, that’s over 75 million people that BMI would have put in the wrong category.
And it doesn’t end there. Not only is BMI a fairly crappy measurement, I also felt icky about labelling children obese. I had had several parents tell me over the phone that their child was now calling themselves fat since being told of their BMI results. One mother of a now 7-year-old was in counselling for some disturbing disordered eating behaviours, like running around the block over and over and over again in a bid to ‘lose weight’. She told me that it had started after his BMI had been calculated. She had, understandably, refused to have her younger daughter measured.
The whole idea that pointing out to parents that their child is overweight will motivate them to make positive changes is unfounded. In fact, it usually backfires.
One Australian study found that when parents perceive their child to be overweight, the child is actually more likely to gain proportionately more weight throughout childhood. This finding was independent of the actual weight of the child. Which means that if a parent thinks their child is fat, even when their child is objectively healthy, they become fat in the long-term.
That study didn’t delve into the mechanisms of how this works, but one hypothesis that’s quite likely is parental restriction of food. Whether consciously or subconsiously, parents start to without food from the ‘overweight’ child, creating feelings of deprivation and food obsession. The child can develop disordered behaviours like binge eating because of this.
Plus, why should skinny kids get a free ride? They could be setting themselves up with unhealthy habits that would last a lifetime, but we were essentially telling their parents that it’s all good because they’re thin.
It didn’t sit right with me. I thought we should talk to all families about eating together, cooking, active play, sleep hygiene, etc. Not just the ones with an ‘overweight’ child. My approach disregarded weight and focused on wellbeing instead. It was solutions-focused instead of problem-focused.
But my approach relied on parent’s self-reports and their subjective feelings. It didn’t matter that it is far more meaningful than BMI. For the man holding the purse-strings for the contract, it came down to the fact that BMI is easily calculated and spits out a number that you can write on a report.
And he had had enough of me expressing my opinion (with zero finesse on my part) in every meeting. He was getting seriously annoyed.
“There is nothing wrong with telling someone they need to lose weight. When I see my doctor and he tells me that I need to lose a few, I think that’s a good thing.”
He was overweight by any metric. But I hadn’t yet developed the art of getting someone on my side. I should have been opening up his eyes to the fact that being weighed, measured and told to “lose a few” had never served him. It helped no one but the bloated diet industry.
The words, “And how has that been working out for you?” were on the tip of my tongue but my manager looked at me with pleading eyes, silently willing me to shut up. We needed this man on our side.
I folded my arms grumpily and slumped against my seat. How could it be that the health system was so anti true health?
Several months later, after a bit of pleading, my somewhat reluctant manager backed me when I wrote an opinion piece for the New Zealand Medical Journal entitled Taking BMI off the table. Parents contacted me from all over the country when it came out, thanking me for standing up for their children. Health professionals emailed to express their delight in my practical approach. And the media lapped it up on what would otherwise have been quite a slow news day.
But that was 5 years ago now, and we still calculate BMI with our 4-year-olds. Because apparently the people at the top still want their easy number, despite the mounting evidence that it does more harm than good. I am holding out hope that if enough of us are persistent enough, one day things will change.