The Verdict Is In: The BMI is a Poor Predictor of Health

Recently a new study was published on something most of us have known for quite some time: the BMI is not an accurate or reliable predictor of health.

But here’s what the latest evidence reviewing the effectiveness of BMI in diagnosing ill health found: it grossly overestimates the number of “unhealthy” people whose BMIs are in the overweight and obese categories.

Just to recap, the BMI is your weight in kilograms divided by your height in meters squared. It was developed as a way to compare weights in large populations, not a medical diagnostic tool. People in the 18-24 range are considered “normal weight” while people in the 25-29 range are considered “overweight” and over 30 are “obese.” To give you an idea of how arbitrary these ranges are, they were all adjusted downward in 1998, upon recommendation by the NIH Obesity Task force, despite all the available scientific evidence that actually pointed toward raising the ranges. According to one Task Force member, “We were pressured to make the standards conform to those already accepted by the World Health Organization” (Health at Every Size, Lindo Bacon). And the International Obesity Task Force who recommended to the WHO the cut-off of 25 for the normal weight category received funding from pharmaceutical companies who made diet drugs. So the ranges are, you know, totally scientific.

It’s totally shocking, then, that this simple math equation, whose ranges were defined more by politics than by science, doesn’t totally tell us everything we need to know about a person’s health, right? That was sarcasm, by the way.

I’ve written before about how problematic it is to rely so heavily on mathematical equations in relation to our bodies, especially when it comes to weight. There’s a ton of stuff we still don’t know about weight regulation (witness the continued insistence on weight loss for health when it’s been shown over and over to not work long term for most people) and while we mostly have the same parts and general bodily functions needed to live, there can be a lot of variability from person to person.

So what this study found was that using BMI alone, “an estimated 74,936,678 US adults are misclassified as cardiometabolically unhealthy or cardiometabolically healthy.”

Oops!

This is not surprising, but now we’ve got yet more evidence to back it up. Yes, it’s one paper. Let’s get some more research behind this so we can finally put the BMI as a health measure to death. In fact, want to help? I just found out about the ongoing study called Health Registry of Obesity (HERO) by the same study authors of the above-mentioned BMI paper.

By the way, I don’t believe that good health is a measure of worthiness or an obligation, so if you are fat and unhealthy or thin and unhealthy, you have the exact same rights to medical care and everything else that healthy people do, without being penalized. I am interested, however, in stopping the lie often perpetuated that fat=unhealthy and thin=healthy. People are being denied important, life-changing operations (kidney transplants, knee surgeries) simply because they are in the wrong BMI category, despite otherwise good health. (Ironically, no one hesitates to perform bariatric surgery on fat patients. Hm….). This needs to stop.

I know many doctors; most of them want to do the right thing (at least the ones that I know), but usually they are under pressure to move fast and work cheaply. The BMI represented a cheap, quick shortcut to preventative health care for them. But now we know it’s bunk. And we have real, useful tools at our disposal: blood pressure, blood sugars, lipid panels, insulin resistance, c-reactive protein (a measure of inflammation in the body). These are the indicators that the study authors used, and they are what our health professionals should be looking at before they declare us sick or not sick. Because this is not only a huge problem for fat people who are over-diagnosed and prescribed an intervention that fails 95% of the time, it’s a huge problem for the normal-weight people who are not being diagnosed at all.

My guess is, this paper won’t be the end of the BMI in medical care. It’s probably going to take a lot more scientific study (much of which already exists), head banging, fist wringing, and just plain shouting to get through a resistant medical establishment. But it’s a good step in the right direction.

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