Every clickbait article in the world tells you that BMI is a terrible metric and an awful way to measure obesity. Is it, though? No. BMI is far better than you think and its use in medicine is fully justified.
However, BMI does have a true fatal FLAW that most people aren’t aware of…
If you value truth more than comfortable lies, this video is made for you. “Good news about your bad habits” is a type of clickbait where someone tries to reassure us that our bad habits are just fine, really. I thought I would highlight an example of this in action, that is, BMI and criticism thereof, most of which are profoundly silly, driven by clickbait, wildly exagarrated concerns and people wanting to be reassured about their bad habits.
Why does BMI provoke such a reaction and stern opposition? That is remarkably simple to figure out – BMI is trivial to calculate, which makes it hard to ignore and it tells you that you’re fat. If given a magic body sculptor that effortlessly let you choose your body type, almost no one would choose to be, so realizing that you are is bad news for most people, something they don’t want to hear.
Of course, if you disagree with that outlook, that’s fair. They’re your choices and if you accept their consequences, then it’s not my place to tell you how to live your life. However, I do want to encourage you to make informed choices about your future and being in denial about what BMI means for you is not making an informed choice. If you’re truly at peace with your decisions, you should not have any problem accepting what they mean in reality.
I know that this topic is very charged for some people. If that’s you, take a deep breath and let go of that tension. I’m not a threat to you. You’re safe. The only thing that can happen here is that you’ll know more than you did before.
Bad Argument #1: Athletes
I’d like for you to imagine the following scenario. You are a doctor. Someone comes into your clinic. They want a comprehensive health check-up, so you oblige and perform a battery of tests for them.
You find the following:
The patient has an enlarged heart.
They have an abnormally slow heart rate – known as bradycardia.
The blood tests you perform show signs of kidney and liver damage.
The patient has been regularly coughing a tremendous amount recently.
While no doctor would perform a muscle biopsy or a similarly invasive test just because, this is a thought experiment, so let’s assume that they did – their muscle biopsy shows signs of intramyocellular lipids. A lot of them. That is, fat in their muscle cells.
Last, but certainly not least, they have a BMI that indicates they’re overweight.
Sounds rough. How bad is it?
Depends. Is the patient a strength and endurance hybrid athlete, who ran a marathon a few days prior to taking the tests or otherwise had an epic workout very recently?
If so, it’s probably not a big deal. The enlarged heart with an unusually slow heart rate is a condition known as athlete’s heart – it’s benign, arguably even beneficial. Transient elevations in kidney and liver markers are in line with expectations for hardcore endurance workouts. An excess of intramyocellular lipids would indicate insulin resistance normally – a sign of developing diabetes – but athletes have high levels of intramyocellular lipids while maintaining their insulin sensitivity. This is known as the Athlete’s Paradox. Coughing after exercise, known as post-exercise asthma is actually more common in elite athletes than the general population.
All in all, our athlete is unlikely to be in dire straits. As a responsible doctor you tell your patient to take it easy for a week, do no workouts and you’ll run the tests again to be sure, but there’s no reason to panic yet.
What if they’re sedentary and overweight, however? Then you’ve got some seriously bad news to break to your patient. Multiple failing organ systems – heart, liver, kidneys. Diabetes on top of that and likely a serious infection.
I assume you wouldn’t ignore these signs of disease just because they can be benign in athletes, so what sense does it make to ignore BMI?!
Bad Argument #2: Body Composition
The prevalence of obesity, PEDs and steroids have skewed people’s perception of what is achievable and normal as far as physique development goes. To be lean by body fat percentage while being overweight is the sort of thing that takes years of dedicated resistance training exercise, if it’s possible for you at all. It’s not easy to get that jacked. You don’t get there by maybe occasionally doing some push-ups.
To put it simply: if you’re overweight, but you don’t have a body that could convincingly play a superhero in a Hollywood production, you’re not a special athletic exception, you’re just overweight.
Bad Argument #3: BMI Is Racist??
Technically, yes, I suppose. Obesity has a different impact on people of different ethnicities – but that only makes BMI racist in the same sense in which sickle cell disease and skin cancer are racist.
This argument, in truth, a blatant attempt at manipulation. You can intimidate people by calling them racist, but you can’t do that to medical fact. Biology doesn’t care what you call it.
Bad Argument #4: BMI Is Sexist?
Men and women are, in fact, at higher risk of metabolic disorders at different levels of body fat. Men are obese at 25% body fat, while women are at around 33% body fat.
Men, however, tend to carry more lean mass too – which means that BMI roughly works out across genders
Bad Argument #5: The Inventor of BMI
There are a whole bunch of arguments that focus on the qualities or opinions of the inventor of BMI. They don’t matter. Dismissing inventions or ideas based on their origin is known as the genetic fallacy – judge an idea based on its own merits.
If you disagree and believe that we should care about the origin of an idea, Henry Ford supported Hitler – and the former is responsible for the invention of the modern factory, so I wish you the very best of luck in living up to those principles.
Bad Argument #6: Some Thin People Are Unhealthy, Some Fat People Are Healthy
This is not specifically about BMI, but that point comes up often enough to be worth addressing. Let us ask ourselves the question: are there some smokers who are healthier than some non-smokers? The answer is obvious – there sure are.
Does that mean anything for whether or not smoking is healthy? Of course not. Smoking is always unhealthy and excessive body weight is no different.
Bad Argument #7: But My 90-Year Old Grandma
Some people also have grandmas who lived to 90 while smoking. This does not mean they dodged the health effects of those habits. That smoking grandma who lived to 90 likely would’ve lived to a hundred if not for the smoking – and she’d have enjoyed a higher quality of life in her preceding decades.
Bad Argument #8: Metabolically Healthy Obesity
You may have heard this idea that BMI is flawed because there’s such a thing as “metabolically healthy obesity”. That state exists. Approximately 30% of obese people don’t have metabolic abnormalities, so no big deal, right? Does that mean that BMI is flawed?
This is, of course, absurd. Would you play Russian Roulette with 4 bullets in the revolver? If not, a 30% chance of being metabolically healthy should not reassure you either.
But it gets worse.
In this study, MHO evolved to normal obesity in almost fifty percent of patients at a median of twelve years follow-up. In this study, it was 44% of patients at nine years follow-up… but those are just the appetizers, so let’s skip the foreplay here – it’s time to look at a meta-analysis that compiles dozens of studies.
Given a decade, 85% of obese people are metabolically unhealthy. What do you think happens in another decade or two? How much do you want to bet on winning rounds of Russian Roulette over and over and over again?
Semi-Bad Argument #9: Just Use BF%
It is true that body fat percentage would gives you a better picture of your health than BMI. Here’s the problem – how do you measure it?
A truly accurate measurement would require the use of an MRI, but using them for this purpose is a non-starter. No one is going to use MRIs to measure fat, it would be absurdly expensive and wasteful.
Every other method of measurement is plagued by some level of error – potentially a serious one. Even the DEXA scan, which is the next best method(from a cost-to-benefit perspective), is not quite as perfect as we’d like.
If we’re criticizing BMI because it can give you an incorrect result, the same can be said of every other practical method of body fat measurement… and, it doesn’t matter in practice, because we don’t have an epidemic of bodybuilders. We have an extreme epidemic of overfat people – which brings us to the last point.
THE REAL FLAW OF BMI
Now, the moment you’ve been waiting for. What is the actual flaw with BMI? To get to that, I’ll need to introduce some background.
What you’re seeing on screen is a chart of the relationship between BMI and body fat – as determined by a DEXA scan.
Now, let’s make this chart a bit more clear. Above this line, we have overweight people by body fat, which is generally given as 17 to 18% in the literature. This line? Obese by body fat, 25% for males. The 25 and 30 markers for overweight and obesity by BMI are also shown.
Let’s get started with people who are false positives. How many people who are obese BMI but lean by body fat percentage do we see? NONE. Zero. Nada. Not even one. Now, I’m not saying that real people like that don’t exist – IFBB pro bodybuilders are, of course, real people. Consider, however, how rare they must be. This chart contains information on seven and a half thousand people, it’s based on NHANES data, a representative survey on the people of United States – and not one person in this group is “obese lean”.
Try to appreciate the magnitude of this situation. If you choose a random, representative sample sample of over seven thousand people, you might find that not even one of them is obese lean, as has happened here.
Now let’s take a look at these guys. Overweight, but lean. I call this the superhero group – if you’re in here, you can play a superhero in a movie. How many of them are there? 23. In this entire huge sample, you have twenty three people who are a true false positive. One in three hundred.
Now this? This is more of the supervillain group. These are the guys who are monstrously muscular, to the point where they are obese by BMI, but only overweight by body fat percentage. They’re still not healthy, but maybe not quite as unhealthy as BMI says. How many of these guys are there? 21.
In this entire huge sample, you have about 0.5% of people for whom BMI is slightly misleading.
Which leaves us with these people. Uh oh. We’ve covered false positives, now it’s time for false negatives.
These guys are normal weight by BMI, some are even underweight by BMI – but overweight by body fat percentage. There’s nearly seventeen hundred of them, 1691. This group is obese by body fat percentage, but they have a normal BMI. There’s over a thousand of them(1050).
This group is overweight by BMI, but obese by body fat percentage – there’s 1828 of them.
So what’s the takeaway here? What’s the real flaw in BMI? BMI is too nice and lenient. 4.5k out of 7.5k people – that’s sixty percent – are fatter than BMI says they are.
Those are not merely my own conclusions based on available data – the scientific literature agrees. BMI does not over-diagnose obesity. If BMI says you are too fat, you almost certainly are. The real problem with BMI is underdiagnosing overfat.
What happens if you include people who are overfat, but normal weight by BMI? It turns out that over 90% of Americans are overfat – and the rest of the world is following suit.
As I said in the beginning of this video, I’m not here to tell you how to live your life – it’s your life, your choices, your consequences – but you should be making those choices with eyes wide open.
Thank you for your attention. If you’re interested in more informed living and the truth, like and subscribe for more content.