Why BMI is flawed – and how to redefine obesity
nature.com
nature.com
For example, the max 'healthy' weight for a 6'10" person is 230lbs.
That's more or less Kevin Durant's draft stats and he looked like this:
'weight (lb) / [height (in)]^2 x 703'
and all the talk of "it's the cheapest and easiest way" ignores the fact that we could add other cheap measurements (like hip and shoulder circumference, or Age) or we could reexamine the formula without adding new variables to make it better.
It's fine if BMI breaks down at the 99.99th percentile, doctors can use their eyes.
Glossary:
ADP: air displacement plethysmography
BIA: bioelectrical impedance devices / bioelectrical impedance analysis
Reference: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8528114/
https://upload.wikimedia.org/wikipedia/commons/0/05/Arnold_S...
We studied a sample of 6,510 white subjects (2,154 men, 4,356 women), aged 18–80 years, including patients visiting our department. The study was performed to valuate the usefulness of a new equation: BF% = –44.988 + (0.503 x age) + (10.689 x sex) + (3.172 x BMI) – (0.026 x BMI^2) + (0.181 x BMI x sex) – (0.02 x BMI x age) – (0.005 x BMI^2 x sex) + (0.00021 x BMI^2 x age) where male = 0 and female = 1 for sex, and age in years, developed by multiple regression to predict BF%
This online calculator claims to use this algorithm:
https://www.omnicalculator.com/health/body-fat
Reference: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3263863/pdf/383...
Pot belly guy measures his waist at his ribs, pear shaped lady rests the tape above her saddle bags, tape isn't level this time, tape is slack that time, etc etc
It might only require tape and might have useful health correlates given good values, but getting those good values is just not something epidemiologists or even clinicians can rely on in the field.
When I was younger and in shape bulked up with single digit body fat it was not useful. I was obese/overweight in BMI only.
Now that I am more sedentary due to office job and less time because I have kids it’s a useful gauge to help find my ideal weight.
Like you said, the complaints are always from the population for whom it is accurate. They're usually in denial and typically vastly overestimate how much muscle they're actually carrying underneath their visible layers of fat.
Check the reddit/bulkorcut subreddit for some examples. If i had to spitball, i'd say only 20-30% of people that post their actually have a good idea of where they are in terms of BMI and body fat. There is an alarming amount of people that i'd consider their physique in the top 1% to top .1% in the world that believe they are fat. And there is an extremely alarming amount of people who are likely 25%+ and even sometimes 30%+ body fat that think they're in a healthy BMI range and ask if they should bulk.
TL:DR - BMI is more accurate than not for people who need the scale. And if anything, it underestimates obesity for people who may not weight enough to be overweight, but still sit at 30%+ body fat.
Something I've observed is that people don't perceive weight on an absolute scale, but relative to other people. Really skews things depending on their local obesity rates.
That is not necessarily true. There are plenty of casually fit people around. They tend to be active but not necessarily all over fitness literature. They tend to be lean and have above average muscle composition, though are not necessarily "muscular".
These people will tend to receive an "overweight" analysis at the doctor's, and they might actually believe there's something wrong with them. I've seen that happen. I had to explain why BMI is bullshit to many friends who came out of an appointment thinking they had to change their diets despite being quite fit and healthy already.
You may argue that the doctors should know better but the unfortunate practical reality is that a good amount of them either don't, or just don't care enough to clarify it to patients. It's a lot easier to print out a report and hand over a set of general recommendations based on it.
> These omissions are why the AMA adopted a policy in June to downplay the clinical use of BMI, calling it “an imperfect measure” that has been used for “racist exclusion” and that has caused “historical harm”.
That sounds like the root justification for avoiding BMI.
Mine seems to correctly report my body fat going up or down as I gain or lose weight.
If I'm getting stronger and the body fat readings are consistently going down that's what I care about.
So if you take someone (or two people) slim, small frame, and compare them piling on muscle vs. fat, the former is obviously healthier, and yet will see them be 'overweight' per BMI far sooner.
I don't have any clinical experience, IANAD, so take with mountains of salt etc., but I'd have thought a simple change that would go a long way would be to have say three BMI ranges instead of one (well, for a given country anyway!) categorised by eye-balling or self-reporting.
We need a simple proxy method for estimating the healthy weight range.
Obviously not as good as actually measuring muscle mass, or anything more scientific, but I think it would be a pretty trivial way of making the BMI estimate another chunk better.
The assessment you're trying to make just isn't reliable.
Perhaps add another variable like volume to distinguish fat and muscle (measured as Archimedes in a bathtub), but as you notice it break the reason that makes BMI so popular. From the article:
> “BMI doesn’t cost anything — it’s cheap and quick,”
> "That’s especially concerning with the exploding demand for new anti-obesity medications. In the United States, a person can be prescribed semaglutide for weight loss simply if their BMI is 30 (the lower bound of the obesity category) or more. With a BMI of between 25 and 30 — the overweight category — a person needs just one weight-related condition, such as high blood pressure, to get a prescription. “If you have 15 minutes in a visit,” Stanford says, “you’re going to follow the BMI … instead of really taking the time to see ‘Do they need the medication?’”
For a list of more accurate measurement approaches:
> “BMI is a somewhat crude metric for determining health risks,” says Susan Yanovski
Of course it's crude, it's a single number, as opposed to thousands of potential risk factors unrelated to BMI.
> Adult BMI charts don’t reflect this variability. And the cut-offs between BMI categories — which are the same in most countries that use the index to diagnose obesity — are “something between science-based and arbitrary”
Again, not the point of BMI. You can have 40% fat or 42% fat in your body, you're still obese, and that means increased risk and a call to take action.
> And because BMI was developed using measurements from white people, people of colour “don’t quite fit into these narrow parameters”
...Commenting this just means stooping down to their level, so I won't.
I understand that personal weight is an emotional topic for people, but holy crap is this article an awful strawman-fighting pile of garbage.
Nature, you can do better than that.
You do have one, right?
While variations may be present, the fact is that someone who is race X and position Y on the BMI scale in the obesity category still has the same risk factors from increased weight as someone who is race Z and position Y.
You do have hard numbers about what the differences are in terms of percentages for the risk factors.
right?
I asked a specific question. Saying “yes, lol u” is a pretty lame response.
Sure, there are and will be variations among populations (and individuals), but that variation is less meaningful when 99.999% of obese people are at unhealthy weights.
Instead, it appears to be a con to excuse unhealthy behaviors, particularly by playing the trump card of "that's racist!" as the Nature article does.
In the long run, I suppose I support a significantly less stringent definition of obesity, to enable broader access to GLP-1s, but not because, unlike a fair number of posters, I’m desperate to tell people they’re fat.
Without even reading the article I already knew there would be some identity politics nonsense underpinning it. Because it’s Nature, and they have no credibility anymore.
For an adult male the test is to:
- Bench press 80% of your weight
- Squat with 100% of your weight
- Dead lift 150% of your weight
Yes, this is muscle/power specific and not good at all for children and the elderly (and women...), but being a lower-bound and "auto-scaling" (because % of body mass) and not requiring any medical equipment...
Perhaps give it a try next time you're at the gym :-)
What is this supposed to be an approach for? Overall fitness level?
Although im going to add a disclaimer for anyone reading this that wants to give this a try.
According to StrengthLevel, which is community sourced stats, for an average male on their site which skews exclusively towards people who do strength training here are the bodyweight ratio's for beginners, who are usually classified as AT LEAST a month of training to 6 months
- Bench press .50x of weight - Squat .75x of weight - Deadlift 1.00x of weight
Moving up to novice which is 6 months to 2 years the numbers are in line with what you shared. So people who haven't lifted should definitely not try to test their strength by going straight to novice numbers on the bar. They WILL hurt themselves if they put anywhere close to that weight and have no spotter or spotter bars.
First lemme be explicit that I didn't imagine anyone sane who has not been to a gym ever - or even in a long time - to plop in ~100% of their weight for anything. Warm up and going there in increments was presumed by me as the standard procedure.
> - Bench press .50x of weight - Squat .75x of weight - Deadlift 1.00x of weight
Yeah, I wasn't sure about the percentages, just the general relation between them. My fault here most likely! I don't remember how many lives ago I heard about this "test". Your numbers sound reasonable too.
But it was clearly after I done my work (out, over 3 years) of getting from "nothing" to "something".
> What is this supposed to be an approach for? Overall fitness level?
I'd say more of the power (strength) level. There is a lot more to fitness.
Apologies for not putting in more details in my original post -> now I _bow_.
I imagine the reason it isnt used is because the surface is complex. Its not as simple as standing on something writing a number, standing next to something and writing a number. You need to keep the angle correct and make sure the starting location is correct.
Wonder if you could use 2 side photos to calculate it, or 1 overhead.
I cut down to 173lbs, and did the cycle again. The following year, I timed my cut better so I was only 185lbs at the time of the screening, got 20/20.
The value of BMI in epidemiology is fantastic because it applies well for almost all normal people and is easy to capture all over the world.
The clinical value is weaker, but mostly because people like to rationalize that they too are also in exceptional athletic form. By definition, most are not.
"For instance, Asian people tend to be at a greater risk of conditions such as heart disease at lower BMIs than are white people, probably owing to differences in body-fat percentage and distribution. The WHO recommends that Asian populations use lower BMI cut-offs for overweight and obesity, which several Asia-Pacific countries now do."
I'm fat. It's that simple, I'm fat. Why am I fat? Because I ate too much and exercised too little.
Why did I eat too much and exercise too little? Well, there's not really one reason; portion size norms are kind of bad here, I ate a lot at night, I stress-eat sometimes, and my upbringing didn't instill good exercise or eating habits for the most part. Doesn't really matter that much, at least not in my case.
How do I know that I am fat? By looking down. I have fat all over my body. And I've lost a lot of weight recently, but I can look down and see that there is more to go before I am rid of the remainder of clearly excess fat on my body. I understand that some people do have a tendency to eating disorders and a distorted view of themselves, but that's not a problem I've ever had, so I'm fine relying on my own senses.
So needless to say, I don't really give a shit about BMI. I think most people ought not put that much stock into it, because most people can, if they're being honest with themselves, tell if they're overweight by looking in a mirror.
But... I think that if anything, the big problem with most people is not that people are too concerned with BMI, it's that people are not concerned enough about the side-effects of their terrible diets and lack of exercise. I do not think that our standards are overall too high, I think our standards are too low, at least here in America. Granted, I'm not exactly a health nut, but seriously: when I was talking to my mother, and I said I have a goal to get down to at least 180 lbs, she thought I would look anorexic at 180 lbs. I had to show her pictures of people who weigh 180 lbs to get her to believe that that's a completely reasonable target weight.
Some segment of the population definitely are too obsessed with BMI and weight loss, and probably the same for some health specialists. But for all of the hand-wringing about that issue, and movements like HAES, somehow people seem to miss the part that your horribly unhealthy habits are not more healthy just because BMI is an estimate and not some sort of absolute fact. Just taking a shot in the dark, if your BMI is >35, I would guess that regardless of the population-level inaccuracies of BMI, it's very likely a good indicator of an unhealthy lifestyle, or at least, a dramatically unusual physique. If I am wrong, I'd love to know.
But what's more important is not eating absurd portions of junk food every day, and getting up and walking once in a while. The whole culture here of "dieting" as a thing you do to lose weight rather than as a lifestyle adjustment is crazy. I know it's kind of cliche by now, but as someone who mostly ignored this sort of thing for years, even I must admit that my view of what a healthy diet and lifestyle looked like was very distorted by weird American cultural things. Amusingly, the only thing that actually opened my eyes to a lot of it was realizing how much I didn't agree with some of the things that HAES advocates on the Internet said. In a roundabout way, it helped me stop rationalizing my weight problems internally and realizing that my weight problems were lifestyle problems, by virtue of almost spite.
I'm not sure how much of a hot or cold take this rant was, because I generally do my best to avoid reading too many online flame wars about lifestyle things, so I apologize ahead of time if this rant is just full of either flamebait or cliches, I genuinely do. But, this article really annoys me, because wow, I don't understand how BMI continues to be such a massive source of concern as obesity continues killing the shit out of us and ruining our quality of life. I'm starting to feel like there's got to be some crazy conspiracy going on.
I think it depends on the person. I diet once a year and that diet keeps me at 6pack abs for almost the entire year until around November/December. A 3-4 month diet allows me to eat whatever I want the rest of the year. This gives me a chance to grow muscle and not put brainpower into food.
When you are far overweight and you can't quite see the finish line, maybe it is a lifestyle thing.
But the culture of fad diets and "90 day weight loss" routines as a cure for their unhealthy lifestyles is ridiculous. The lifestyle a lot of people are living is not healthy to do for an entire month, less most of the year. And I'm not sure I'd say that's "most people" in America, but I'd venture to guess it's a whole lot of them.
I exercise 2x a week at worst, 6x at best.
I can be literally fat (210lbs) and still have 4 abs showing. Perks of 400lb deadlifts. Its that if I want all 6 to be showing, I need to be around 180lbs. Get down to 170lbs and that buys you a few months.
I can imagine it’s hard in the states (although here in the UK we’re on the same track) with huge portion sizes, lots of fast food and lots of processed food. It’s hard, and expensive to cook and eat healthy these days.
I lost a bunch of weight recently. What really worked for me was, - fasting between 6:30pm and 10:30am. No late eating was a game changer for me - exercise. I do HIIT classes 4 times a week - trying to cook healthier - cut down alcohol
you can't give any advice - everything is "correlation does not equal causation" or "unless you have a peer reviewed paper you can't tell me anything"
eat vegetables and exercise. if you can't even make that limited leap of obvious common sense, then just accept your reduced expected lifespan and overall poor quality of life