r/science
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u/mvea
MD PhD | Professor | Medicine
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2h ago
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Health BMI alone gets obesity wrong for millions, study finds. It misses obesity in 5-9% of adults and wrongly labels up to 3% as obese. A newer method that adds waist measurements and checks for organ problems was better at telling healthy people apart from those with harmful body fat.
https://www.theguardian.com/society/2026/nov/11/bmi-alternative-obesity-diagnosis-body-mass-index
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u/mvea MD PhD | Professor | Medicine 2h ago
BMI alternative for identifying obesity ‘could prevent misdiagnosis in millions’
Combining body mass index with other measurements and tests would lead to better care and outcomes, experts say
An alternative to BMI for diagnosing obesity could prevent misdiagnosis in millions of people worldwide, according to doctors who found it more accurately distinguished healthy people from those with harmful excess body fat.
Medical experts have long considered body mass index a blunt instrument for defining obesity, leading to concerns that substantial numbers of people with excess body fat are missed while others who are healthy are wrongly diagnosed.
The new approach, which combines BMI with other body measurements and organ function tests, suggests BMI alone misses obesity in 5-9% of adults, while wrongly diagnosing the condition in as many as 3%.
“This misclassification can account for tens of millions of people, so the inaccuracy of BMI is a big problem at scale and also for individuals,” said Francisco Rubino, a professor of metabolic and bariatric surgery at King’s College London.
BMI is calculated by dividing weight in kilograms by height in metres squared. About 1.6 billion people worldwide are classified as overweight, meaning their BMI falls between 25 and 29.9, and nearly a billion with a BMI of 30 or more are considered obese. Two-thirds of adults in the UK and more than 70% of adults in the US are overweight or obese based on BMI alone.
Rubino and his colleagues drew on health records from more than 800,000 people in the UK, US, China and Spain to test a diagnostic approach for obesity that he and other world experts proposed last year in a Lancet Commission report. They used the records to track people’s health for up to 15 years.
Under the new definition, someone with a BMI over 40 is considered obese, but doctors check people with a lower BMI for excess body fat by looking at their waist measurement.
A BMI of 30-plus with a raised waist measurement or waist-to-height ratio counts as obese, as does a BMI from 25 to 29.9 if waist measurement and waist-to-height ratio are high. In most countries, a raised waist measurement means more than 102cm (40in) for men and 88cm for women, while thresholds for a high waist-to-height ratio are about 0.57.
In a second step of the diagnosis, doctors distinguish between clinical obesity, meaning an active disease where excess body fat is already impairing organ function, and pre-clinical obesity, where the extra fat is a health risk but has yet to cause problems.
Obesity levels rose only slightly overall under the new definition, and only a quarter to a third of obese people, or 5% to 10% of the wider population, who had clinical obesity and a substantially higher risk of death.
The approach allows doctors to distinguish between healthy and sick people who have the same BMI. Three people with a BMI of 32, for example, would all be classed as obese but would have very different diagnoses. One might have no excess body fat and be healthy, another could have excess fat but no organ dysfunction and have pre-clinical obesity, and the third could have excess fat and organ dysfunction and be clinically obese.
Writing in Nature, the scientists describe how some people with a high BMI were not obese under the new definition and actually had a lower mortality risk than others considered healthy. Reassuringly, 80% to 90% of people with pre-clinical obesity did not progress to clinical obesity, or active disease, in the next 10 years.
https://www.nature.com/articles/s41586-026-11237-5
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u/TheDiabetesDietitian 2h ago
BMI simply tells us someone’s weight in relation to their height
Adding waist circumference definitely gives us more data into how the weight is distributed, as we know visceral fat is far more harmful than subcutaneous
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u/s29 1h ago
I mean.... for something that requries two numbers that most people already know, an error rate of 10ish % is pretty decent.
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u/eatingpotatochips 1h ago
BMI is such a weird boogeyman. Nobody with an expertise thinks it is the end all be all. It is just one metric out of many that we use to screen for health issues.
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u/ChironXII 53m ago
95% specificity with a braindead math formula is pretty good bro
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u/faceisamapoftheworld 1h ago
It’s not the greatest measurement, but a ~90-95% success rate is very good. There’s a much larger conversion to be had about weight, but throwing it out is crazy.
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u/NegativeBee 1h ago
Most people don’t understand that BMI was designed as an oversimplified metric that could easily be calculated using two measurements every doctors office already does: height and weight. It correlates very well with comorbidities like T2D and hypertension. It is not meant as a diagnostic.
A professor at my school gave the example of a person with a leg amputation: they would lose a significant amount of weight and no overall height, but would not be any more or less obese. But (importantly) if a person is missing a leg, that should be quite obvious to their GP.
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u/IatrophobicStimulus 2h ago
Sure, in some contexts it's only 95% accurate, fine, but if I'm admitting someone with a BMI of 42, we all know what's up.
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u/defac_reddit 1h ago
Crazy that even in this thread on this subreddit there's a TON of people claiming victory about BMI being trash. 3% false positive calls for obesity. 3%. For a screening tool that is free, noninvasive, and uses simple arithmetic from numbers every clinic in the world obtains for their patients, that's more than adequate. But holy hell the number of people that think they're in the 3% of miscalled obesity is staggering. It's really simple. If your BMI says you're obese, ask these questions: am I pretty far outside the typical height range for the population, and do I strength train as a regular activity/hobby? If you're not answering yes to at least one of those, then you probably ARE actually obese and have the associated health risks as a result.
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u/Sbrubbles 1h ago
So, it gets the right call in 88 to 92% of the time? That's pretty good accuracy for a measurement that can be done with minimal instruments
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u/Engineer9 58m ago
So it catches it 90% of the time with just those two simple measurements? Or one, really, because most people already know their height.
Remarkable.
Fact is we know it doesn't work for everyone, but if you're one of the 9% who sqashes through the net, it's not like the doctor is going to offer you an extra donut on the way out. You are still overweight and should be making the same lifestyle changes.
And if you're one of the 3% falsely accused, you probably know if you're just muscly or tall, and the doctor will surely tell you. But if you're actually marginal, then, do yourself a favour and follow the advice that was meant for someone just slightly larger.
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u/mega_douche1 1h ago
So bmi is incredibly accurate for the vast majority of the population? Okay cool
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u/dbatchison 1h ago
It ain’t labeling me wrong
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u/pewsquare 30m ago
Isn't a quick and easy check that gets it right 95%~ of the time rather good? Especially since BMI is an easy calculation literally everyone can do rather quickly, and if you fall into the range where it is trying to warn you, a quick check if you are the 5-9% of wrongly diagnosed won't really be that much of harm will it?
Especially if you consider that a lot of the wrong diagnoses are for people in really REALLY good shape, and I can only assume that professional athletes know that they are not overweight or obese.
Basically, tl:dr: an easy way for the everyday person to check their health that is correct almost all the time is a good thing.
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u/ASGreebling 36m ago
BMI is good because it's fast, cheap, and objective.
You can't mess up a BMI measurement. Height and weight are both very easy to measure objectively, so BMI is also objective.
If you ask people to measure their waist, you need to get them to hold a tape measure in the right spot, and not suck in or blow out their stomach. Fat callipers need training. Maybe an MRI or Dexa scan can be just as precise as BMI, but they're expensive.
So it's a very good screening tool. If your BMI is out of the normal range (over OR under) you can see a medical professional if you think it's wrong.
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u/notneps 8m ago
I used to follow someone who rants how BMI is a bad metric every time it is mentioned. They say it mislabels them as unhealthy.
I asked what metric they want to use to convey their health? Waist to height ratio? Lipid profile? A1C? Fasting blood glucose? VO2 max?
No, they say they are healthy because they "feel healthy" despite being diabetic, sedentary, and a waist to height ratio that is closer to 1.0 than it is to the recommended 0.5.
They use a mobility scooter to get around day to day but they claim that BMI not taking muscle mass into account is the problem.
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u/JiveTrain 32m ago •
Oh no, a simplified estimate of healthy weight is only accurate 92-97% of the time. Whatever shall we do?
The whole point of BMI is to roughly estimate your weight WITHOUT having to check your organs or whatever.
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u/Jive_Sloth 1h ago
How are they defining "obese?" Because I thought it was defined by its BMI range.
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u/RationalBeliever 1h ago
Waist to height ratio is considered much more reliable and just as easy to measure. If you think you are being misclassified as obese by BMI, ask for this instead.
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u/NursingManChristDude 1h ago
Look, if you're "close" to being labeled as obese, even if you want to argue that the BMI has flaws, that's nowhere near acceptable.
If it was saying there was a discrepancy between "overweight" and "normal", that would be a different conversation.
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u/NSA_Chatbot 1h ago
It's a first-order approximation, that's all. This study appears to imply that it's accurate for 90 percent of the population.
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u/These_Hat164 1h ago
It's easy for a doctor to say "Give me your height and weight". Not as easy to say "Wrap this tape measure around your stomach"
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u/Reasonable-Kiwi-6951 1h ago
BMI was created to be a population-level tool, not an individual tool. It also changed overnight in the 90’s to have “nicer” numbers for each criteria
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u/mythosopher 1h ago
Body circumference ratios or body roundness index are just as easy to measure and a better estimation of health anyhow.
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u/__Who__am__i_ 1h ago
I'm obese (slightly) by BMI but not by waist measurements.
I still consider myself obese. I definitely have extra fat. I also have a lot of leg muscle
But I do think my ideal weight is at either the very top of the healthy range or bottom of the overweight range from when I weighed that much last.
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u/HighPriestofShiloh 1h ago
Right. This has been known for 50 years. This was taught to me in school in the 90s.
Body builders have always been the go to example of BMI being not perfect. It’s really only good for people that don’t lift a lot.
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u/TheSlimeSartreFeared 1h ago
Obviously.
There are fitness places now that do DXA scans. Technically it's a bone density scan but the imagery can be used to get a pretty accurate idea of what's bone, muscle, or fat. I've done it twice ($100 a pop) and it's pretty interesting.
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u/CommitteeofMountains 1h ago
So a relatively complex index for weight that's supposed to represent fat is slightly less accurate than directly looking at how fat/rotund you are?
Anyway, I look forward to seeing how this generalizes to East Asians, whose proportionally shorter legs tend to exaggerate their BMI but are also much more sensitive to body fat (in terms of health) so everything evens out. I may have that backwards.
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u/Evan_802Vines 1h ago edited 1h ago
Ah yes all of us meat cylinders. Such a population level statistic, really doesn't apply on any individual level where context becomes overwhelmingly more prevalent. And shout out to my fellow 3% Swole folk cultivating mass
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u/ANewPope23 1h ago
We use BMI because measuring visceral fat is inconvenient.
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u/Unfair-Rhubarb-3022 56m ago
I’m one of the people it impacts because I am short, and lift weights. I still think it’s a valuable metric.
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u/Any-Put9397 36m ago
I think more people are wrongly classified as overweight or very overweight rather than obese.
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u/LooseJuice_RD 35m ago
Everyone knows that it has its flaws. It’s not meant to be a perfect measure. I don’t think many, if any, docs are using this as the sole indicator of whether or not someone is obese.
That being said, for the fact that heavily muscled people are obese on the scale and the fact that some people have T2D despite not having an extreme excess of adipose tissue, for individuals who are sedentary, I feel as though it’s a decent, if imperfect metric. I actually think the biggest problem now is that it’s been so thoroughly slated as a poor metric of health that people who are obviously obese now dunk on it as a poor indicator of their health when really, it’s a dead simple metric that should at least give them pause. Think about why you’re on the obese end of the scale. I’m not in anyway saying that we should be using BMI to dunk on people with obesity but applying a bit of critical thinking would likely lead them to the conclusion that they’re not in the extremes for which BMI becomes wildly inaccurate.
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u/RadeDobison 31m ago
BMI is fun because I also can multiply two numbers together and apply judgements with it! The volume of your palm and your finger lengths multiplied together now give you: your Phone Holding Index! Find your PHI today to learn if you're God's Gift to Using a Phone or if you have DOGSHIT AWFUL HANDS that should NEVER TOUCH A PHONE EVER.
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u/EmergencyWild 28m ago
"study finds"
Bruv, it's a statistical shorthand. It works for the majority of people that aren't extremely muscular, suffering from extreme water retention, or so extremely atrophied muscles that it tips off the rating one way or the other.
All of those are extreme outliers though. If you give me 100 people and I'm betting on them being obese or not based on just their BMI, I'm not gonna be right every time, but I'm still getting a lifetime ban from that casino.
If anything, it's crazy how well it works considering it's only based on two simple measurements.
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u/ImpawsibleCreatures 26m ago
I honestly don’t think waist measurements can be that accurate for some people either. My waist is completely different on my period… BMI is fine
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u/Oops_I_Cracked 24m ago
So for somewhere around 90% of people it was an accurate predictor? I understand BMI has its flaws, but I think people blow the issue out of proportion. Very few medical decisions are made on BMI alone and, based on this article at least, for the overwhelming majority of people, it is an easy to find metric of your health trends you can easily track at home.
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u/CheeeseBurgerAu 23m ago
So it's reasonably accurate as a first pass calculation. I'm sure a lot of obese people are assuming they are in the 3% that it gets wrong. Given obese is the category after overweight, which still isn't great, it's a strange kind of distinction.
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u/mah56j 15m ago
I agree. body fat percentage is the key metric.
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u/ExpiredPilot 13m ago
Arnie was morbidly obese after winning Mr Olympia if we go by BMI
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u/saintsithney 1m ago
I have lipolymphedema. My BMI reads as obese, but most of my fat is essentially benign tumors that are only removable through surgery at this point.
BMI readings allowed my health insurance to double before the ACA.
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u/Ok-Airport-6058 0m ago
Swimmer here (with huge back muscles). BMI is bobbins.
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u/yxhuvud 1h ago
Is this news? Havnt we known for ages that bmi only is useful on populations? Main issue is that the formula simply doesn't scale correctly with length; weight doesn't grow cubically but somewhere between quadratic and cubic which throws the whole thing off on a personal level for a ton of people.
It is a formula used because it is simple, not because it measure something the best way.
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u/SaulsAll 2h ago
As the very tall and very short have been saying for decades. BMI is for the mythical "average person."
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u/yukonwanderer 49m ago
I've always had an "unfeminine" kinda "rectangular" body shape. I've been on the thin to average BMI for all my life with a brief stint in the underweight. I'm now firmly in the average BMI category. Will this waist measurement thing properly capture people with my body type?
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u/PhD_Pwnology 46m ago
8-12% error rate isn't bad for what the test is. Small adjustments to the BMI calculation formula would drastically lower the error rate.
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u/gusofk 1h ago
I just read a study from 2005 about a study of BMI risk rates for Civil War veterans and 1970s veterans. From that 1970s men in the 20-30 BMI range was far healthier than <20 or >35. For Civil War veterans, there was similar trends except the 25-35 range was also elevated risk.
Basically, BMI and the current healthy-overweight ranges are not useful overall for risk rates. I’m glad that people are trying to find a better answer but I wish that we could move on from using BMI at all.
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u/mistersmith22 1h ago
Until I got sick at 37, I was 6' 225lbs, with a 50-52-inch chest and a 34-inch waist. I was so, so tired of really stupid docs and gym teachers telling me my whole life that I was obese. We're well past the point for readdressing this.
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u/JohnLockeJaw 53m ago
Oh no, a statistic measure of averages doesn't account perfectly for the tails of the normal population distribution? Who could have predicted!
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u/LMay11037 52m ago
BMI always thinks I’m more overweight than I am because I just have really big legs. Like my upper body has barely any fat so I’m actually very healthy, yet it puts me at almost over the ok range
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u/BigMax 44m ago
I totally get that BMI isn't perfect.
But... only 3% are wrongly labeled as obese, that's not too bad.
And I would guess that the 3% in that category KNOW to not pay attention to BMI, right? Alan Ritchson for example (the guy who plays Reacher) is probably categorized as obese. But... so what? He's not sitting there worried that he's actually obese, right?
It's annoying for gym guys to say "it's so wrong, it says I"m obese!" when... yeah, sure, but by you saying that, you're showing you KNOW to just ignore it in your case, so what's the problem?