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turtle4499 t1_j54omxc wrote

Reply to comment by Hugh-G-Rection-Jr in The Benefits of Taking Vitamin D Might Depend on Your Weight | A reanalysis of a large trial found overweight and obese people might metabolize vitamin D supplements differently, leading to lower circulating levels by Hrmbee

>Uh a BMI of 25 is the cut off where you are no longer in the normal range and get overweight and not obese, obesity starts at a BMI of 30.

Again no its not. That's what I am telling you. BMI does NOT tell you if an individual is obese or not obese it tells you the percentage chance that someone is obese or not obese. The number is a gross oversimplification and always has been. Obesity is defined by body fat percentage. Sources that define obesity via BMI are 99.999999% of the time misquoting there usage in population studies.

At a population level because we know MOST people with a BMI of 35 are obese you can use that to separate out obese and non obese people. The 25 cutoff range is used because its very hard to be obese and have a BMI below 25.

As I stated this is something drs get wrong all the time and it causes this nonsense.

Here is the CDC who is properly describing BMI.

>BMI can be a screening tool, but it does not diagnose the body fatness or health of an individual.

And

>The accuracy of BMI as an indicator of body fatness also appears to be higher in persons with higher levels of BMI and body fatness. While, a person with a very high BMI (e.g., 35 kg/m2) is very likely to have high body fat, a relatively high BMI can be the results of either high body fat or high lean body mass (muscle and bone). A trained healthcare provider should perform appropriate health assessments to evaluate an individual’s health status and risks.

BMI is thus a indicator of odds of being obese and not a indicator of being obese except at the high ranges because at those ranges the odds of being obese increase high enough that few who get by are rare. At 35 you would be passing someone like brock lesnar/ dwayne johnson as obese. They are roided to the gills and would be a low percent of people with a BMI of 35 or above.

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OrneryLawyer t1_j55evcx wrote

You are really invested in stressing that someone high BMI might not be an obese person.

Sure, buddy. Anyone with eyes can tell if your 30 BMI body is due to being like the Rock or you know, just being a fat slob.

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turtle4499 t1_j55h6ux wrote

And when you make a study where you don't get pictures of the 10k people in it just using BMI doesn't give you the visual information. Congrats you figured out the problem.

BMI has a variable positive predictive value understanding what ranges are good for what situations is important. You would not want to use the same scores for screening patients that you would for measuring risk factors of obesity.

The closer you are to the lower bound there is a reduction in confidence. Which is why in research associating things related to being fat you don't use the 25-30 cutoff because it has too many false positives. Which systematically biases that population to have understated risks of being overweight. Studies that use better adjusted obesity numbers have confirmed this effect.

BMI at a cutoff of 25 is not intended to have low false positive rate its intended to have low false negative rate. Thats why its called a SCREENING TEST. Because almost 100% of people with obesity will have a BMI above 25 that includes many people particularly in the 25-30 range that are not obese. The range that is used is purposefully over capturing because that leads to better screening.

I am really invested in stressing the proper use of BMI. Because using it incorrectly is what causes people to ignore the number out of hand. This is a science sub where people discuss science like proper understanding of the statistical meaning of a BMI score.

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OrneryLawyer t1_j55mg3n wrote

You are overthinking this. If a patient stands in front of you with a 30 BMI, will you be able to tell at a glance if the person is obese or not?

What causes people to ignore the number out of hand is not "incorrect use of BMI," it's self delusion.

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Hugh-G-Rection-Jr t1_j55p008 wrote

Maybe someone hurt him with the BMI? I mean you need serious mental gymnastics to go over the point and overcomplicate a basic information that much… “incorrect use of BMI” and the whole study thing really nailed it for me, from where I am only fatasses that can’t cope with being fat say crap like that. Look I know this sounds hard but the people who made this stuff know a lot better than someone on reddit who is fixating on a borderline score that would give a false pozitive/negative, really man? Really? You think people with a bmi of 25.2 are going to go insane because they are classed as overweight (25-30 means overweight, like in a little bit over? not obese or going to die just a little fluffy?). I hope in the next comments he gets what screening test actually means and stop saying 25-30 is obese, please link me where cdc says that on the BMI 25 is obese not overweight also that obesity starts at 25 not 30 (TWO TOTALLY DIFFERENT THINGS) and I’m giving in my practice licence tomorrow morning. Please tell me all studies are perfect and please tell me a serious study where if there even aren’t pictures you don’t have staff to evaluate the subjects.

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Hugh-G-Rection-Jr t1_j55pfaj wrote

My bad, “society that bases too much on calculus and not enough on statistics” - while shitting on a statistic discovered and worked upon from 1850 while citing what fits the anti dr, anti scientists “i know better without having the slightest ideea what the numbers I’m ready represent” is pretty much explanatory to why. Also the real reason why that number is the way it is isn’t because false results, look down the street and tell me 3/5 people aren’t fat…

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SerialStateLineXer t1_j58vn89 wrote

What do you mean by "obese?" The medical definition is having a BMI in excess of 30. Arguably this is not a very good definition, but it's the definition that's used. So when you say that BMI is an imperfect proxy for obesity, it's not really clear what you mean by "obesity." Are you talking about a body fat percentage threshold? Waist circumference? Waist height ratio?

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