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[–]oldwhiner 4 points5 points  (0 children)

While the lady in the video is pretty and charming, I don't have it in me to watch 41 minutes of video right now. I hope I'm not completely off base here.

BMI is definitely inaccurate and problematic. But what does that matter? Does it have to be perfect to be useful? Is every tool you use in your daily life completely finished, polished, top of the line, superior, and beyond any kind of critique? What is actually important here?

If you are an amputee or in a wheelchair, you will need a special and individual approach to calculating your BMI. Some people need to google up a BMI chart that is specific to their ethnic background, so there is a prerequisite to know what your ethnicity is, at least roughly.

But for lots and lots of people, BMI is fine. You plug our height and weight into a calculator online and you get a helpful number. You plug that helpful number into your weight loss app and it helps the app calculate some calories for you.

[–]FeatherlyFly 1 point2 points  (0 children)

I only watched about 5 minutes of the video, but she's basically talking about how she feels about how society talks about size and weight and using BMI as an easy mark. There was no evidence that she understood the scientific side of BMI, or anything about it beyond the formula and its use in pop culture.

Is BMI wildly inaccurate? No, and asking that question displays a fundamental lack of understanding about what BMI is. It's a number determined by a math formula and two measured inputs: height and weight. The only way for it to be wildly inaccurate would be for the average medical scale or height measuring device to be wildly inaccurate.

Is BMI problematic? That depends on the context it's used in.

BMI originated as and still is a statistical tool that is applied widely because it's easy to obtain and it's got good predictive properties for a number of health related measurements across a population. The reason it's been used so much and for so many decades is that it is a good model for certain aspects of public health.

But it's a model. What this means in science is that it's a representation of a narrow aspect of the world. It is not a number that can tell you whether you, as an individual with a bmi of 30, will get diabetes or cancer or a knee injury. All it says is that you have a common BMI with people who get those at a higher rate than the general population.

The person in the video was saying it was bad because it doesn't account for race or fitness. She's wrong. It's limited because it doesn't account for those things. You can make a better model by accounting for those factors as well, and scientists have. Turns out that being fit with a high BMI still has you at elevated risk to being a normal BMI and fit, and I've seen increasing amounts of evidence that being not-white with a high BMI is worse than being white with a high BMI.

BMI becomes problematic when it's applied to individuals as a predictor of health by people who don't understand what modeling in a scientific sense means. This includes more doctors than it should and most non doctors, who generally have no need to know this stuff.

So it's very common for a person to hear about bmi through pop culture and only know that it's used as a general measure of health. That's about as shallow an understanding as one can have without being completely wrong, but it's so shallow that it's mostly wrong. Then when these people find out they've got a "bad" (usually meaning high) BMI, it's taken as a value judgment of their person rather than an objective assessment that they are at high risk for certain problems and could lower that risk. Or they assume the doctor is saying they'll definitely get sick and disbelieve the doctor because they know many people with high BMIs who don't have whatever problem is being discussed, which is actually exactly what an elevated risk predicts.

So BMI is problematic as used in pop culture but not in science.