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Why BMI is flawed — and how to redefine obesity (www.nature.com)

submitted 1 year ago by [email protected] to c/[email protected]

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The main diagnostic test for obesity — the body mass index — accounts for only height and weight, leaving out a slew of factors that influence body fat and health.

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[–] [email protected] 28 points 1 year ago (1 children)

BMI isn't really flawed, it's misused.

It wasn't created to be used with individuals. It was created as a statistical tool to compare populations.

[–] [email protected] 7 points 1 year ago* (last edited 1 year ago) (2 children)

It doesn't really work at that either.

You can't use it tell if soccer players are fatter on average than rugby players or if Japanese people are fatter than Samoans. Or even if men are fatter on average than women.

But these are population questions.

[–] [email protected] 11 points 1 year ago (1 children)

I think that's just more examples of it being misused. BMI may be useful to compare populations in certain instances. It has value in being easily calculated based off data that is often easy to obtain. But, there are many situations where it would be inappropriate to use BMI for statistical comparison. That doesn't mean it's entirely useless.

[–] [email protected] 5 points 1 year ago* (last edited 1 year ago)

I mean BMI is also useful for comparing individuals in certain instances. That's why doctor's use it.

It's a crappy measure, but it's good enough for a range of use. You just have to be thoughtful enough to say, yeah it's not going to work for this person.

I'm not anti BMI, I'm just opposed to people repeating statements like "BMI is useful to compare populations" which don't really mean anything.

[–] [email protected] 7 points 1 year ago* (last edited 1 year ago) (1 children)

You can use it after controlling for the variables that would otherwise skew it.

Comparing different sports with different muscle requirements, or different genders with different muscle development, would be a wrong use.

Comparing averages of samples of Japanese vs. Samoans, with the same ratio of males vs females, sporty vs non-sporty, and a similar age distribution, would be a viable comparison.

INB4 genetic differences between Japanese vs Samoans:

"Over the period of 1978-2013, in a population of approximately 200,000 Polynesian people, the prevalence of obesity increased from 27.7% to 53.1% in men (2.3% per five years) and 44.4% to 76.7% in women (4.5% per five years)"

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9012561/

[–] [email protected] 3 points 1 year ago

You should have kept reading about BMI.

This isn't about speculative genertc factors it's about medical boards arguing that the thresholds need to be set differently for these populations.

Use lower BMI thresholds as a practical measure of overweight and obesity in people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, as they are prone to central adiposity and their cardiometabolic risk occurs at lower BMI:

https://cks.nice.org.uk/topics/obesity/diagnosis/identification-classification/

Similarly, new Zealand used to have higher thresholds for obesity for Maori and Polynesian (which includes Samoa), but because a range of issues including diabetes is such a problem for these populations they brought it back down. It still doesn't work reliably as a risk factor for a range of stuff.

https://medicalxpress.com/news/2020-06-bmi-inconsistent-obesity-maori-pacific.html