While BMI as a measure of obesity is far from perfect, it mostly fails in a false negative direction. False positives are quite rare; you have to be really quite buff in order for BMI to tell you you’re obese when you’re not.
That is to say, I believe BMI-based measures will generally suggest lower rates of obesity than by-eye estimation, not higher.
Is that so? From the way BMI is defined, one should expect a tendency to misclassify tall normal people as overweight, and short overweight people as normal—i.e. a bias in opposite directions for people on either end of the height continuum. This is because weight scales with the cube of height, but BMI is defined as weight / height².
After reading around a bit, my understanding is that the height exponent was derived empirically – the height exponent was chosen to maximise the fit to the data (of weight vs height in lean subjects). (Here’s a retrospective article from the Wikipedia citations.)
The guy who developed the index did this in the 19th century, so it may well be the case that we’d find a different exponent given modern data – but e.g. this study finds an exponent of 1.96 for males and 1.95 for females, suggesting it isn’t all that dumb. (This study finds lower exponents – bad for BMI but still not supporting a weight/height³ relationship.)
I don’t find this too surprising – allometry is complicated and often deviates from what a naive dimensional analysis would suggest. A weight/height³ relationship would only hold if tall people were isometrically scaled-up versions of short people; a different exponent implies that tall and short people have systematically different body shapes, which matches my experience.
In any case, my claim above is based on empirical evidence, comparing obesity as identified with BMI to obesity identified by other, believed-to-be-more-reliable metrics – those studies find that false positives are rare. Examine.com is a good source, and its conclusions roughly match my impressions from earlier reading, albeit with rather higher rates of false negatives than I’d thought.
I still don’t think you’re wrong. Will is correct when he says that it is more likely someone with a BMI of 25 or lower is actually overweight than someone with a BMI of 25 or higher is just well-muscled, but that isn’t the same as estimating by eye.
The point, as I understand it, is that if you live in a country where most people are overweight, your understanding of what “overweight” is will naturally be skewed. If the average person in your home country has a BMI of 25-30, you’ll see that subconsciously as normal, and therefore you could see plenty of mildly overweight people and not think they were overweight at all—only people at even higher BMI’s would be identifiable as overweight to you.
Will is correct when he says “It is more likely someone with a BMI of 25 or lower is actually overweight than someone with a BMI of 25 or higher is just well-muscled”, but that isn’t the same as estimating by eye.
Relatively minor in this particular case, but: Please don’t claim people said things they didn’t actually say. I know you’re paraphrasing, but to me the combination of “when he says” with quote marks strongly implies a verbatim quote. It’s pretty important to clearly distinguish between those two things.
I agree “BMI gives lots of false negatives compared to more reliable measures of overweight” is not the same thing as “BMI is more prone to false negatives than by-eye estimation” – it could be that BMI underestimates overweight, but by-eye estimation underestimates it even more. It would be great to see a study comparing both BMI and by-eye estimation to a third metric (I haven’t searched for this).
But if BMI is more prone to false negatives, and less prone to false positives, than most people think, that still seems to me like prima facie evidence against the claim that the opposite (that by-eye will underestimate relative to BMI) is true.
While BMI as a measure of obesity is far from perfect, it mostly fails in a false negative direction. False positives are quite rare; you have to be really quite buff in order for BMI to tell you you’re obese when you’re not.
That is to say, I believe BMI-based measures will generally suggest lower rates of obesity than by-eye estimation, not higher.
https://examine.com/nutrition/how-valid-is-bmi-as-a-measure-of-health-and-obesity/
Is that so? From the way BMI is defined, one should expect a tendency to misclassify tall normal people as overweight, and short overweight people as normal—i.e. a bias in opposite directions for people on either end of the height continuum. This is because weight scales with the cube of height, but BMI is defined as weight / height².
After reading around a bit, my understanding is that the height exponent was derived empirically – the height exponent was chosen to maximise the fit to the data (of weight vs height in lean subjects). (Here’s a retrospective article from the Wikipedia citations.)
The guy who developed the index did this in the 19th century, so it may well be the case that we’d find a different exponent given modern data – but e.g. this study finds an exponent of 1.96 for males and 1.95 for females, suggesting it isn’t all that dumb. (This study finds lower exponents – bad for BMI but still not supporting a weight/height³ relationship.)
I don’t find this too surprising – allometry is complicated and often deviates from what a naive dimensional analysis would suggest. A weight/height³ relationship would only hold if tall people were isometrically scaled-up versions of short people; a different exponent implies that tall and short people have systematically different body shapes, which matches my experience.
In any case, my claim above is based on empirical evidence, comparing obesity as identified with BMI to obesity identified by other, believed-to-be-more-reliable metrics – those studies find that false positives are rare. Examine.com is a good source, and its conclusions roughly match my impressions from earlier reading, albeit with rather higher rates of false negatives than I’d thought.
Thanks for sharing this, I guess it looks like I was wrong!
I still don’t think you’re wrong. Will is correct when he says that it is more likely someone with a BMI of 25 or lower is actually overweight than someone with a BMI of 25 or higher is just well-muscled, but that isn’t the same as estimating by eye.
The point, as I understand it, is that if you live in a country where most people are overweight, your understanding of what “overweight” is will naturally be skewed. If the average person in your home country has a BMI of 25-30, you’ll see that subconsciously as normal, and therefore you could see plenty of mildly overweight people and not think they were overweight at all—only people at even higher BMI’s would be identifiable as overweight to you.
Relatively minor in this particular case, but: Please don’t claim people said things they didn’t actually say. I know you’re paraphrasing, but to me the combination of “when he says” with quote marks strongly implies a verbatim quote. It’s pretty important to clearly distinguish between those two things.
Fair enough. I’ve edited it to remove the quotation marks.
I agree “BMI gives lots of false negatives compared to more reliable measures of overweight” is not the same thing as “BMI is more prone to false negatives than by-eye estimation” – it could be that BMI underestimates overweight, but by-eye estimation underestimates it even more. It would be great to see a study comparing both BMI and by-eye estimation to a third metric (I haven’t searched for this).
But if BMI is more prone to false negatives, and less prone to false positives, than most people think, that still seems to me like prima facie evidence against the claim that the opposite (that by-eye will underestimate relative to BMI) is true.