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Comment by franciscop | Hacker News Reader

Isn't it strange that in [1] the beginning of the "Normal" BMI has more risk of death than the beginning of the "Obese"? I'd even venture to say that the "Overweight" range has, in average, lower Mortality Risk than the "Normal".

PS, another plus for the Metric System. The Imperial System form includes Feet+Inches, while the metric one only needs Centimeters for the height.

[1] https://www.infino.me/join/

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The finding that people with an "Overweight" BMI have a lower rate of all-cause mortality is consistent and has been called the "obesity paradox." A very interesting study came out this month that addressed it.[0] Basically, they did a prospective study of 55,000 (mostly elderly women) where they measured their body fat percentage directly using DEXA scans. It turns out that when you control for body fat percentage, it's actually better for all cause mortality to have the highest BMI category and the lowest body fat percentage. Of course, the measure "BMI after controlling for body fat percentage" is kind of unintuitive, since BMI is supposed to be a crappy proxy for body fat percentage. But basically, as long as your body fat percentage is low, being bigger is healthier than being smaller.

0: http://annals.org/article.aspx?articleid=2499472 (this paper used to be available, because I read the whole thing, but I guess it's paywalled now)

Thanks for posting this! I wonder if part of this paradox is also explainable because people with high muscle mass will tip towards the high end of the scale.

Very cool!

BMI is a terrible metric - it's just a function of weight and height and a terrible indicator for health - most professional atheletes would have high BMIs due to a high muscle to fat ratio (muscle tissue being denser than fat tissue). It's not abnormal to be "overweight" - "obese" at 10% body fat depending on what sport you're doing. (long distance atheletes are normally quite lean, sprinters are bulkier)

It's really not a terrible indicator for health despite this argument getting trotted out every time the subject of BMI comes up on the internet. Obviously, as you pointed out, there's individuals for whom BMI is inaccurate but they represent a very small portion of the whole population. At a population level, it tracks very nicely with health outcomes.

If you're a powerlifter or a sprinter, don't change your habits just because your BMI is high. On the flip side, if you sit behind a desk all day, don't blame a high BMI on the metric being inaccurate.

The metric is very convenient and a useful rule of thumb which is therefore perhaps useful for some proportion of the general population: especially for those with limited medical knowledge and/or equipment - but it's really not a niche minority who is unaccounted for by the BMI.

Muscle density and bone density, amongst other factors, will vary by ethnicity enough that researchers are discussing the value of one singular set of windows for the BMI[0], and although I don't have evidence I would also say that it really doesn't take much exercise for a person to skew their BMI by a noticeable amount - I would venture the top 30% of all exercisers would see their BMI affected by their habits.

So while there is value in the BMI for being easy to collect its data and make recommendations based on its value, in this day and age you'd really expect a more granular and precise measurement for use in research and disseminating health information.

It shouldn't be such a big leap for smartwatches / fitbits to measure body fat %, for example.

[0]http://www.ncbi.nlm.nih.gov/pubmed/19221673?dopt=Citation

> PS, another plus for the Metric System. The Imperial System form includes Feet+Inches, while the metric one only needs Centimeters for the height.

Don't be absurd. There's no reason one couldn't choose to input inches only, or decimal feet, or metres, decimetres and centimetres, or decimal kilometres.

Its been contested in a few papers now that overweight people can outlive normal weight, according to these BMI subcategories. But I am not surprised that things can be fuzzy, since ultimately its a continuum.

The low end of normal includes people with eating disorder (anorexia used to have very high mortality rates) and some people with drug addiction.

Note that for women the low end of normal is probably sub-optimal.

Most people in Europe measure their height in meters plus centimetres.

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