[chris (OP)]

Ari wrote to ask:

I've both read, and been told repeatedly, by friends in various medical and bio/fitness professions, that BMI is a wildly inaccurate way of measuring personal fitness - that on an individual basis, it basically assumes that we're two-dimensional; that it gets wildly skewed if you're shorter or taller than the average (below 5'5" or above 5'9").

It seems to have been co-opted by for-profit fitness businesses and products, the general body-shaming cultism favoured by marketing firms the world over, and used to exclude and exploit patients (particularly American ones) on private insurance based on (incredibly misleading) assumptions about health risks.

If this is true, why do so many apps, public health initiatives, and individual human doctors rely on the BMI and quote it as a determinant of healthy body weight? Or is this just a case of going along because this is what the medical profession has decided is standard, even though it doesn't make sense?

[Bored chemist]

BMI was developed so that doctors could tell their fat patients they were fat, while sounding convincingly scientific (and less rude).
It's a crude tool- but it does that job.

It also gives some sort of handle on answering questions about "what is a healthy weight?" and so on.

[chris (OP)]

But would it also not be true to say that a 100m sprinter would probably also register as "obese" according to BMI?

[alancalverd]

Arnold Shwarzenegger in his prime, and George W Bush (airforce reserve pilot and distance runner) were both "obese" on BMI count. I think Usain Bolt is borderline.

I was offered a "50 year check and lifestyle advice" by my GP. I explained that my mum had already told me to avoid cigarettes, fried food,  alcohol, and promiscuous sex, and to get plenty of fresh air and exercise. He said "true, but I can get an extra 50 quid for writing it down". 

[atrox]

I personally consider the BMI a quick but rough tool that is able to give you a hunch of in which direction your bodymass goes. However, I do not consider it to be very accurate, as there is many factors that determine the ideal weight of a person.
I would not use it as the ultimate truth but instead also consider the context. Of course the BMI means something else for a obvious muscleman than for someone who never worked out at all consists basically of fat depots. And of course there is all shades inbetween those extremes. So in the end you never know if you are considered as overweight because you are indeed to fat or because you also have some muscles under some fat depots, but it is actually still fine. But I guess, if your are trying to be honest to your self you will know the course of your own alarming BMI. But for the cases that are not alarming but just considerable it might be difficult to decide just basing on the BMI if you should really take care about it or not.

3 years ago I started a quite demanding teamsport and lost a lot of weight. The lowest weight I got there was as per BMI still slightly overweight (25,2) but I started to feel really uncomfortable and also when I look at pictures from that time I still think I look ill there. My personal ideal weight, where I feel comfortable and have some buffer in both directions before I start feeling uncomfortable, is considered with a BMI of 27.0 as overweight. I just take it as it is but try not to be too irritated about this.

[Bored chemist]

It seems that others doubt the validity of BMI
http://www.thedailymash.co.uk/news/health/body-mass-index-replaced-by-just-looking-at-people-2013012958016