> It's such a simple measure and it seems to be useful for predicting health issues.
It is good for predicting in general populations, but can easily fall apart with individuals. It doesn't take into account what the makeup of your mass is. Muscle is denser than fat, so a muscular man and a fat man of the same height and weight would have the same BMI, but vastly different health profiles.
>It is good for predicting in general populations, but can easily fall apart with individuals.
While it can "easily fall apart with individuals", the difficulty lies in finding an individual to measure that will make BMI fall apart.
And that's because the huge majority of us is of the kind of complexion and body type where BMI works well.
So, sure it can give false positives, but for a quite small fraction -- besides any other rough medical heuristic does the same (e.g. signs of having the flu can also be caused by 200 other reasons).
The "individual measure" you're looking for is body fat percentage.
A vastly more accurate proxy is waist circumference.
BMI was developed largely because the proxy data (height and weight) are so readily available, especially in databases. Few doctors take waist measurements, and doing so is slightly more complicated.
> the difficulty lies in finding an individual to
> measure that will make BMI fall apart
I'm 5'11 with a 36 inch waist. This gives me a waist-to-height ratio of 0.507, which has me just under or over the healthy cut-off, depending on who you ask[0].
I was 212lbs this morning, giving me a BMI of 29.6, which has me at the border of "Overweight" and "Obese Class I (Moderately obese)", and is high risk.
I don't think you'd think I was a weird shape if you looked at me.
I see how it is a bad measure for some cases, but I don't think that makes it a bad tool. It's like financial advice that says how much someone should save for retirement or spend on rent based on income. Someone with a normal job progression will probably be fine following it, but for doctors who take out huge loans and eventually have a huge increase in income it's not helpful and a more specific plan should be created. For BMI, most people should aim for the middle of the range and if they think there's a reason that they don't fit the scale then a doctor can be consulted.
Why do people keep bringing up this point. Even blood tests are not perfect tools but we still make use of them. There are always some false positives and negatives. It is a combination of all the available results that a doctor will use to make a determination. If your BMI is high but you look muscular, have normal BP and cholesterol and no other concerns, they may feel that you are in good health.
I think the sort of person to be incredibly muscular is probably also one of the most likely groups to get body fat measurements, and be educated about the limitations of BMI. Because that person is almost certainly a bodybuilder, serious athlete, etc.
It is good for predicting in general populations, but can easily fall apart with individuals. It doesn't take into account what the makeup of your mass is. Muscle is denser than fat, so a muscular man and a fat man of the same height and weight would have the same BMI, but vastly different health profiles.