Currently, diagnosis of whether someone is overweight or obese is made by weighing them, measuring their height and referring to medically standardised Body Mass Index tables.
(Feel free to correct me if the above is wrong as I have no medical training and haven’t ever seen a doctor about weight issues).
Okay, that seems straightforward. But my understanding of the (supposed) health risks caused by overweight and obesity is that any health problems which may develop are caused by
Lack of cardiovascular fitness,
circulation problems
excess body-fat.
Many athletes who have bulky muscles, and thus have a lot of lean weight, but very low body-fat levels, would come out as obese according to the BMI tables.
Some “thin” people with BMI less than 25 do no exercise and have attenuated muscles, and thus are actually carrying a higher body-fat amount than you’d expect.
So why don’t doctors whip out a set of body-fat calipers to use on you instead of just relying on your physical weight? Or would that be impractical?
The problem is, it’s not easy to get an accurate measure of body fat. Fat doesn’t just live right under the skin or in love handles - it’s all throughout your body, in between your organs, and so forth. So even a good pair of calipers won’t do the job accurately.
Last I heard, the water tank was still the way to get an accurate reading of body fat. First they weigh you outside the tank, in air. They you get in the tank, and they weigh you there. Since fat is lighter than water, and muscle heavier, the difference between your wet and dry weights gives a reasonably accurate reading of your percentage of body fat.
Maybe someone else will be along shortly to explain that even better.
Anyone, the point is, getting a truly accurate body fat reading is not that easy, therefore, we use alternative measurements such as BMI to give an approximation in a short time period. BMI is even less accurate (as you point out) but it can function as a crude screening tool and has the advantage of being cheap and non-invasive. The downside is that it’s achieving almost a cult status, whcih is doesn’t deserve, since it’s far from a highly sensitive and accurate tool.
Yeah, the point of BMI was that it was a relatively easy way to indicate that you might have a problem. But it was never meant to be an absolute. (a friend of mine in the Secet Srevice got in trouble for having a BMI above the overnment standard despite having low bodyfat. He had to get a doctor to clear things up.)
One measurement being looked into is waist size. That tends to be a better predictor of diabetes (article) than BMI. Which makes sense. If you’re at a heavy weight but mostly muscle, you’ll have a lower weight than someone at the same weight who is fat.
Broomstick is correct about the difficulty in accurately calculating body fat %. The caliper method is wildly incorrect much of the time, as are those bathroom scales that have body fat gauges on them. This is why you’ll sometimes find people who claim they are 10% body fat, or some such, even though they are noticeably bulky.
Even the water tank method has its issues. The predictive model is based on the average measurements of five reference corpses, but none of them are anywhere close to the average.
The BMI very specifically does not apply to athletes, but that doesn’t seem to be widely disseminated.
BMI is highly accurate, as long as you’re talking about a group of a few thousand people. When you start making predictions about individuals, that’s when you see a loss of accuracy.
Not sure, but among the folks who talk about such things, there’s general agreement that dissection is a highly accurate method of measuring body fat.