One of the problems is one of the old "why are you looking near the lamppost for your lost keys?" "That's where the light is!" variety -- the things that are easier to measure get used because they're easier, not because they are accurate. (For BMI, everyone knows their height and weight and they are easy to get from corpses for mortality data)
When people talk about obesity they are generally talking about excess fat. There is some concern with overweight-ness from internal organs getting physically bigger, but I haven't heard one way or the other whether that is good, bad, or indifferent. There is some evidence that having extra muscle makes the heart work harder, but our society doesn't seem to care. Likewise extreme calorie restriction may greatly lengthen life, so having any body weight at all may be a liability, but few seem willing to eat the right 800 calories every day, and those guys may be wrong anyway.
Body fat percentage is harder to measure, but probably better for addressing the kind of obesity people are really concerned with. Ideally you'd get a full body composition breakdown, but I don't know how to measure that.
When I was a kid there was software that came with calipers and asked you to measure bits of yourself, and produced a reasonably accurate body fat % number. There are some scales that claim to do it based on some sort of electric current thing. And measuring your density (float you in a tub of water to get your volume, then divide) somehow gets to that number too, though I don't know how they differentiate bone vs. muscle vs. other internals. Google...
But the real kicker is that there's the question of 'is all excess fat bad' and 'is some excess fat (e.g. fat around organs vs. skin-layer fat) worse than others' and 'what is bad' anyway. I think we're all clear on the idea that having 50+ lbs of fat is probably obese, but what does that actually mean for health? This is what was implied in my #3. We don't know, and our classification schemes are making it hard to find out, because we lump together mortality of all people who are some weight X at some height X^2 * C.
Re: This is my burning ball of fury
When people talk about obesity they are generally talking about excess fat. There is some concern with overweight-ness from internal organs getting physically bigger, but I haven't heard one way or the other whether that is good, bad, or indifferent. There is some evidence that having extra muscle makes the heart work harder, but our society doesn't seem to care. Likewise extreme calorie restriction may greatly lengthen life, so having any body weight at all may be a liability, but few seem willing to eat the right 800 calories every day, and those guys may be wrong anyway.
Body fat percentage is harder to measure, but probably better for addressing the kind of obesity people are really concerned with. Ideally you'd get a full body composition breakdown, but I don't know how to measure that.
When I was a kid there was software that came with calipers and asked you to measure bits of yourself, and produced a reasonably accurate body fat % number. There are some scales that claim to do it based on some sort of electric current thing. And measuring your density (float you in a tub of water to get your volume, then divide) somehow gets to that number too, though I don't know how they differentiate bone vs. muscle vs. other internals. Google...
But the real kicker is that there's the question of 'is all excess fat bad' and 'is some excess fat (e.g. fat around organs vs. skin-layer fat) worse than others' and 'what is bad' anyway. I think we're all clear on the idea that having 50+ lbs of fat is probably obese, but what does that actually mean for health? This is what was implied in my #3. We don't know, and our classification schemes are making it hard to find out, because we lump together mortality of all people who are some weight X at some height X^2 * C.