Sometimes where I work, an argument will go around in circles for a while until somebody says, "Process check!" and then asks a very simple question that either cuts to the heart of the matter, or at least gets at how the group is going to come to a conclusion.
I saw a reference to BMI in a post that Jeff referenced. So here's a quick process check on that stat, using my own body as reference.
I am 5' 10.5 inches tall (and I insist on that half an inch, thank you.) With a current weight of 149 lb, my BMI is 21.1 which is considered "Normal weight" by the Obesity Education Initiative. My body fat, as measured by skinfold caliper, is 8.2% which is considered "freaking awesome" and "a sick result" by Jeffrey D. Erno.
Playing with the BMI calculator I find that the Normal Weight range of 18.5-24.9 BMI would allow me, at my height, to weigh anywhere from 134 to 176 lb. Sounds reasonable - lots of people at 5' 10.5" would be doing great at 176lb.
Now let's look at that 176lb as an extrapolation of skinfold measurement, for my body. My first ever skinfold measurement was in March 2008, and I was at 12.7% and a total weight of 154 lb. That makes my lean body mass at that time 134.4 lb, with 19.6 lb of fat. I could get to 176 lb from that point by adding 22lb of fat, and BMI would say I have normal weight, right?
Lean Fat Total Percent
134.4 19.6 154 12.7%
134.4 41.6 176 23.6%
Wow. Other paleo types have said it before, but I wanted to do the process check anyway - BMI stinks as a measure of fitness. I should note as a further point that at 160 lb (estimated body fat 16%, BMI=22.6) I had stage I hypertension: now I am 110/70.