[quote IamTomHanks]For 99% of the population it's a pretty damn good guide.
Try more like 50% to 60%.
www.ncbi.nlm.nih.gov/pmc/articles/PMC2877506/[/quote]
I'm not sure this paper is saying quite what you think it's saying.
"BMI-defined obesity (≥ 30 kg/m2) was present in 21% of men and 31% of women, while BF %-defined obesity was present in 50% and 62%, respectively. A BMI ≥ 30 had a high specificity (95% in men and 99% in women), but a poor sensitivity (36% and 49 %, respectively) to detect BF %-defined obesity. The diagnostic performance of BMI diminished as age increased. BMI had a good correlation with BF % in men (R2 = 0.44) and women (R2 = 0.71), but also with lean mass (R2 = 0.50 and 0.55, respectively)."
It's all down to the difference between specificity and sensitivity.
If we take BFI (actual measurements of body fat directly, rather than inferred indirectly from BMI) as "truth", what this is telling us that BMI does pretty well in terms of "if your BMI is above 30, it's very likely your BFI - your actual body fat - really is above the healthy range." Where it falls down as a test is that there will be people who are unhealthily fat even though they're in the 25 to 30 range of BMI. IN other words, people who are obese in reality according to BFI measurements that the BMI test misses.
In short: if your BMI is telling you you're obese, it's very likely that you are in fact obese (professional rugby prop forwards aside). But (the danger of complacency) if your BMI is telling you you're merely overweight, don't rest on your laurels, because you could in fact be obese in terms of health risks, without the BMI picking it up.
I've heard this referred to as being a "TOFI" - thin on the outside, fat on the inside. Because the people who fall into the "okay looking BMI, terrible BFI" category usually turn out to have lots of visceral fat, the worst sort in terms of risk factors for things like diabetes.
(Incidentally, that's my body type - lots of cosy layers of fat wrapped round my liver etc., look okay from the outside in clothes - being an "apple" body shape rather than a "pear" or "hourglass" bodyshape is a give-away for that risk.)