BMI is a bad measure. All the fat acceptance nutters will tell you that.
And they’re right. The evidence indicates that it actually under-reports obesity.
A smaller scale study (1,691 persons) using DEXA scans (seen as a valid body fat measuring device) found that there was a 34.7% discrepancy between BMI and DEXA for women and 35.2% for men. However, BMI appeared to misclassify women as less fat as they were by DEXA; notable misclassifications include 20.3% of women being obese via BMI while DEXA showed 37.1%, 24.8% of men being obese via BMI compared with 38.4% of men being obese via DEXA. These results have been replicated in which persons in the normal BMI range were actually obese according to body fat percentage (20% of men, 9.2% of females) and more persons in the overweight BMI range were actually obese by body fat percentage (67.2% of men, 84.2% of females). High obesity rates in this study may be partially explained by socioeconomic issues, as it comprised Mexican persons (n=538) living in the southern USA. Finally, another study utilizing DEXA on a sample size of 1,393 persons found that 26% of persons were classified obese by BMI while 64% of persons were obese by DEXA; a misclassification rate (false negatives) of 25% for men and 48% for women was noted.
While I am certainly biased against the FA movement and all their ridiculousness and thus more likely to favor evidence that benefits “my” side, there is hardly any evidence in favor of their claims and loads of evidence against them.
Back where I grew up these people would have been called “powerful stupid.”