BMI is a highly insensitive measure of obesity prone to under-diagnosis, while direct fat measurements are superior because they show distribution of body fat.

• In a single-center study, 66% of patients classified as obese on the basis of DEXA scanning had BMI values in the non-obese range, according to Eric Braverman, MD, of New York Presbyterian Hospital/Weill Cornell Medical Center.
• Among more than 1,000 patients, 56% were obese according to the DEXA results, versus 20% using the standard BMI-based definitions.
• Scoffing at BMI as the “baloney mass index,” Braverman said it’s “very likely that obesity is a much bigger epidemic than the 300 million people acknowledged by the World Health Organization.”
• BMI is just a mathematical equation based solely on height and weight that is too general for diagnosing anything, especially in such an exacting field of clinicians.
• Braverman said that if any endocrinologists rely on math to calculate thyroid stimulating hormone, he would be laughed at.
• DEXA scans, on the other hand, provide a direct measurement of body fat percentage and can spot fat exactly, in every part of the body.
• It’s particularly effective, Braverman said, for that part of the population that is known as “thin-but-unfit.”
• Their condition is known as normal-weight obesity, in which the BMI is low but they have a high percentage of body fat, especially compared with more favorable tissue like muscle.
• These patients are at higher risk of dyslipidemia, as well as hypertension among men and cardiovascular disease among women.
• The researchers found that DEXA identified 56% of patients as obese while BMI identified 20% as such.
• Among those classified as obese by DEXA, only 34% were classified as obese by BMI and 5% of patients identified as obese by BMI actually weren’t obese according to DEXA scans.
• These individuals were muscular and large, so they look like they’re high weight but they really had high muscle mass.
• This means that we may have more health problems, as individuals are delaying treatment because they don’t think they’re obese and think they’re thin and ‘just a little flabby.

http://www.kevinmd.com/blog/2010/05/body-mass-index-bmi-measure-obesity-accurately.html