About 39% of all Americans who are currently classed as slightly overweight are probably, in fact, obese, researchers from of Weill-Cornell Medical College in New York reported in the journal PloS One. Using just BMI (body mass index) to gauge how fat or lean people are is an approximate measurement, its current usage has most likely underestimated the number of obese individuals in the country, the authors wrote.

When assessing people’s body weight status, a blood test that measures leptin levels to BMI would more accurately identify obesity, the authors wrote. Obese people have a higher risk of developing several diseases and conditions, including cancer, heart disease, and diabetes type 2.

Researchers, Eric R. Braverman, MD. and Nirav R. Shah, MD, MPH, found that 39% of all those who had been classified as overweight were, according to their tests, obese. They examined X-ray imaging scans that measured body fat directly, rather than BMI scores alone.

BMI (body mass index) is a measurement of a person’s height in relation to their weight. A BMI of 25 to 29.9 means the individual is overweight, while 30 or more means they are obese. BMI has long been criticized as inaccurate and incomplete.

A 100-meter Olympic sprinting champion who is 6ft 2inches tall may weigh more than a couch potato of the same height. However, the athlete has more muscle (lean tissue) and less fat than the other person. Muscle is heavier than fat. The athlete will come out with a higher BMI than the couch potato, although his/her waist-hip-chest measurements would clearly show that the overweight or obese person is the other. BMI does not take into account fat-muscle ratio.

The authors say the current method of taking just BMI readings tends to misdiagnose a considerable number of obese people, who are mistakenly classified as just overweight. These people are consequently at a higher risk of developing obesity-related diseases because no intervention is made.

The authors examined DXA (dual-energy X-ray absorptiometry) scans of 9,088 adults at a private clinic in New York between 1998 and 2009. Three-quarters of them were Caucasians, 63% female, 37% male, and their average age was 51.4 years. DXA measures muscle mass, bone density and body fat.

They found that:

  • 64% of the patients were classed as obese according to the DXA results
  • 26% of the patients were obese according to current BMI only methods

Braverman explained that BMI is particularly inaccurate when assessing older females. He says he called BMI baloney mass index (BMI stands for “body mass index”).

The researchers believe that if a doctor has to use just BMI, a better cut-off point might be 28 for adult males and 24 for adult females.

Although DXA testing would be much more accurate, the authors explain that the scans are costly.

Measuring blood levels of leptin is not costly, and could be an additional test to the BMI measurement which would provide a better assessment of the patient’s body weight status, the researchers believe.

In an Abstract in the Journal, the authors concluded:

“Our results demonstrate the prevalence of false-negative BMIs, increased misclassifications in women of advancing age, and the reliability of gender-specific revised BMI cutoffs. BMI underestimates obesity prevalence, especially in women with high leptin levels (>30 ng/mL). Clinicians can use leptin-revised levels to enhance the accuracy of BMI estimates of percentage body fat when DXA is unavailable.”

Update August 17th, 2013 – For the first time in thirty years obesity rates in America have remained unchanged, says a study from Trust for America’s Health and the Robert Wood Johnson Foundation.

Written by Christian Nordqvist