But researchers say that there may be a more accurate way of measuring obesity -- one that suggests the prevalence of the condition is a lot higher than we currently think. Their study is published in the journal PLoS ONE.
The New York researchers found that Dual Energy X-ray Absorptiometry (DXA) scans better diagnose obesity than BMI, since they can measure body fat, muscle mass and bone density simultaneously.
When using this method, up to 39 percent of overweight Americans (meaning they have a BMI between 25 and 29.9) are, in fact, obese.
According to the latest data from the CDC, 34.4 percent of adults (ages 20 and older) are overweight, based on solely their BMIs. Therefore, up to an additional 13 percent of U.S. adults may actually be obese instead of overweight, when using the DXA scans.
The research included 9,088 people -- 63 percent female and 37 percent male -- whose BMI, DXA, insulin and leptin were all taken between 1998 and 2009. Their average BMI was 27.3, with a 31.3 average percent body fat.
"The BMI is an insensitive measure of obesity, prone to under-diagnosis, while direct fat measurements are superior because they show distribution of body fat," study researcher Dr. Eric Braverman, M.D., who is the founder and president of the nonprofit PATH Foundation NY, said in a statement. Braverman worked with Dr. Nirav Shah, M.D., M.P.H., who at the time of the research was an assistant professor at the New York University School of Medicine. Shah is now the New York State Commissioner of Health.
The differences in how many people have obesity between BMI and DXA are especially evident for older women, and people who may have high levels of leptin, a hormone protein, in their blood. Leptin is linked with body fat.
"Currently, no other blood test or biomarker has been correlated with the rate of obesity," the researchers wrote in the study. "The use of both DXA and leptin levels offers the opportunity for more precise characterization of adiposity and better management of obesity."
Clarification: This article has been revised to clarify just how many people may actually be obese instead of overweight, considering this new way of measuring obesity.