Researchers say that nearly 4 in 10 adults who are classified as merely overweight are, in actuality, obese.

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t's no secret that America has a bit of a weight problem. The most recent statistics say 35.8 percent of Americans are overweight, while 35.7 percent are obese. But a new study published in the journal PLoS One suggests that those estimates are wrong, and that we may be severely underestimating how many of us are obese. Is the obesity epidemic actually worse than we thought? Here, a brief guide:

How did the researchers determine how fat we really are?
Doctors normally look at body mass index, or BMI, for a quick indication of whether a patient is obese. (Calculate your BMI, which is a ratio of height to weight, here.) But the problem with BMI, researchers say, is the measure only gives a general sense of your weight; BMI doesn't account for how much of the weight is muscle and how much is fat. The new study, led by Dr. Eric Braverman, went a step further, and measured the percentage of body fat in 1,400 men and women using a dual-energy X-ray absorptiometry (DEXA) scan capable of providing a breakdown of bone, fat, and muscle mass.

And what did they find?
About half of the women who would be classified as merely overweight according to their BMI were actually found to be obese when their body fat percentage was taken into account. The same was true for a quarter of men in the study. Overall, 39 percent of those categorized as "overweight" by BMI qualified as obese when their body fat percentage was taken into account.

Why is this significant?
This is why "some people call it the 'baloney mass index,'" Dr. Braverman tells Health.com. "People aren't being diagnosed [as obese], so they're not being told about their risk of disease or being given instruction on how to improve their health." Doctors really need to come up with a better method of classifying weight, says Dr. James Hospedales, and simply lowering the BMI obesity cutoff won't work. "We'd also be calling an increasing number of people obese who aren't, which could lead to issues with stigma, insurance policies, and other problems," he says. "We have to think quite carefully about the pros and cons."

Sources: CBS News, Health.com, TIME