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If you were overweight or obese, would you know it? You’d think so, but research suggests that people aren’t good at recognizing when they’ve hit an unhealthy weight. That’s why they need an outsider to tell them.

That outsider should be a physician, but a recent study in the Archives of Internal Medicine reveals that many doctors are falling down on the job — and that’s a problem. (More on Time.com: Where BMI Fails, Researchers Suggest A New Measure of Body Fat)

The study looked at data on 7,790 adults ages 20 to 64 who participated in the National Health and Nutrition Examination Survey (NHANES). Only 45% of participants who qualified as overweight (with a BMI between 25 and 30) and 66% of those considered obese (with a BMI of 30 or higher) reported being told by their doctor that they had a weight problem.

What’s more, 37% of the overweight patients and 19% of obese patients who didn’t have a talk with their doctor about weight didn’t know they were overweight. (More on Time.com: What Does Race Have to Do With Obesity Counseling?)

Reported the Wall Street Journal‘s Health Blog:

Because Americans have gotten heavier in the past few decades, many may compare themselves to friends and family and not realize they’re overweight, Robert Post, an author of the study and now an attending physician with Virtua Family Medicine Residency, tells the Health Blog. (He conducted the survey while at MUSC.) These results suggest it’s very important for physicians to tell their patients if their BMI puts them in the overweight or obese category, even if it would seem to be obvious, he says. They can use “a neutral tone,” he says. An accompanying editorial by Robert Baron, of the University of California, San Francisco, makes the comparison to smoking, saying that patients urged by a physician to quit were significantly more likely to do so than those who weren’t. He suggests physicians treat BMI as “a routine vital sign” and highlight troublesome results on a medical chart, then relay the information to the patient the same way they would an abnormally high blood-pressure or cholesterol reading.

Of course, it’s worth pointing out that as a metric, BMI has its critics: it is an imprecise measure of body fat content — and fails to account for a person’s accumulation of belly fat, which is associated with the greatest risk of heart disease and diabetes. Still, BMI is the only standard measure of obesity that is currently used nationwide, and until a more accurate gauge is developed, it’s the best way we have to track and treat the obesity epidemic.

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