Many people who are overweight and obese either don’t realize it or are in denial — and too few doctors are setting them straight, according to a new study in the Archives of Internal Medicine.

Researchers analyzed data on roughly 5,500 people who took part in government health surveys between 2005 and 2008. One-third of the obese participants and 55% of overweight participants had never been told by a doctor that they were overweight, the study found. This is like going to see your doctor with an obvious cancer and that doctor is too shy or busy to tell you the truth.

If a doctor did comment on a patient’s weight, it seemed to make an impression. Nearly 20% of obese people whose doctors hadn’t brought up their weight described themselves as “not overweight,” compared with just 3% of those whose doctors had addressed their weight. Obese and overweight patients who discussed the issue with doctors were also more than twice as likely to have tried to lose weight in the previous year. This is what I tell my patients straight out “Your diet is a direct result of the definition of obesity”.

“If people are told by their doctor that they are overweight, it corrects their perception,” says the lead author of the study, Robert Post, M.D., research director of the Virtua Family Medicine Residency in Voorhees, New Jersey.

Overweight is defined as having a body mass index between 25 and 29, and obesity is defined as a BMI of 30 and up. (BMI is a rough estimate of body fat based on the ratio of a person’s height and weight.)

Doctors may be reluctant to broach the subject of weight for a number of reasons. For instance, busy physicians might not want to fall behind schedule by adding another topic to their list of things to discuss during an appointment and many doctors have negative attitudes toward their heavier patients, whom they see as unlikely to stick to a diet and exercise program, he adds.

Even though almost two-thirds of U.S. adults are now overweight or obese, Dr. Moulavi believes that as Americans have grown heavier, most of the population has a skewed perception that this is “norm” of what constitutes an ordinary weight now.

In fact, most of the overweight study participants accurately estimated their BMI. But many didn’t see their weight as unhealthy or recognize the need to shed some pounds.

In addition, studies have shown that smokers whose doctors remind them of how unhealthy smoking is are encouraged to quit and are more likely to do so successfully than those whose doctors stay mum. Simple reminders and encouragement to lose weight could have a similar effect on overweight and obese patients, Dr. Moulavi says.

Know your BMI and act on it if it’s above 25.