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Dr. George Fielding, a pioneer of weight loss surgery in Australia, remembers how patients treated him in the late 1990s, when his weight reached 330 pounds on his six-foot frame. He would meet new patients, dressed in Armani suits and feeling on top of the world, and then be abruptly upended.

“They’d say, ‘Mate! You’re sitting there telling me I need it? You need to have your surgery!’ ”

Despite being an internationally recognized expert on lap band and gastric bypass surgeries, Dr. Fielding knew that his appearance was dissuading some patients from using his services. And years of yo-yo dieting and extreme exercise hadn’t helped him keep the weight off.

“People would come for consultations and say they would do it, and then they didn’t come back,” said Dr. Fielding, who now works at New York University’s Langone Medical Center. “I knew they were probably thinking, ‘Wow, he’s fat! Is he any good?’ ”

So Dr. Fielding wasn’t at all surprised by the results of a recent study in the International Journal of Obesity showing that patients secretly — or not so secretly — look with disdain on doctors who are overweight or obese. Just as overweight people are stigmatized in an array of professional and personal settings, overweight doctors are seen as less credible than “normal weight” doctors, and patients are less likely to follow their medical advice, the study found.

The study’s stinging conclusions resonated, too, with Dr. Deep Ramachandran, a pulmonary critical care and sleep doctor at Genesys Regional Medical Center in Grand Blanc, Mich. After administering tests exploring his patients’ breathing irregularities, he often has to tell them a blunt truth: they need to lose weight.

That can be a particularly tough message for this messenger to deliver. Dr. Ramachandran, 42, says he has a body mass index of 38 and should lose at least 60 pounds.

“It’s easy for doctors to assume that once we put on the white coat, patients will listen to what we say,” he said. “But this study says the white coat doesn’t protect you. You have to own your problems and communicate with patients in ways they can understand.”

So he often tells patients that he, too, struggles with weight loss and knows how hard it is.

“How do we deliver a message that may be seen as hypocritical? I don’t know the answer,” Dr. Ramachandran said. “But doctors are human beings first and scientists second. We are subject to the same maladies as the rest of society. It’s an incredibly difficult problem, and a challenge.”

The bias against overweight people is so socially accepted, said Dr. Rebecca M. Puhl, the lead author of the study and the director of research and weight stigma initiatives at Yale University’s Rudd Center for Food Policy and Obesity, that “despite all the doctor’s training and expertise, it can jeopardize the doctor’s ability to have a conversation about health care with the patient.”

Patients seem to hold doctors to a stricter standard: “If you’re a health professional, people think you shouldn’t be struggling with weight,” said Dr. Puhl, who is a clinical psychologist and has conducted extensive research into weight stigma.

For the study, the Yale researchers randomly assigned 358 participants — about 57 percent female and 70 percent Caucasian, with an average age of 37 — to complete one of three online surveys about a hypothetical doctor, responding to questions like whether they would choose the doctor as a provider, follow the doctor’s advice about exercise and losing weight, or consider the doctor credible and trustworthy. The only difference among the versions — indeed, the only identifying characteristic about the gender-neutral doctor — was whether the doctor was “normal weight,” “overweight” or “obese.”

The researchers also included a widely used “fat phobia” list of 14 pairs of adjectives that ranks the respondent’s perceptions on a scale. Typical pairs include “lazy/industrious,” “good self-control/no self-control” and “attractive/unattractive.”

Those who answered the surveys about doctors described as overweight or obese were significantly more likely to change providers than those with doctors described as being of normal weight, regardless of the weight of the patient. The higher respondents’ fat phobia scores, the more likely they were to place their trust in a physician of “normal weight.”

Dr. Fielding, the Australian bariatric surgeon, knows firsthand the change in attitudes that can occur in patients and colleagues when they deal with doctors of different weights. Since having lap band surgery 13 years ago, at age 43, he has maintained his weight at 210 pounds, following a regimen of regular, moderate exercise and 800 calories a day. “I was treated completely differently by patients and doctors,” he said. “They weren’t distracted by the fact that I was fat.”

He acknowledged that the respect now automatically conferred on him sometimes ignited an ancient flicker of resentment. “I’m the same guy, still George,” he said.

Dr. Puhl suggests that medical professionals have a front-line position to change the bias against people who are overweight and should make sure that patients understand the many factors that contribute to excess weight, including genetics, food chemistry and the biology of satiety.

“Then shift the conversation from weight loss to health behaviors that patients can improve, regardless of body weight,” she said. “I’ve met enough people who are thin and unhealthy because of drinking or smoking, or are overweight by the B.M.I. but are very healthy. And that is true for doctors as well as patients.”