As a woman whose height and weight put me in the obese category on the body-mass-index chart, I cringed when Michelle Obama recently spoke of putting her daughters on a diet. While I’m sure the first lady’s intentions are good, I’m also sure that her comments about childhood obesity will add yet another layer to the stigma of being overweight in America.

Last August, Dr. Delos M. Cosgrove, a cardiac surgeon and chief executive of the prestigious Cleveland Clinic, told a columnist for The New York Times that if he could get away with it legally, he would refuse to hire anyone who is obese. He probably could get away with it, actually, because no federal legislation protects the civil rights of fat workers, and only one state, Michigan, bans discrimination on the basis of weight.

Dr. Cosgrove may be unusually blunt, but he is far from alone. Public attitudes about fat have never been more judgmental; stigmatizing fat people has become not just acceptable but, in some circles, de rigueur.

I’ve sat in meetings with colleagues who wouldn’t dream of disparaging anyone’s color, sex, economic status or general attractiveness, yet feel free to comment witheringly on a person’s weight.

Over the last few years, fat people have become scapegoats for all manner of cultural ills. “There’s an atmosphere now where it’s O.K. to blame everything on weight,” said Dr. Linda Bacon, a nutrition researcher and the author of “Health at Every Size: The Surprising Truth About Your Weight” (Benbella, 2008). “If we’re worried about climate change, someone comes out with an article about how heavier people weigh more, so they require more fuel, and they blame the climate change crisis on fatter people. We have this strong belief system that it’s their fault, that it’s all about gluttony or lack of exercise.”

  It’s no secret that being fat is rarely good for your career. Heather Brown (no relation) has experienced this firsthand. A few years ago, she applied for a grant-writing job with a small nonprofit in the Boston area. After a successful phone interview, she was invited to the office.

“As soon as I shook the interviewer’s hand, I knew she would not hire me,” Ms. Brown said. “She gave me a look of utter disdain, and made a big deal about whether we should take the stairs or ride the elevator to the room where we were going to talk. During the actual interview, she would not even look at me and kept looking to the side.” Ms. Brown, 36, who now works as an assistant dean at a college near Chicago, said she never even got a “No thank you” letter after the interview.

That story is all too familiar to people like Bill Fabrey, an advocate who in 1969 founded the National Association to Advance Fat Acceptance. The organization’s archives, he says, are full of stories from people who say they lost jobs or promotions because of their weight, or were not hired in the first place.

Some of the most blatant fat discrimination comes from medical professionals. Rebecca Puhl, a clinical psychologist and director of research at the Rudd Center for Food Policy and Obesity at Yale, has been studying the stigma of obesity for more than a decade. More than half of the 620 primary care doctors questioned for one study described obese patients as “awkward, unattractive, ugly, and unlikely to comply with treatment.” (This last is significant, because doctors who think patients won’t follow their instructions treat and prescribe for them differently.)

Dr. Puhl said she was especially disturbed at how openly the doctors expressed their biases. “If I was trying to study gender or racial bias, I couldn’t use the assessment tools I’m using, because people wouldn’t be truthful,” she said. “They’d want to be more politically correct.”

Despite the abundance of research showing that most people are unable to make significant long-term changes in their weight, it’s clear that doctors tend to view obesity as a matter of personal responsibility. Perhaps they see shame and stigma as a health care strategy.

If so, is it working? Not very well. Many fat people sidestep such judgments by simply avoiding doctor visits, whether for routine checkups, preventive screenings or urgent health problems.

Indeed, Dr. Peter A. Muennig, an assistant professor of health policy at Columbia, says stigma can do more than keep fat people from the doctor: it can actually make them sick. “Stigma and prejudice are intensely stressful,” he explained. “Stress puts the body on full alert, which gets the blood pressure up, the sugar up, everything you need to fight or flee the predator.”

Over time, such chronic stress can lead to high blood pressure, diabetes and other medical ills, many of them (surprise!) associated with obesity. In studies, Dr. Muennig has found that women who say they feel they are too heavy suffer more mental and physical illness than women who say they feel fine about their size — no matter what they weigh.

Even if doctors don’t directly express weight-based judgments, their biases can hurt patients. One recent study shows that the higher a patient’s body mass, the less respect doctors express for that patient. And the less respect a doctor has for a patient, says Dr. Mary Huizinga, the study’s lead author and an assistant professor at Johns Hopkins School of Medicine, the less time the doctor spends with the patient and the less information he or she offers.

Fat stigma affects everyone’s health — fat, thin or in between. Last fall, Lincoln University in southern Pennsylvania announced that it would weigh and measure all freshmen, and require those with a B.M.I. over 30 to enroll in a special fitness class. Fat rights advocates protested it as discrimination: If the fitness class was that important to student health, shouldn’t everyone take it?

Lincoln’s administrators backpedaled after a storm of bad press. But the controversy underscores the fact that fat stigma isn’t about improving people’s health, as doctors like Delos Cosgrove contend. If it were, the conversation would be about health rather than numbers on the scale and the B.M.I. chart.

Dr. Bacon tells the story of an overweight teenage girl whose high school was going through a “wellness campaign.” Hallways were plastered with posters saying “Prevent teenage obesity.” After the posters went up, the girl said, schoolmates began taunting her in the halls, pointing at the obese girl on the posters and saying, “Look at the fat chick.”

She said heavier students were now made to feel guilty about their lunch choices, but the thin ones could eat anything they wanted without comment — even if it was exactly what the fat kids were eating.

“Stigmatization gives the thinner kids permission to think there’s something wrong with the larger kids,” Dr. Bacon, the nutrition researcher, said. “And it doesn’t help them look at their own health habits. There’s got to be a way to do this more respectfully and more effectively.”

Harriet Brown teaches magazine journalism at the Newhouse School in Syracuse.

By HARRIET BROWN