LOS ANGELES — As American health authorities prosecute an all-out war against obesity, a small cadre of researchers is warning that the nation’s 78 million obese adults and 12.5 million obese children are already suffering collateral damage. The message that they will become victims of self-inflicted disease, poor role models for their families and a drag on the economy unless they lose weight has left many obese Americans feeling depressed, defeated and ashamed, these experts warn.

Ironically, some of the campaigns aimed at obese Americans could sink efforts to help them improve their health by eating better and exercising more, the experts wrote last week in the International Journal of Obesity.

Anti-obesity campaigns viewed as stigmatizing “instill less motivation to improve health,” while the messages that appeared most effective at encouraging behavior change didn’t mention obesity at all, according to the research team from Yale University’s Rudd Center for Food Policy and Obesity.

The study comes as state and federal public health officials grapple with an obesity crisis that threatens to swamp efforts to contain health care costs and prolong Americans’ life spans. In a bid to reverse surging rates of obesity in the United States and the industrialized world, public health officials have spawned a slew of campaigns that take a variety of approaches.

Many encourage behavior change with helpful tips such as “eat a variety of colorful fruits and vegetables every day,” as a program backed by the Centers for Disease Control and Prevention advises.

But other campaigns have been less upbeat. In Georgia, a controversial series of video and billboard advertisements remind parents that “fat kids become fat adults,” and that “being fat takes the fun out of being a kid.” An Australian anti-obesity campaign pointedly warns viewers, “the more you gain, the more you have to lose.”

Such messages are broadcast amid widespread stigma against the obese: Heavy workers earn less, are more likely to be passed over for jobs and promotions, and are more likely than their thinner peers to be viewed as lazy and undisciplined, researchers have found. A poll released last month by Harris Interactive/HealthDay found that 61 percent of Americans do not consider negative remarks about a person’s weight to be offensive.

Even among physicians, obese patients elicit feelings of prejudice and blame. A 2003 survey, published in the journal Obesity Research, found that half considered their obese patients awkward, ugly, unattractive and unlikely to follow their advice. In addition, one-third of doctors also viewed obese patients as weak-willed, sloppy and lazy.

Against this backdrop, it’s little wonder that some public health campaigns would employ guilt and shame to motivate people to lose weight, said Rebecca Puhl, the Rudd Center’s research director and leader of the new study.

“There tends to be a sense that maybe a little bit of stigma isn’t such a bad thing, that maybe it’ll give overweight or obese viewers a little motivation,” she said.

But such views do not account for shame’s boomerang effect.

“When children or adults are made to feel stigmatized, shamed or teased about their weight, they’re likely to engage in binge eating and unhealthy weight-control practices, and to avoid physical activity,” Puhl said. “We find that people actually cope with stigma by eating more food.”

To Nina Savelle-Rocklin, a San Fernando Valley psychotherapist who specializes in treating those with eating disorders, the link between shame and overeating is clear.

“Shame is about feeling bad about who you are,” Savelle-Rocklin said. That message “is unbearable and intolerable” to most, and those who quell negative emotions by eating “are going to turn to food. … It’s just a recipe for disaster.”

University of California, Los Angeles psychologist Matthew D. Lieberman, who studies the neuroscience of persuasion, said the latest study is in line with research showing that public health campaigns can only be successful if they “fit with our sense of ourselves.”

When he’s in the lab watching the persuasive process unfold on brain scans, the messages that spur people to action are the ones that activate a region of the brain involved in thinking and reflecting about one’s self. Negative thoughts aren’t likely to recruit the neural systems that convert a message into action, Lieberman said.

Puhl and her colleagues asked 1,014 volunteers to evaluate 30 public service announcements from several countries aimed at curbing obesity. The team found that obese subjects who viewed the anti-obesity campaign messages were likely to perceive shame and stigma more strongly and more often than their slimmer counterparts.

Past research suggests that their long exposure to others’ negative attitudes toward them may prime obese viewers to experience stress when the subject of weight is raised. They commonly respond to such stress by consuming more calories, said Puhl.

“It’s not a helpful public health tool,” she said. “It reinforces the problem and makes the situation worse.”

Far more effective were messages that suggested specific steps that would improve health, conveyed a sense of empowerment and left references to obesity unspoken.

In the study, the slogan that got the highest marks for motivation was “Eat well. Move more. Live longer.” Part of a British campaign, Change4Life, it was rated stigmatizing by less than 30 percent of participants, and 85 percent said it would move them to change.