The research shows even women who claim to be "at one" with their bodies and have never suffered eating disorders feel panic-stricken when seeing overweight people.
According to brain scans of women shown pictures of overweight strangers, activity is triggered in a part of the brain processing identity and self-reflection.
But the same tests on men revealed they showed no interest in their own weight.
Scientists from Brigham Young University in Utah found women felt scared of being overweight without consciously thinking about their figure.
They say the results show how women are conditioned to be afraid of being fat because they are constantly bombarded with messages and images of a thin ideal.
Neuroscientist Mark Allen, who led the study, said: "These women in our study had no history of eating disorders and project an attitude that they don't care about body image.
"Yet under the surface is an anxiety about getting fat and the centrality of body image to self."
The research, published in the May issue of the psychological journal Personality and Individual Differences, is part of a long-term project to improve treatment of eating disorders by tracking progress with brain imaging.
Dr Allen said when anorexic and bulimic women view an overweight stranger, the brain's self-reflection centre – known as the medial prefrontal cortex – "lights up" in ways that suggest extreme unhappiness and, in some cases, self-loathing.
The motivation for this new study was to establish a point of reference among a control group of women who scored in the healthy range on eating-disorder diagnostic tests.
Surprisingly, even this control group exhibited what Dr Allen calls "sub-clinical" issues with body image. The results prompted Dr Allen to do the same tests on a healthy group of men but found they did not think about their own weight when looking at images of fat men. Dr Allen said that the results showed that even confident women were much more likely to have eating disorders than men.
However, Dr Mary Brown, lecturer in psychology at Robert Gordon University in Aberdeen, said that it was inaccurate to assume men had no concerns about obesity.
"Men are conditioned to be more detached and would react by thinking, 'Oh, that's an overweight person'.
"But I would question how the researchers concluded men weren't bothered. Men would be more inclined to react adversely if a photo of their own face was superimposed on the overweight body. This is because men are more influenced by feeling unfit and are stereotyped just like women in terms of taking action and being able to take part in sport."