When most people around us are overweight or obese, it's hardly surprising that we no longer notice it. Fat has begun to look normal. In one sense, that's great - the stigma that overweight people have suffered in the past because of the way they look must surely die away. But it's a dangerous road. If we don't know we are overweight, we may be at risk of sleepwalking into crumbling joints, heart problems and diabetes.

The latest evidence that we no longer see big as unhealthy comes from the US, where researchers from the Rush University Medical Center in Chicago asked African American women volunteers to look at a "body image scale" made up of drawings of women of different sizes and identify which women on the scale were overweight, obese and "too fat". They were also asked which picture featured a woman of approximately the same size as themselves. This is the visual scale they were given:

African American women body image scale. Photograph: Adapted with permission from Macmillan Publishers Ltd

The scale is taken from Obesity. Pulvers et al. (2004). Development of a culturally relevant body image instrument among urban African Americans. Obesity Res. 2004;12:1641–1651.

[For anybody who wants to guess which of the women are of normal weight, overweight and obese, the answers are at the bottom.]

This was not an enormous study - 69 African American women were recruited from a low income neighbourhood of Chicago, with a mean age of 38, all of whom had at least one child. African American women were chosen because they have the highest obesity rates of any demographic group in the USA. The mean BMI of this group was 32, which is classified as obese - normal weight is 18.5-24.9, overweight is 25-29.9, obese is 30-34.9. Over 35 is considered severe or morbid obesity.

The women in the study, published by the Journal of Nutrition Education and Behavior, were pretty much in agreement over which women in the drawings were overweight and obese. But the researchers found that the only sizes they thought were "too fat" were the last two drawings - numbers 8 and 9. So the women were not concerned about being overweight - in their view, it was only serious obesity that could be bad for your health.

And when it came to identifying their own body weight from the drawings, many of them got it wrong. The 56% of overweight women (BMI 25 or greater) and 40% of obese women (BMI 30 or greater) did not classify their body size as either overweight, obese, or too fat. From the perspective of these women, overweight began at a BMI of around 35 - not 25 as the experts say.

What is going on here is that the cultural belief of the women as to what "too fat" looks like and therefore what is unhealthy is at odds with what the medical profession thinks. Elizabeth Lynch, who led the study, said:

The fact that women felt that overweight body sizes were not too fat suggests that being told they are overweight, even by a physician, may not be sufficient motivation for them to attempt to lose weight.

The researchers point out that there is a long history of African American women feeling more satisfied with a large body size than non-Hispanic whites. Nonetheless, as people get more and more overweight in the US, Europe and in all other countries too, there is a lesson here. The researchers suggest that health messages should be accompanied by pictures of what healthy and unhealthy weight actually looks like. It's something we are all losing sight of.

So which women are of normal weight? Numbers 2, 3 and 4. Number 1 has a BMI of 16 and is therefore under-weight. The scale then rises in roughly 3 BMI points for each drawing, so 6 and 7 are obese and 8 and 9 are severely obese.