Nearly 66 per cent of Americans are now overweight or obese, compared with 62 per cent in 2004 and 53 per cent in 1994.

Australians are not far behind: Nearly 64 per cent are obese or overweight, found a report by the Australian Institute of Health and Welfare last December.

Yet the number of Americans trying to lose weight had fallen, says a research letter in the Journal of the American Medical Association'sMarch 7 edition.

Lead author Andrew Hansen from Georgia Southern University said his research had shown that the percentage of adults who were overweight or obesity - who were also trying to lose weight - had decreased from 55 per cent in 1988-1994 to 49 per cent in 2009-2014.

Adults who were overweight but not quite in the obesity range "put forth the least weight loss effort", he said in a Q and A with online research portal, Research Gate.

"The largest decrease in weight loss effort was among black women. These decreases in weight loss attempts happened at a time when overweight and obesity increased from about 52 per cent in 1988-1994 to 66 per cent in 2009-2014," Dr Hansen said.

"With acceptable body weight shifting to a heavier weight, some do not see a need and view themselves as just right," said Dr Hansen.

"Often, individuals will compare themselves to others around them ... So, if most people in a person's social circles are overweight, then overweight is viewed as normal to them."

In Australia, the AIHW found weight varied by postcode: North Sydney had the fewest overweight or obese people, at 53.4 per cent of adults, compared with country South Australia, where 73.3 per cent were obese.

Jane Martin, the executive manager of the Obesity Policy Coalition in Victoria, said being a healthy weight was now less normal than being overweight.

For those obese people trying to lose weight, it was often a 'white knuckle ride' to overcome obstacles and temptations, such as vending machines.

???"Not only is overweight normalised, highly processed unhealthy food is normal," she said.

"If you are in a group with unhealthy weight, you stick with the group, and you have the same habits, (and) maybe it is more comfortable to be like them."

Levels of obesity were often highest in areas of disadvantage where unhealthy meals were easier and more affordable.

The use of vanity sizing by retailers - where yesterday's size 14 becomes today's size 10 - was another way of tricking people into thinking "it is all okay".

With so many people overweight, an international group called the World Obesity Federation is urging the media to avoid stereotypical images by providing photos showing active overweight people at work or shopping.

It is campaigning against "photographs that place unnecessary emphasis on excess weight or that isolate an individual's body parts (e.g. images that disproportionately show abdomen or lower body). "

It says these stereotypes contribute to discrimination against overweight people at work and school.

One American study found obese women earned 12 per cent less than normal weight women.

Images like these showing torsos without heads, and rolls of body fat, are discouraged by the World Obesity Federation because they stigmatise overweight and obese people. Photo: iStock

Embarrassment also made some people less likely to seek medical help.