The swimsuit season is fast approaching and every US magazine appears to be touting a miracle diet for losing the winter blubber before hitting the beach. Despite the body beautiful image of the Baywatch babes, many Americans have huge problems when it comes to discarding clothes and shedding pounds.

Now it's official: America is the fattest society in the history of the world. The National Institutes of Health will next week issue a new standard for measuring a healthy weight that will label 55 per cent of adult Americans "overweight".

A total of 100 million Americans will be classed as "overweight" under the new guidelines to be announced in full on Wednesday.

The number of Americans in the obesity category - people too overweight to, say, fit into an airline seat - has risen 350 per cent in the past 30 years. According to the latest figures, some 22.5 per cent of Americans tip the scales as obese.

"Unprecedented access to high fat and high calorie food is one of the many factors contributing to obesity," said Kelly Brownell, a professor of psychology at Yale University. "There has been a explosion of mini-markets, drive-in restaurants and fast-food outlets inside railway stations and hospitals offering large serving sizes. This is all part of a toxic food environment resulting in an epidemic of obesity."

Some of the obese may suffer from so-called fat genes or hormonal problems. But the bulk of Americans' vast weight is due to eating too many double burgers and supersize fries.

Prof Brownell says: "In some cases there is a psychological attachment to food, which is difficult to break."

The new formula devised by the National Institutes of Health defines anyone with a body-mass index - a formula used to estimate body fat - of more than 25 as overweight.

Under the new standards, a 5 foot4 inch woman weighing 145 pounds (10s t 5 lbs) would be overweight, compared with 159 pounds (11 st 5 lbs) under previous standards. The overweight level for a 5 foot10 inch man would drop to 174 pounds (12 st 6 lbs) from 193 (13 st 11 lbs).

Physicians are also advised to measure their patients' waistlines. Men with waists of more than 40 inches and women whose waists exceed 35 inches face an increased risk of serious health problems.

"Unfortunately, people in America are very overweight and it incurs health risks such as diabetes, high blood pressure, arthritis of the knees and certain kinds of cancer," said Xavier Pi-Sunyer, an obesity specialist who heads the National Institutes of Health task force.

Yet as the American belt expands, so has impatience with experts telling people what to eat. Fat people are on the march, at least metaphorically. The National Association to Advance Fat Acceptance, which describes itself as a human rights group, has hit out against the new guidelines.

This group runs a dating service for the overweight as well as a computer bulletin board where 500-pounders (35 st 10 lbs) can trade information on where to buy shorts and shirts. The association is concerned that increased pressure to lose weight will force people to try doomed diets and discourage them from going to the doctor for fear of being harassed about their size.

DR PI-SUNYER, of St Luke-Roosevelt Hospital in New York, said: "It is right to alert people to the health risks which increase when they get to a certain weight. We are telling them not to gain any more weight. We are trying to take a preventative role.

"I think that fat is a 21st century problem. Few people are not physically active at work or at play. You guys in Ireland and England are catching up on us."

According to the latest book by travel writer Bill Bryson, the average American walks a mere 350 yards a day.

The nation's sedentary lifestyle and outsized appetite is putting pounds on the dollar economy. Health problems associated with obesity cost $60-$80 billion a year, or 10 percent of all heath spending.

"We have to take social action against bad eating habits," stressed Prof Brownell. "Just like smoking has become socially unacceptable, we should begin doing the same thing with unhealthy food."

US experts recommend that even normal weight people should be vigilant. The scales may lie, but the tape measure and the full-length mirror don't.