As more of us become obese, the food manufacturers want us to think it's our own fault, the politicians try to avoid the issue and the diet industry gets rich on the misery it creates. So where do we go from here?

In May 2012, fire engines, police and an ambulance were called to the family home of a teenager called Georgia Davis in Aberdare, south Wales, in order to get her out of it. Nobody could dream up a more horrifying and humiliating nightmare for a girl of her age. A team of more than 40 people was involved in demolishing an upstairs wall of the semi-detached house and constructing a wooden bridge to get a specially reinforced stretcher into her bedroom. Georgia weighed 400kg (63 stone), said some reports. Nobody really knew – she was too heavy to get on the scales. Georgia's rescuers put up tarpaulins to shield her from the camera lenses as they extracted her through a 10ft square hole in the brickwork and took her to hospital. She was covered by a sheet, because she could no longer get into any of her clothes.

The 19-year-old was morbidly obese and her organs were failing. Her mother, Lesley, called the ambulance because Georgia could no longer stand. For months she had not moved from her bedroom, where she spent her days on her laptop and watching TV. Eventually, like Alice In Wonderland inside the little house after drinking something she shouldn't, she grew too big to get out of the door.

Georgia is the extreme marker of a massive problem that has its roots in the way we live today and affects all of us. Two-thirds of us are overweight. A quarter of us are obese and in real danger of damaging our health and dying prematurely. But we are in denial. Obesity looks like Georgia, we think. It doesn't look like us. Her troubles were served up as entertainment in the tabloids. In photographs, her face, above the mountain of flesh, is curiously passive.

Obesity is not something to gawp at, and it is not a problem just for other people. It affects most of us. It's not about the way we look, or the size of dress or trousers we wear. This is about a very real threat to our health. Obesity is a life-shortening condition. Life expectancy in the UK, which has risen steadily since records began, may for the first time be about to fall. Moderate obesity cuts life expectancy by two to four years, and severe obesity could wipe an entire decade off your life, said the Lancet in 2009. The costs to health services and to the world's economies of vast numbers of people becoming sick and unable to work are already huge and increasing. The National Heath Service is spending £5bn a year treating heart attacks, strokes, diabetes, cancers, liver failure, hip and knee joint problems, and other consequences of obesity, and the bill is expected to reach £15bn within a few decades.

Experiments have shown that what we now think is normal is actually overweight and even obese. In 1999, researchers asked nearly 1,000 men and 1,000 women their weight, height and how they would describe themselves on a scale from "very underweight" to "obese". They repeated the exercise eight years later, and those surveyed weighed dramatically more, but fewer realised it. Only 75% correctly considered themselves overweight, compared with 81% eight years earlier. We just don't want to see it.

Georgia Davis was 15, weighing over 200kg, when she was branded Britain’s fattest teen by the Sun. Photograph: Ken McKay/REX

Georgia was 15 when she first hit the front page of the Sun, weighing more than 200kg (32 stone) and branded Britain's fattest teen. The question everybody eagerly asked was what had she been eating – how big a mountain of food? How many cakes at one sitting? Why she should want to was very much of subsidiary interest. Georgia and her mother, who is also obese, spoke of comfort eating after Georgia's dad died when she was five. In later stories it emerged that before she was 10, she had become the carer of both Lesley, who had heart disease, and her stepfather Arthur Treloar. Social services discussed removing her from the family, but she resisted. By any stretch of the imagination, Georgia had a tough childhood.

We can avoid being overweight by exercising personal responsibility, politicians say, voicing the script written by the food industry. We choose what we put into our mouths. We ought to know what will make us fat and have the self-restraint to stop eating. But can you really make that judgment of Georgia at the age of seven, who even then weighed 70kg (11 stone) and whom Lesley admits she fed with condensed milk as a baby, weaning her on to tinned potatoes and later filling her up with fried eggs and chips?

Georgia sold her story to the tabloids and TV to get the money to go to a weight-loss camp in North Carolina, where she lost half her body weight. She came back to find nothing had changed at home – her mother bought fish and chips because there was nothing to eat in the house. Georgia's own account, as told to a tabloid newspaper: "Around eight weeks after returning from camp, I drifted off the plan. I felt really alone. My parents weren't doing it with me at home and my friends weren't doing it at college, so there was no motivation to continue. I started reverting to my old ways. I wouldn't eat for half a day, then start bingeing into the night. I knew things were getting out of control, but I didn't want to return to the US because I missed my family too much and I was desperate to go to college and be a normal teenager."

There has been no comprehensive plan from any political party to tackle the obesogenic environment. The unwillingness to talk about fatness allows politicians to avoid the issues or offer half-hearted responses; above all else, it enables them to avoid what they fear would be a damaging confrontation with the powerful economic players within the food and drink industry. Politicians are also afraid they will be accused of taxing the poor if they hike the prices of cheap foods – an argument often put forward by their industry friends. One government after another has opted for talks and voluntary agreements on food labelling and marketing to children. The deals that have been struck have been partial and ineffective.

We are ill-served by many of the bodies held up as experts in food and nutrition, because they are not impartial. They take money from big corporations, often with a clear conscience, because they don't see harm in processed foods and drinks. The impressively named British Nutrition Foundation published a "facts behind the headlines" paper to debunk allegations that sugar is toxic. The foundation claims independence, but is full of scientists who believe in working with industry, and it is funded extensively by the world's largest food corporations. Its "sustaining members" include Coca-Cola, Danone Waters and Dairies, DuPont, Kellogg, Nestlé, PepsiCo, Tate & Lyle, Associated British Foods (which includes British Sugar) and Unilever.

The paper said that "overall, evidence does not support the claims made that sugar increases the risk of obesity, diabetes and heart disease". The foundation preferred the views of the European Food Safety Authority (EFSA), which suggested the data on the effects of consuming large amounts of sugar were "limited and mainly short-term". The EFSA has been embroiled in controversy over conflicts of interest.

And then there is the UK government's own advisory body, SACN, the scientific advisory committee on nutrition. On this are a number of scientists who work with big food companies and see nothing wrong with it. Chairing the SACN working group reviewing carbohydrates in our diet – including sugar – is Professor Ian Macdonald from Nottingham University. He is a paid adviser to Coca-Cola and Mars, although he stepped down for the duration of SACN's sugar inquiry. He sees no reason he should not work for industry. In fact, he thinks it's a good thing. He believes it gives him perspective. "I have explained my associations with industry to the Department of Health, and they are quite happy with the relationships," he told me. He has advised Coca-Cola that they have an issue, he said. "I told the chief executive of Coca-Cola in Atlanta that he has a real problem, because his company is perceived as being a cause of the problem and they need to do something about their promotion of full-strength Coca-Cola and its status within the business. His senior colleagues winced when I said that to him in the board that I sat on, but he took it on the chin and they are beginning to do something about it."

The trade bodies Sugar Nutrition in the UK and its global counterpart, the London-based World Sugar Research Organisation (WSRO), both argue that there is no good scientific evidence to blame sugar for obesity. They cite one study after another, most of them industry-funded, on the benefits of sugar. For all their bullish stance, they look to be on the back foot. The WSRO's director general, Richard Cottrell, who was previously director of Sugar Nutrition, no longer seems inclined to share his objective views of the science. "I don't speak to the press," he said when I called him, and put the phone down.

‘Over the years I dieted myself up from a sensible size 10 to the 20 I am now, with enormous amounts of self-loathing. Now I think of the years of my life I put on hold.’ Photograph: Johanna Parkin for the Guardian

The food industry and its friends and collaborators would like us to think it is all our fault. You can eat whatever you like, they insist – there is no such thing as bad food. It is your fault if you overeat and get fat. They are partly responsible for a blame culture that delivers the overweight, burdened with shame and self-loathing, into the hands of the diet gurus.

Yet the diet industry, with its gimmicks, motivational books and celebrity endorsements, is one of the biggest frauds of our time. In the UK, it is estimated to be worth £2bn. There are vast amounts of money to be made from the raising and dashing of people's hopes. When you look at the scientific studies carried out on people trying to lose weight, it's hard not to think that all the blockbuster diet gurus are charlatans – if not, one can only assume that they are incredibly hopeful and optimistic people. They must be wonderful at parties. Because all the research points the same way: it tells us that our bodies, having got fat, will do everything possible to keep that weight on. It's the fear of starvation. We are like hibernating animals that store up food so they can survive a hard winter. Except that, these days, winter never comes. Restaurants and takeaways are full of food. The pizza delivery is on its way.

Kelly Brownell at the Yale Rudd Center in the US invented the term yo-yo dieting, also known as weight cycling. People embark on very low-calorie diets and all goes well for a number of weeks or months. But then they get the plateau effect. The excess weight stubbornly refuses to shift. Depression and fatigue set in, making it impossible to continue. Weight goes back on and disproportionately what used to be muscle is replaced by fat. It is bad for health and harder to lose next time. So could diets actually shorten your life? Brownell and colleagues looked at this back in 1991, in a study published in the New England Journal of Medicine. Brownell's team looked at the data produced by a large and very well-known long-term trial in the US called the Framingham Heart Study, which followed the lives and monitored the changing health of more than 5,000 people in a town of that name in Massachusetts from 1948. They found that people whose weight fluctuated a lot had a greater risk of heart disease and early death than those whose weight was steady. And this was most marked among the youngest – aged 30 to 44 – who are both the most diet prone and the least likely to fall ill for other reasons. While they point out how hard it is to get the evidence to prove that diets shorten lives, they warn of the possibility that yo-yo dieting may actually cause chronic disease.

Angela Meadows, from Birmingham University's psychology department, believes that diets, not the excess of junk food around us, are actually the root cause of the obesity epidemic. It is going on diets that causes people to gain weight, she says. "I started dieting when I was probably 12, 13 years old, when I wasn't fat. I dieted myself out like most people – if they are dieting, they tend to get very big, and they get bigger and bigger with every diet, and there is more self-loathing, more blaming, more comfort eating, more withdrawal, so you've got that avoiding, coping behaviour.

"The worse you feel about yourself, the less likely you are to go out and actually exercise, and people do sit at home and comfort eat, and then they get bigger and their problems get worse." Meadows' view, and that of the Health at Every Size movement, is that it is possible to be fat and fit – if you are active enough, your weight does not matter, because it is no longer a health issue.

"So over the years I dieted myself up from probably a sensible size 10 or whatever to the 20 I am now with enormous amounts of self-loathing. Now that I've stopped doing that, it's like I've got my life back. I think of the 30-odd years of my life that I put on hold and wasted. My mother's 72 and still won't leave the house unless her arms are covered. She still doesn't eat properly. She's dieted all her life as well. She was thin when she got married and got bigger. Generations of women have been wasting their lives on this, and for what?"

It ended, Meadows says, when she began "eating intuitively", which she describes as listening to her body's signals. She says she does not now respond to the food environment around her – she is not at the mercy of advertisers. "Now that I no longer have that sense of deprivation, I no longer think I should do this and I shouldn't eat that, because then, when things are stressful or when that's just in front of you, that's what you turn to. 'I've had a hard day, I've been good, I deserve it.' If you're quite attuned to your body, you don't respond in that way."

Psychology professor Jane Ogden, who treats people with such morbid obesity that they cannot get out of a wheelchair, thinks we are getting to a point where the personal responsibility argument will not hold. "With obesity, perhaps the tipping point will come when it starts to be seen that the next generation is just following in their parents' footsteps and that child obesity is a form of child abuse. Somebody can have a BMI of 40, but if they have a kid, then they also are fat before the age of 10. That isn't acceptable any longer."

And the huge financial burden on the NHS may eventually force ministers into more action. "The government has to be much more nanny state in terms of policing the food industry, taxing snack food, taxing fizzy drinks, banning fizzy drinks, banning sugary foods, and not just in school dinners but also in work canteens and hospital food. Every kind of food provision has to be much more controlled by the government. Then they have to put money into cycle paths and street lighting and redesign their cities so that it is much more easy for people to be physically active."

But Ogden says she can see the other side of her own argument. "I do believe all that, but then there is the libertarian in me who says, 'Does that mean they ban Cheesy Wotsits?'

"I mean, my children sometimes have Cheesy Wotsits, but my children are perfectly thin because they don't have them all the time. Foods are bad only if you have too many of them."

The British Nutrition Foundation published a 'facts behind the headlines' paper to debunk allegations that sugar is toxic. It is funded extensively by the world’s largest food corporations. Photograph: Johanna Parkin for the Guardian

There have been few changes to the environment where Georgia Davis grew up that facilitate healthy living. Aberdare has a covered market near the station, where fruit and vegetables and meat and fish are for sale, alongside knitting wool and clothes and gimmicks and gadgets. But the main shopping streets offer every kind of cheap takeaway, from pizzas to pies to chips to curry. The Pop-In Cafe will do you homemade chicken curry, with both rice and chips – as well as a can of fizzy drink, tea or coffee – for £4.60.

To reach the housing estate where Georgia was brought up, you have to climb a steep road that runs up the far side of the valley. A lot of cars are passing, where once adults and children would have had no option but to walk. There is nobody on the pavement, but it is a wet day.

Georgia is not in the yellowish-cream semi-detached house, one of many identical homes on the estate. The shape of the hole made in the upstairs outside wall is still faintly visible under new brickwork and a coat of paint. Her mother appears at the door. "She doesn't live here any more," she said. "But she won't speak to you. She has a contract with the Sun."

• This is an edited extract from The Shape We're In, by Sarah Boseley, published by Guardian Faber at £12.99. To order a copy for £8.99, including free UK mainland p&p, call 0330 333 6846 or go to theguardian.com/bookshop.