Bethany Walton leads me inside the art school she attends in Lincoln, powering enthusiastically up the stairs ahead of me. We are bound for the fine art room, whose paint-daubed denizens have headed off for lunch, leaving their sketches pinned artlessly around the room. With a flash of green nail varnish, Bethany shows me her latest work. She is on a one-year art foundation course, specialising in photography, and her recent images include a monochrome still-life of an egg box, open and filled with hollow shells. "I need to do that one again," she says thoughtfully, motioning to a shadowy area. "It's not quite dark enough."

Articulate and funny, with a soft, sing-song voice, 19-year-old Bethany is, in many ways, an average indie kid, kicking around in her trainers and enthusing about her favourite bands - the Killers, the Klaxons, the Kooks.

Last April, though, Bethany's outlook was far less positive. Her weight had reached 34st 7lbs (219kg), and doctors had given her a life expectancy of 30.

Bethany was a little over nine pounds when she was born - within the normal range - but her weight started to become an issue at an early age. By six, she had noticed that she was bigger than her peers. "I didn't fit into the school uniform that everyone else was wearing," she says. "I couldn't wear the little white belt that went around the red-and-white checked dress."

At eight, she was prescribed anti-depressants on the basis that they would suppress her appetite. They didn't work, although she continued taking anti-depressants for the next 10 years.

She was brought up by her mother and her grandmother, whom she called Nana. She never knew her father. But Nana died when Bethany was 12, and she was overcome with grief. Already 15st, Bethany put on another eight stone over the following year.

She was given diet plans by her doctors, but her weight continued to rise until, aged 18, Bethany reached and passed the 34st mark. At this stage, if she failed to lose weight, her prognosis was singularly bleak. Doctors warned, she says, that she would probably get diabetes and joint problems. "and I could end up in a wheelchair and die of a heart attack or heart failure by the time I was 30."

Over the past few years, public concern about child obesity has risen exponentially, though probably not quite as fast as the problem itself. Statistics bounce across the newspapers: obesity in six-year-olds has doubled in the past decade; 13.4% of primary school children are obese; a quarter of schoolchildren are overweight when they reach secondary school. In recent months, we have seen a rash of stories about seriously overweight children, starting with Connor McCreaddie, an eight-year-old boy whose mother had to attend a child protection conference when his weight rose to 14st. Then there was Samantha Hames, a nine-year-old who also weighs 14st. Despite their age and vulnerability, Connor and Samantha have been portrayed as marauding monsters, junior Godzillas, eating and destroying everything in their paths. So, for example, it has been noted that Connor has broken four beds and five bikes. And to make it clear just how big Samantha is, the Sun chose some piquant images. "She weighs nearly THREE times most girls her age - the equivalent of TWO Victoria Beckhams and more than beefy soccer star Wayne Rooney."

For those who choose to go public, this treatment seems par for the course. Bethany experienced it herself last year as the subject of a BBC Three documentary, the baldly titled The 34-stone Teenager. Appearing on GMTV, she was shocked to see the description "Britain's Fattest Teenager" flash up on screen. She rolls her eyes: "I just love that title." She was also hurt by a Sunday Mirror headline that claimed "Size 32 Bethany has never been dancing or had a boyfriend." Actually, she says, "I did have a boyfriend at school ... but he really hurt me, so I didn't want to talk about it."

Speaking to Bethany, all the lurid headlines and empty stereotypes (the notion that the obese are stupid, lazy, or greedy) fall away. Ask her about food, for instance, and there are no tales of fast-food palaces, of stocking up on chips, fried chicken or kebabs. "I've only been to McDonald's or any other fast-food places maybe three or four times in my life," she says, "and I don't eat burgers. Mum has always been really adamant: 'You're not eating any of that rubbish.'"

She doesn't deny that over-eating is the cause of her condition ("We had tests done to see if there was something physically wrong, but there wasn't") but her problem has always been portion size - eating far too much at each meal. Although her mother started trying to control the amount she ate when Bethany was young, she would steal chocolate from her three brothers (all of them 10 or more years older than her) when they came home from boarding school in the holidays. "This was when I was four or five," she points out, "and I wasn't allowed chocolate, so I used to steal it, like any young kid would. I just didn't understand the consequences. I always thought, my mum will make it all better - and it wasn't until I was 10 or 11, around the time that Nana died, that I realised, oh, mum can't fix everything. She couldn't make Nana better, and she isn't going to be able to help me with my weight."

Next week, Bethany is the subject of a follow-up documentary, having been shadowed by a film crew since last April, when she had an operation called sleeve gastrectomy. Performed by keyhole surgery, the operation involved the reduction of her stomach by 90%, the insertion of a line of staples and cutting away some of the excess organ. What is left of Bethany's stomach is shaped like a sleeve, and only the width of a thumb.

The approach to Bethany's condition is interesting, given the guidelines issued by the National Institute for Clinical Excellence (Nice) last December, which suggested that obesity surgery should be available to the small number - and it is a tiny minority - of teenagers who are severely obese. The guidelines caused some controversy when they were issued, perhaps not entirely surprisingly. After all, the idea of performing major surgery on children, for what should be a preventable condition, is highly emotive.

But what else can be done when a weight problem has spiralled? Could Bethany, as some people have suggested, not just have tried another diet? She sighs. "If I had gone on a typical diet, it was going to take me five years to lose all that weight. To have the willpower to keep going for that long, without any lapses - or certainly any major ones - would involve being almost superhuman. People who say 'Oh, just go on a diet' should look at the reason why the person is eating. Is it an emotional factor or is it a physical one? For me, it was an emotional thing If you find something that makes you feel better when you're in tears and you're helpless, then you're going to want it, aren't you?"

Still, it wasn't an easy decision to have the surgery. Such procedures typically have a mortality rate of 0.5%. Bethany was afraid of dying ("The time I must have spent with my best friend, Hayley," she says, "asking 'What if I die? What if I die?'") but the alternative was simply to "get fatter, not go to college, not do art and die miserably in my 30s or 40s with diabetes and heart problems".

How did she feel after the surgery? "It was horrible. I've never been in such pain - I felt like I'd been snapped in two." Taking her A-levels a few months after the operation, Bethany sailed through her art exams, but struggled with sociology and human biology. "I didn't realise that being sat down for three hours was going to hurt so much."

Her diet in the first few weeks post-surgery was entirely liquid - "water, milkshakes, non-lumpy soups" - and she had to introduce solid foods into her diet slowly, testing each one to see whether she could digest it. She hit a sticky patch when it turned out that one of the easier foods to digest was cheese sandwiches - something that she had gorged on before the operation - but since then she has been more careful. "I tend to like healthier food now. I try to eat as many green things as possible. I try and get my five a day ... Mum cooks meat-and-vegetable-type meals in the evening, and I have mine on a tea plate."

The operation had immediate physical effects - she lost almost two and a half stone in the first six weeks - but the emotional effects were stark. She had to ditch her anti-depressants just before the surgery, and was also left without the crutch of comfort eating. Suddenly, all her emotions were "Out. Everything feels really raw ... and I didn't cope very well for a while ... the anti-depressants had been masking my feelings and diverting my thoughts. They change you as a person, I think. I wasn't the real Bethany, then. And I think now I am."

In the film, Bethany's mother repeatedly says that her daughter should have had more psychological support over the years and you can't help agreeing. She is bright, friendly, driven, and also someone who feels things deeply. Aside from some counselling after her grandmother's death - "which didn't really help" - the main strategy for treating Bethany's obesity seems to have been to encourage her to diet, which always failed. She found herself in the same cycle of behaviour that many people do. Something - a trauma, boredom, or simply a love of food - triggers the urge to eat too much, and, as the person gets bigger, the unhappiness and isolation caused by being overweight becomes a trigger in itself. Bethany has had to cope with years of bullying - being called "fat bitch" or "fat whore" in the street, or being approached by "drunk men saying 'All right love, you look easy.' It was disgusting." She also had to cope with abuse at school, which led her to leave on her 16th birthday, before her GCSEs. (She took a year off "to recuperate really" before taking her GCSEs and A-levels in two years on a specialised college course.)

In the year since the surgery, Bethany has lost eight stone - the amount she put on in the year after her grandmother's death - and she clearly feels excited about the future. "I feel that I can do things now. I can wear more clothes, although I'm struggling at the moment, because most of the fashions are just a long top with leggings, aren't they?" Her health is better, too. She used to find walking difficult but now, "I can walk around for hours and hours and hours."

There are downsides to her achievement. There is a possibility that, once Bethany has lost all the weight, she will have to have cosmetic surgery to trim off the excess skin. "I look at my friend Hayley, you know - and I sound like a 40-year-old saying this - but everything's firm, everything's where it's supposed to be, smooth and perfect. On me, it's not. I could get to 25 and be 12st and have scars all over me. I could look like a right car accident ... So I'm getting rid of an old problem, but it's creating another one. You hear about people having cosmetic surgery and their leg rots off, or they die, or they end up with one eyebrow stuck on their left buttock or something. I do sometimes feel like I've wrecked my body already," she says sadly, "and I'm only 19."

Then there is the difficulty of adjusting psychologically to a body that is changing at a pace. "I am worried about that," she says, because "I don't know what I'm going to look like. I don't know the person under here - she's only just starting to come out and the people around me haven't met her yet. I don't even know her."

It's a complicated time for Bethany but, for now, the weight is still coming off, and her future has been transformed. She is heading for the University of Lincoln in September to start a degree course in photography, which she hopes to follow with an MA in London. On the wall above us, an art teacher has scrawled, "Can we transcend?" In Bethany Walton's case, it seems the answer is yes.

· The 34-stone Teenager: Six Months On is on BBC Three on Monday at 9pm.