In January 2004 Jo Godley, a training consultant from Newark, Nottinghamshire, joined a slimming club. Like so many people at the beginning of a new year, she was keen to lose a bit of weight. Actually, keen doesn't even come into it. Jo was desperate. She had learned to deal with the fact that the only place she could shop for clothes was Evans, with its yards of nylon and horrible elasticated waistbands. She had learned, too, to stage manage situations at work where she simply could not fit her behind into the relevant chair (at conferences she would stay standing or turn and flee, on the pretext of illness). No, the final straw came after she had the second of her daughters, when she would find herself marooned on the floor long after she had finished changing a nappy or building a tower of bricks. 'It was awful,' she says, softly. 'I just couldn't get up.'

At this time, Jo weighed 23 stone. She decided there was nothing for it: she would have to resort to weight loss surgery or be fat for the rest of her life. As a fat twentysomething, she had often wondered how people could ever be as big as 30 stone. Now, at the age of just 33, she could easily imagine herself swelling to those proportions. It would happen slowly but surely, and the idea of it scared her far more than the surgeon's knife. In April 2004, Jo underwent a duodenal switch, a radical procedure in which both the size of her stomach and her body's ability to absorb nutrients were drastically reduced. Result? She is now a 12-stone shadow of her former self.

The weight fell off effortlessly, like sandbags dropped from the basket of a hot air balloon. No sooner had Jo discharged herself from hospital than she took a taxi to her mother's house in Sheffield, where she ate - albeit cut up into tiny pieces - roast beef and Yorkshire pudding. At first, eating normally was hard: toast would stick in her throat, and she would often have that 'Christmas dinner feeling - you're so full, you can't move'. But she was soon back to her old self, a creature who disdains fruit and vegetables and cottage cheese in favour of crisps, biscuits and Chinese takeaways. Yet still the weight disappeared. She lost two stone in the first three weeks after the operation, and a stone a month thereafter. The changes to her body were dramatic, and prolonged. For the first time in her life she had collar bones, knobbly wrists and a proper jawline. In the course of a year she had to buy six entire new wardrobes. Catching sight of herself in a shop window, she saw only a stranger. Was she thrilled by all this? Is chocolate ice cream the best thing ever?

But Jo still had to live her life. As a fat person, it was all too easy to imagine that the lot of the slim was always a happy one. But as I discovered when I set out to find out what life is like for the suddenly skinny, it isn't quite as simple as that. Weight loss surgery might be a quick fix for the body but it doesn't change everything else. Dramatic weight loss has a nasty habit of exposing not just a person's collar bones but a lot of other stuff, too: their relationships, their place in the hierarchy at home and at work and, perhaps most significantly, their feelings. A compulsive eater who has had bariatric surgery is still a compulsive eater; that part of their personality remains intact long after their stomach has been stapled, as does the reason for their over-eating - 'head hunger', they call it. Through thin eyes, the world looks different, but not necessarily better.

Jo does not regret her operation, not for a minute. The great moment - the fantasy moment - came when she bought a pair of leather trousers in size 14. But, to her evident amazement, the benefits of her new shape have been far more wide-reaching and insidious than merely a smaller bum. 'People look at me more,' she says. They look at me directly. They respect me more, listen to my opinions. I have more credibility. I don't know whether it's a conscious thing or not, but when you're big, people think you have no self control.'

Jo has a senior job at Nottingham City Council, and her new colleagues have never known her as anything other than a size 14. Does she feel that being slimmer has given her a career boost? 'I think it has. I'm more confident. The look of me is more pleasant. I fit in more.' Socially, people who used not to look at Jo twice are now very friendly. How does she feel about that? 'I'm very conscious of it. It's all: "Oh, come out with the kids for a barbecue." But, to be honest, that upsets me far less than all the people that I no longer see.'

And here's the rub. For reasons she cannot entirely fathom, since she lost weight, Jo has lost touch with some of her old friends. 'I haven't changed at all personality-wise; I've always been quite confident and outgoing. But some of the friends I had when I was big I no longer see. They don't return my calls. If I do see them, all they want to talk about is my weight. It's really disappointing.'

Does she think they are jealous? She isn't sure: not all of them are big themselves, though she admits that when she was in her twenties, certain of her friends were chosen purely because of their size. 'I used to have one girlfriend who was 26 stone. We would go on holiday together, and she'd often say: "I would never go away with any of my thin friends, only you." She laughs. 'We hated each other, but we were comfortable with each other.'

Then again, perhaps she has changed more than she thinks. Her husband recently bought her a convertible - a car that in many ways, has come to symbolise her new life never mind celebrate it. 'It's bright yellow, and it's me as a teenager or in my twenties: wearing short skirts, going out, flirting, being extreme.'

Like many people who have been overweight since they were a teenager, Jo Godley believes she missed out on being young. Her body kept her at arm's length from one-night stands and bad boys and going out on the pull: 'I would always go for old, safe, stable men who told me I had child-bearing hips. I was always in long relationships.' But now the weight has gone, she half-wonders what it would be like to try all those things after all. 'What would it be like to be with some cocky guy, or with someone who treated me rough? I would like to see what kind of arseholes I could meet. I do love my husband, but I want him to give me 30 per cent of my life to go off and have fun with my friends. But that wouldn't be fine, and I won't do it. I won't throw off my comfort blanket.'

Has her weight loss had an effect on her marriage? Her voice is a whisper now. 'He loves big women. He's plump himself. He finds me less attractive, and it's made him self-conscious about [his own body]... It has affected us quite negatively, I suppose. It's terribly disturbing. I look at him and think: is that the man I married? It's a very horrible, selfish thing.'

There are no accurate, up-to-date figures for how many people undergo weight loss surgery in the UK. There were 400 operations on the NHS in 2002, but many more have such procedures done privately (this is what Jo did). However, according to Ken Clare of Weight Loss Support, a charity that works with people who have had, or are thinking of having, bariatric surgery, obesity is such a serious problem that some 1.5 million people are now eligible for operations on the NHS under the criteria laid down by the National Institute for Clinical Excellence (NICE recommended that bariatric surgery be available to the morbidly obese - people with a body mass index of 40, or 35 if they also suffer from other significant weight-related disease such as diabetes or high blood pressure; BMI is calculated by dividing weight in kilos by the square of height in metres). Of those 1.5m, NICE believes that 4 per cent might seriously consider surgery. Radical surgery is an increasingly popular way of treating a peculiarly stubborn 21st century disease.

Stomach surgery was first carried out on animals, and then on people, in the late Sixties. Today there are three main techniques: gastric by-pass, in which a small pouch is formed using the top of the stomach and then reconnected to the small intestine; duodenal switch, in which two-thirds of the stomach is removed and two-thirds of the intestine by-passed; and gastric banding, in which an adjustable silicone ring is looped around the stomach, then attached to a port sewn into the abdominal muscles. To tighten it, the doctor inserts saline directly into the port, and the ring constricts. In the UK, gastric banding will set a private patient back £6,000, while a duodenal switch costs between £10,000 and £12,000. If carried out properly, all three techniques boast good long-term success rates. However, going under anaesthetic is a huge risk for the morbidly obese. Mortality rates are relatively high: for gastric banding, the least radical procedure, the figure is 0.5 per cent; for gastric by-pass operations and the duodenal switch, that rises to 1 per cent.

All three operations, moreover, have serious physical side effects. Post-surgery, some patients find they can no longer tolerate wheat; others lactose. Patients who have a band inserted find they are prone to feeling sick, while those who choose to have a duodenal switch suffer bowel problems, including terrible diarrhoea and stomach spasms. All patients find themselves deficient in certain vitamins, and are destined to rely on supplements for the rest of their lives. And then there is the issue of mental health. One American doctor has reported that about 30 per cent of her clinic's patients convert a passion for food into a passion for the bottle, while in this country there is anecdotal evidence to suggest that some patients develop a compulsion for cosmetic surgery (most of those who have had weight loss surgery are stuck with an excess of baggy skin; some undergo an operation to get rid of it, and this is when they get hooked). 'Surgery seems very alluring,' says Deanne Jade, director of the National Centre for Eating Disorders. 'But it focuses on the immediate problem, not life afterwards.'

Jade was a member of the committee which compiled the 2002 NICE guidance on surgery for morbid obesity. She believes that patients who undergo surgery should also be offered at least a year's counselling, though this is rare at present. 'Some patients are freed of their cravings,' she says. 'But others aren't. And life does change. People may feel under more pressure. They can no longer hide behind their weight, and may need to learn new relationship skills, social skills, even shopping skills. When you are overweight, your persona develops accordingly. Life might be a bit of a joke, for instance, or you may not be a sexual person. Then the boundaries change. Compulsive eaters are not good at recognising their other important needs. They lack self confidence. They go through life like a bagatelle ball, moving away from what hurts them. Food for them is a refuge in the storm.' So when that refuge finally disappears, life can seem even tougher than it did before? 'Sometimes, yes.'

Sarah Smith certainly thinks of food as a refuge. There have been times in her life when it felt like the only thing that gave her any pleasure at all. Sarah, 42, is a human resources director in a big City law firm. To the casual observer, she is an incredibly successful woman. She has a salary of £100,000 and lives in a pristine house in a quiet Hertfordshire village; in the drive sits a gleaming BMW and, beside it, a convertible Mercedes which may be about to replace it. But appearances can be deceptive. Today Sarah weighs about 11 stone. Eighteen months ago she was 22 stone, with a BMI of 54. She was so huge that she was no longer able to stay standing while doing the washing up. Her knees ached, her blood pressure soared. Buying clothes was becoming increasingly difficult. She would wriggle into a size 32 and think: where do I go from here? Not even Evans sold anything bigger than that.

'Chocolate wasn't my problem,' she says. 'My enemy was carbohydrates. I love pasta and rice. They gave me such a feeling of satisfaction. I work very hard in a stressful job and, once a month or so, my treat after a long week was a takeaway. I would reach a certain point on my way back up the M1 where I'd know: if I ring the Chinese takeaway now, by the time I get home the bloke will be at the door. It was a wonderful way to reward myself. I loved the pork curry and fried rice and would order two portions. One wasn't enough. Two was too much, but I can't leave food.'

Sarah was in a longstanding relationship with a married man, and this, too, was the cause of over-eating. 'When you're with a married man, there are a lot of times when you feel sorry for yourself. You find yourself on your own. They haven't turned up. If it was my birthday, or I'd had a good day at work, [I'd have] no one to celebrate with, or to give me a cuddle. So then I'd eat. It was unhappiness. It was the only thing that made me feel good.'

Sarah had tried every diet going. Nothing worked. So in February 2004 she had a gastric by-pass. It was agony - when she came round from the anaesthetic she truly thought she was going to die - but it worked. After seven months she had lost nine stone. Sarah travels a lot with her work, and she soon began to notice she now needed a smaller overnight bag: no more tents, no giant-sized underwear. She was delighted. Cinderella could go to the ball - literally. No longer did she make excuses when invited to family weddings. No longer did she tremble and shuffle off every time she passed a buggy in the supermarket (she used to dread a child saying: 'Look at that woman, Mummy'). One day, as she took a short cut to her office through a department store, a tiny pair of pointy kitten heels caught her eye. She had long since given up on delicate shoes; her feet would only spill over their sides like so much ripe brie. But still she was curious. It was amazing. In went her foot, just like that. She danced a delighted little jig, and bought herself four pairs.

But has her transformation been wholly positive? Sarah has never revealed to her colleagues, and some friends, the real reason for her weight loss: they believe she simply dieted. When I ask her why, she begins to cry. 'I feel very ashamed that I got that fat. I don't want people to think I took the easy way out, or cheated, or that it was about vanity. I hate it that I've failed, because in everything else in my life I've been successful.'

She might be lighter of body but she is more burdened than ever in the sense that she must keep this secret. And then there is all that spare skin. She loves being able to lie in bed and see right down to her pubic bone. She also loves being able to fold her arms. But the skin is horribly disfiguring and she is too afraid to have more surgery to get rid of it. 'I think I'm carrying about a stone in skin,' she says. 'It's across my stomach and the tops of my arms. But the worst is my breasts. They're horrible. They're like rabbits ears with Jelly Tots on the end. I always felt my weight was one reason why I never married. Now I look more acceptable, someone might be interested. But how could I expect anyone to accept these boobs?'

Sarah does not regret her surgery. She is envious of the 20-year-olds whom she has heard are having it now; she feels the last 20 years could have been different had she done the same. But this is not to say that her dreams have come true. 'The one area of my life before that I put down to my weight was personal relationships. Apart from the married man I have been seeing for 25 years, no one else has ever shown any real interest in me. I've never been chatted up - nothing. I thought when I lost the weight that things would be different. I'm tired of being on my own. But I haven't got a queue of men at the front door. I don't know what I look like to other people. Do I still look fat? I had this idea that people who weren't fat got a lot of attention from men. Now I think: am I an ugly person?'

Sarah isn't ugly - far from it - yet I find myself listing several of the world's most beautiful women and pointing out what hopelessly lonely private lives they have. 'Yes,' she says, her face briefly lighting up. 'I'm always telling myself that even attractive people get their hearts broken.'

Jo and Sarah are magnificent women: feisty and funny and painfully honest. But still it is hard to avoid the feeling that they are also proof positive of how destructive and impoverished a culture of instant gratification can be. I want a cake; here's a cake. I want a new body; here's a new body. In this context, words like 'treat' and 'reward' and 'hunger' - words those who have had weight loss surgery use often - no longer mean quite what we think they do. The most positive and uncomplicated of the women I talked to was Audrey Shanks, a 33-year-old IT helpdesk manager from Scotland who, two years ago, opted to shrink her stomach with a gastric band. Before her operation, Audrey weighed 27 stone; she is now 14. Audrey likes her new body, and so does her husband. Although her skin looks like a 'burst balloon', she feels sexier. She can use a public loo and, if she travels by bus, she no longer worries if someone tries to squeeze in next to her. More significantly, she believes her band is keeping her from an early grave. Even so, there is now a disorienting absence in her life. I had a sense of this weird, troubling vacuum when I met Jo and Sarah, for all that they were busy trying to fill it with fast cars and, in Jo's case, more surgery ('After my skin and boobs, I want everything done,' she said. 'Eyes, nose, the lot!'). It is, however, Audrey - still full from her supper - who articulates it best. 'They do the surgery on your body, not your head. But, of course, it would be better if they could just stop the hunger, because that's the thing that never goes away.'

· Sarah Smith is a pseudonym