There isn't much that Emma Allen doesn't know about dieting. She once gave up solid food for four months. It didn't work out. She tried the weight-loss programme NutriSystem, but needless to say, they didn't help either. She was even one of the first generation of Atkins devotees who were required, among other things, to test their own urine.

Yet while she was publicly attempting to shed the pounds, secretly, Emma liked being overweight. As a child she had fantasies of taking a pill that would make her fatter and fatter until she eventually just floated away.

She never told anyone, but when she got pregnant 18 years ago, everything changed. "It was like a religious epiphany," Emma says. "I remember having this incredible feeling that I could think about what was good for me, instead of calories. The possibility of thinking about food differently was a big turning point."

Over the next 10 years, Emma immersed herself in the world of size politics. She paid closer attention to the size liberation movement: a political movement that started in the 1970s and made size an axis of oppression. Groups such as Fat Underground and Fat Activists Together (FAT) fought for anti-discrimination legislation on the grounds of weight. Then three years ago she finally took the decision to do something she had always wanted to do. "I'd had these fantasies all my life and had been restraining them all my life. There came a time when I wanted to explore," she says. "I wanted to know more about what they were about. How would I feel about actually gaining weight, would I enjoy it?" In spring 2007, she took the plunge and gained 33lb, to reach a total weight of 17.5st.

Emma is a 49-year-old professor at a university in the north-west of England. She is also a "gainer" – sometimes known as a "feedee" – who overeats in an active attempt to put on weight. Although there are no statistics on the number of people doing this, gaining is more common than one might think. "They are everybody: every age, every country, every size; I mean, tiny, skinny people wanting to gain . . . it really is a case of, look around you, somebody is having these fantasy scenarios," says Emma.

This week Donna Simpson, a 42-year-old mother from New Jersey who weighs 43st, made headlines by revealing that her ongoing weight gain was part of her plan to become the fattest woman on earth. Pictured with an enigmatic smile and a burger in her hand, the press coverage showed varying degrees of restraint in highlighting the £400-a-week food shops, fast-food binges and unrepentant bid to hit 73st.

Gaining is often linked to feederism; a topic that occasionally pops up as freakshow fodder in magazines, chat shows or documentaries such as Fat Girls and Feeders: a 2003 Channel 4 documentary. This focused on the relationships between men and the overweight, vulnerable women they chose to fatten to immobility and beyond. Yet many women actively seek to gain weight of their own volition.

There are many websites and groups dedicated to gaining but Fantasy Feeder (FF to its members) is perhaps the most comprehensive. There are forums, stories and photographs that show unbuttoned blouses revealing pot bellies, wobbly tummies and impressive mounds of flesh cascading over waistbands. Large bosoms escape the confines of their bras, and rolls ripple beneath over-stretched T-shirts. Before and after pictures show the usual weight transformation journey, but in reverse. The poses are proud, matter-of-fact and often sexual.

There are lots of men on the site, but it is the images of female gainers that catch the eye. In our present landscape of body blandness, they stand out as controversial, bold and visually political. Fat is still, most definitely, a feminist issue for some female gainers."I think being a feminist has affected my relationship to my body and gaining in several ways," says Emma. "I started, very young, bucking the trends of beauty norms, like bra-wearing and shaving and makeup. I always thought that these practices were ridiculous; so that made it easier to go against the norm. Gaining is very liberating."

Others say they like making a statement with their weight because it challenges our stereotypical notions of beauty. Some, like Helen Gibson, a 40-year-old nurse from the Midlands, gain weight simply to please themselves. "It is my right to be fat; nothing about making a point." Yet even she concedes putting on weight after her marriage made her feel free: "Those three months were the most liberating of my life; I could feel the fat going back on. My tummy returned to its former glory – fat, soft and flabby, just how it should be."

Helen's husband knows she is a gainer, as do friends, who are well aware of how much she "adores being fat"; understandably, though, being an NHS employee, she cannot come out of the gaining closet completely. At the latest estimate, 57% of women were classified as being overweight, including 25% who were obese. Overall, obesity and related health issues now account for 9% of the NHS budget. As a nurse, says Helen, she cannot be seen to publicly advocate being overweight. For others, anonymity is the result of not wanting anyone to know, which might explain the profusion of headless pictures on the FF website.

As any gainer will tell you, life outside the community can be harsh. There is still a huge amount of derision and discrimination towards the obese, so the decision to keep their gaining a secret isn't really a surprise. Lauren, a 20-year-old American gainer, says she does not want to offer more ammunition to people by explaining the predilection. "As a fat woman, I have experienced fat discrimination almost on a daily basis," she says. "It's usually not so glaring as an intolerant jerk screaming, 'Diet, fatty!' but smaller, more painful ways: going to parties and no one talks to me, being glared at while I'm eating in restaurants, the snickering in changing rooms in department stores."

For many non-gainers, the practice seems strange because of the health implications – both physical and psychological. Even organisations such as the US-based National Association to Advance Fat Acceptance (Naafa) dismiss gaining on health grounds. Obesity experts say that being overweight can cause everything from heart problems and diabetes to high blood pressure and gall stones. The message is that fat and health don't mix. But Emma disagrees. She says that it would be more useful for people to consider the multimillion-pound diet industry and its "95% failure rate", and feels overweight people are instead blamed for all the world's ills. "I think people worry about health because it's the easiest place to hang fat hatred. The data actually suggests that it has to do with activity, and not size. People respond badly to anything that asks them to reconfigure their presumptions and preconceptions."

Psychologically, gaining is still a grey area. While one would assume purposefully overeating to gain weight is as much of a disorder as not eating, Susan Ringwood, chief executive of Beating Eating Disorders (Beat), says that isn't the case. "It isn't an eating disorder as such, because there is no morbid fear of fatness, or weight gain. In its extreme forms it is more likely to be a personality disorder that is organised around submission/domination and sexual fantasies."

Another theory, says psychotherapist Phillip Hodson, is that intentional weight gain for women could well be an avoidance tactic: they don't want to attract the unwanted attention of men, so they transform themselves into something deemed conventionally unattractive. Most women don't feel this way, but it could be true for a small minority. "I have come across cases where it's quite obvious that women deliberately become large, or remain large, for psychological reasons," he says. "These include trying to avoid attention and becoming sexually invisible. Some women use food to become so different from the stereotype and to avoid all that is involved in fitting that stereotype: from wolf whistles to being propositioned."

It's a thought, but it doesn't appear to mean anything to Emma or Helen who define weight gain in very sexual terms. Although Donna Simpson's press coverage glossed over the sexual aspect of gaining, for them, more fat means more sex appeal; the extra flesh that everyone else is attempting to shed fuels their desires.

Emma goes one step further to say that gaining is an intrinsic part of her sexual identity. She cannot gain at the moment because of MS and diabetes, but still calls herself a gainer.

For most of us, weight gain seems simple: a bit too much butter on your toast and one chocolate biscuit too many can mean the difference between zipping up your jeans or not. But the question of how to gain weight is quite a hot topic on Fantasy Feeder. There are "Eat Yourself Fat" tailormade diet plans to increase your weight, and the advice ranges from eating ice cream before bed to homemade milkshakes and lots more pasta.

While some favour junk food overload, others, like Emma, say that it is the very antithesis of what gaining is about. "For me, it's all about a kind of hedonism; it's about opening the doors and allowing in fleshy pleasures, whether it's food itself, or what happens to my body, or what happens to somebody else's body. I need a big variety, because what's appealing to me are contrasts of textures and tastes and aromas and colours . . . if I have to eat a big bowl of pasta, I'm not interested. I mean, I love pasta, but I'm not going to eat four servings of it."

Instead Emma maintains a healthy eating regime. "I know no one will believe this, but I eat lots of wholegrains, fruit and veg; probably a bit too much cheese, and chocolate – although I now only eat sugarfree candies. Fish, if it's fresh . . . of course. My diet isn't primarily McDonald's and KFC; in fact, it almost never is." Likewise, Helen's love of gaining is as much about the act of eating as the result. "It's the pleasure of food that is the biggest pleasure for me; followed by each extra roll of fat that comes with the amount that I eat," she says. "I adore how I look naked – and I have been known to spend far too much time admiring myself in the mirror."

The presence of online gaining communities has provided people with a support system. Many say it is like coming home. "This is our small part of the world where we are surrounded by people who say, 'You're not weird; it's perfectly fine to feel as you do, in fact, we think you're great because of it,'" says Lauren. "To virtually everyone, it is a liberating, wonderful feeling." Emma says that she is in the privileged position of "coming out" because she has little to lose: her partner will not leave her because of it, and she is unlikely to lose her job. Colleagues don't know, but she doesn't think they will be too surprised, given her outspoken views on fat issues.

As a moderator on the FF site, she comes across a lot of people who on the one hand are desperate to be fat, on the other, desperate to be thin. "Real desires need attention, not curing," she says. "Lots of people in the community want to understand why they have these fantasies and desires, and there's sometimes an undertone of; 'so that I can cure them'. Not always, but there are definitely people who feel that way."

Some, she says, are just as unhappy with their bodies as those trying to lose weight. "Most people who tell you that they're happy with their bodies are lying. There are people who are like, 'Yeah, I'm cool: fat is beautiful – I'm having weight loss surgery . . . certainly, there are women on FF who are dieting."

Being a gainer isn't as straightforward or easy as it might seem, she says. "One comes into contact with messages about weight loss, health and beauty, about, I don't know, 20 times a day. Every time you open your email, a magazine, every time you turn the television on . . . so any attempt to do anything different, takes incredible strength and courage – and we all fall down," including Emma. "Of course it gets me down! I often feel like all men – and women – believe that stereotype is beautiful, even though I know better," she says. "I hammer myself over not being that stereotype, but only when I'm having a bad time and am already vulnerable because of other things going on around me."

If we look around us, says Phillip Hodson, it is clear that regardless of increased pressures to be thin, we are getting fatter as a nation. "The natural figure of the hunter-gatherer has returned: good childbearing hips and a good abdomen," he says. "But I would be worried about people who are saying they want to get fat."

But Helen is not worried. At 16st she still only considers herself to be pleasantly plump. She has a picture in her head, she says, of what she will look like when she is fat. "I am a long way off that, although I am on my way," she says. "With each mouthful, calorie and year, I am on my way to achieving it."

Some names have been changed