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Harriet Jenkins is watching an old video of herself from seven years ago. She's filmed standing on a stage, wearing a little black dress and pearls, with her blonde hair swept glamorously back.

Her name is called and she clasps her hand to her mouth in disbelief as the crowd bursts into applause.

It might have been the Oscars, given her reaction — and she looks every inch the star. At that moment she was. Harriet, a teacher from Southampton, had become a celebrity of the dieting world: Slimming World's Woman of the Year.

Harriet, who's 5 ft 8 in, once weighed 26 st, but she lost 15 st in 15 months, going from size 30 to a size ten.

Harriet Jenkins (pictured), a teacher from Southampton, was once a celebrity of the dieting world: Slimming World's Woman of the Year. Harriet, who's 5 ft 8 in, once weighed 26 st, but she lost 15 st in 15 months, going from size 30 to a size ten

Seven years after losing the weight, it's a different story. Just weeks after the competition, her weight began creeping back up and today Harriet is not a size 10

Seven years later, it's a different story. Just weeks after the competition, her weight began creeping back up and today, while she doesn't reveal her weight, she's clearly as far away from a size ten as she was before the competition.

Harriet is one of several 'Super Slimmers' in a new Channel 4 documentary exploring why weight loss is so difficult to maintain.

These slimmers won some of the dieting world's largest weight- loss competitions, but most of those featured have regained much of the weight.

Around 40 per cent of people who achieve extreme weight loss — up to 20 st — put it all, or more, back on, experts say. And it's not just extreme slimmers: each year 30 million of us try to lose weight, and 10 per cent put it back on within three years.

Harriet Jenkins (right), pictured with a friend before she dropped 10 dress sizes through healthy eating and exercise

Sick of comments about how big she was, Harriet joined Slimming World with a friend. 'Losing weight felt good,' she says. 'And when something feels good, you want to keep on doing it.

'I did want to be slimmer of the week and slimmer of the month. It brought out my competitiveness. I'd waited — and wanted — for years to look like that.'

After 15 months on the diet plan, she did. But just weeks after she'd been made a Slimming World ambassador, Harriet started to pile the weight back on — 'at about the same rate that I lost it — a stone a month,' she says.

'I'm pretty sure I put it back on because I was eating too much.' Another super slimmer whose weight loss quickly reversed after the photo-calls is Jane Hall. 

Jane Hall (pictured left, before losing the weight and right, after) from  Lancaster was crowned Rosemary Conley's slimmer of the year in 2012 after losing 8 st over two years

Since losing the weight Jane has put five stone back on. 'When I lost all the weight, I was convinced I wouldn't dare go out and see anyone if I put weight back on. But it happens so quickly,' she says

The 59-year-old from Lancaster was crowned Rosemary Conley's slimmer of the year in 2012 after losing 8 st over two years.

'When you lose weight in a big competition, you begin to feel famous,' she says. 'You're in the local magazine, national papers, on the radio and TV. I was in one of The Hairy Bikers' programmes and they came round to my house for lunch.

'The year after I won was great. I loved it. It took me an hour to get round the supermarket because everyone wanted to talk to me and tell me how great I looked and how I was inspiring them to lose weight.'

Since then, Jane has put 5 st back on. 'When I lost all the weight, I was convinced I wouldn't dare go out and see anyone if I put weight back on. But it happens so quickly,' she says.

That year, Leroy Wilson, 50, a DJ from Basingstoke, Hampshire, was also being hailed as a weight loss champion after losing 17 st in seven months (he's 6ft 4in) – and going on to lose another 3 stone. 

He had been so overweight doctors told him he could have a heart attack if he didn't lose weight.

Leroy Wilson (left, before his original weight loss), lost 17st in just seven months after being told he could suffer a heart attack

Leroy Wilson (pictured), went on to lose another 3st - being hailed as a weight loss champion

But Leroy, who is 6ft 4in — who only three years ago appeared on the front of LighterLife magazine, suave and slim in a black suit — now weighs 23st

'I used to have one little mantra: do it or die,' he says.

And it worked. Weight loss felt like a new lease of life. 'I could run up and down stairs. I could play squash for two hours. I was skating and playing ice-hockey, it was unbelievable,' he says.

'I went to the funeral of a friend who'd been as big as me and died because he didn't do anything about it, and I thought: 'That could have been me.' '

But Leroy — who three years ago appeared on the front of LighterLife magazine, suave and slim in a black suit — now weighs 23 st. 

This is partly because he has a blood disorder and is taking steroids, but also because he's 'lost focus'. 

'When I lost weight, I became a celebrity, of sorts. I got loads of letters from strangers telling me how inspiring I was, and more interest from women.

'But there's also something weird about being thin when you used to be obese. And the attention made me insecure.'

This might be what partly lies at the root of weight regain: how the slimmer perceives their weight loss.

As Jane Ogden, a professor of psychology at the University of Surrey, explains: 'When people lose vast amounts of weight, they often experience a sense of rebirth. 

When Leroy lost weight, he had turned into a celebrity of sorts. He got lots of letters from strangers telling him how inspiring he was

'Huge increases in self-confidence and well-being are reinforced by positive comments from other people.

'But their body esteem doesn't always keep up with their change in shape and sometimes they still have an image of themselves as being overweight.

'Sometimes weight gain is caused by not liking the new, thinner self, which can be a challenge to their identity and relationships.'

It can occur as a gradual slipping back into old habits. 'This can be caused by emotional eating or work or social eating,' she says.

'It can also occur as a rebound effect after months of denying themselves food and building up a preoccupation with what they weren't allowed to eat.

'When weight loss is maintained, this is often the result of a process of reinvention when people can reinvent themselves as a new thinner, healthier person.'

There is also the problem that extreme weight loss is almost impossible to maintain, says Traci Mann, professor of health psychology at the University of Minnesota.

'Very few people know diets don't work in the long run. 

'Their body is physically changing because of dieting, your metabolism and hormone levels change, so you feel hungrier, but can eat less.

'Because people don't know those things, they blame themselves when the weight comes back on. Regaining it is a predictable consequence of losing it.'

The reason is partly evolutionary, says Dr Thomas Barber, an endocrinologist at University Hospital, Coventry. 

'One of the biggest threats to a species' survival was starvation,' he says.

'Any loss of fat would have typically occurred during a famine, so it makes sense during those circumstances to conserve energy and lay down more fat to mitigate the harmful effects of starvation.

'It does this by enhancing appetite and seeking food to replenish depleted fat stores, largely through a hormone called leptin (the satiety hormone).'

But when we put on weight, too much leptin is produced, causing overeating. Something called 'persistent metabolic adaptation' is also to blame. 

When we lose weight our metabolism slows down, says Dr Barber, meaning we have to eat less not to put on weight.

'That might make people think: 'What's the point in dieting if I'm just going to regain the weight?' But a small minority of people can maintain it,' says Dr Barber.

One is Daniel Wheeler, 31, from Staines, Surrey, who appeared on the cover of Men's Health magazine in 2012 having dropped from 22 st (he's 6 ft 4 in) to 14 st.

Daniel Wheeler (pictured) from Staines, Surrey, appeared on the cover of Men's Health in 2012 after having dropped from 22 st (he's 6 ft 4 in) to 14 st

'Every diet made me lose weight, but they were unsustainable long-term because they were too restrictive,' he says.

Instead he became 'obsessed' with health and fitness and managed to lose 8 st in two years through strenuous exercise and avoiding processed food. 

Having left his marketing job to become a personal trainer, he's made exercise his life.

This is common with the 10 per cent of people who manage to avoid putting weight back on, says Professor Mann.

'They tend to exercise at least an hour a day, every day. They make keeping the weight off the most important thing in their life.'

So, is there any point dieting if we're only going to put the weight back on? The fact that diets 'don't work' should not be taken as bad news, says Professor Mann.

'It's a liberating message as it can free you from being on the horrible cycle, where you diet, lose weight, regain weight and diet again.'

Instead of regaining his weight, Daniel Wheeler (pictured)  became 'obsessed' with health and fitness and managed to lose 8 st in two years through strenuous exercise

The focus needs to be on something sustainable long-term, says Dr Kevin Hall, who is a senior investigator in the physiology department at the National Institutes of Health in Washington.

In other words, it's what we're always told: eat sensibly and healthily, and exercise.

'You have to think about lifestyle changes that ensure you get the maximum amount of enjoyment out of life while still being healthy.'

And you have to live like this for ever, he says.

Leroy is resolute that he will slim down again. 'I know I can lose weight again. Being as big as I was is just something I can't imagine any more.'

A LighterLife spokesperson said: 'Our research shows those who follow our weight loss plan and continue with our weight maintenance plan keep weight off.'

Rosemary Conley agreed, but added: 'We all know life can prevent us from being as disciplined as we'd like.' Slimming World declined to comment.

Super Slimmers: Did They Really Keep The Weight Off? Channel 4, February 14, 8pm.