It’s official - almost one adult in five in the United Kingdom is obese. As sure as new year resolutions follow Christmas over-indulgences, January wouldn’t have been January without a story reminding the nation of its weight problem. We’re getting bigger, apparently. Last month, the House of Commons public accounts committee reported on the ballooning problem of obesity rates in Britain, which have tripled in the past 20 years, so that 21 per cent of women and 17 per cent of men are now classified as obese. Last year, the public spending watchdog, the National Audit Office, reported that obesity-related illnesses were costing the NHS half a billion pounds a year, causing 18 million sick days and 30,000 premature deaths. The Government even held a “body summit” last year in which experts debated policy cures.

But where are all these fat people? Obese means having a body mass index (weight in kilograms divided by height in metres squared) of 30 or more - but is every fifth person you meet the big, fat drain on national resources that these figures imply? By contrast, 15 per cent of men and 27 per cent of women drink more than the recommended amount, and smoking causes 120,000 premature deaths annually. But whereas treatment can help people stop drinking or smoking, everyone has to eat. And while the effects of long-term alcohol or tobacco abuse are mostly hidden, the effects of overeating are obvious. There’s no denying the serious health implications of being very overweight, but what is wrong with being a bit plump?

Not much, says Professor Tom Sanders of the department of nutrition and dietetics at King’s College London, who advises government departments and health organisations. The definition of obesity is, in some respects, arbitrary, he says. While health risks, particularly diabetes, “start to take off” when your body mass index passes 28, he says that because muscle weighs more than fat, fit but bulky people such as professional rugby players would be clinically defined as obese.

Obesity has become a derogatory term, he says. “I don’t dispute the fact that severe obesity compromises health, but there is an increase in weight with age - and part of the increase in obesity is a result of the ageing population. It is better to be a bit overweight than to be a smoker, which can reduce your life expectancy by 10 or 12 years. Being plump will only reduce your life expectancy by six months or a year.”

He also warns that “weight cycling”, the habit of dieting and putting weight on again, “is far worse for you than being slightly overweight”. The best antidote to obesity is to exercise more. Take the Netherlands as an example. “Our dietary patterns are not that dissimilar,” says Professor Sanders, “but the Dutch have a much lower incidence of obesity, probably because they spend an average of 30 minutes a day exercising compared to 20 minutes a week in the UK.”

He is scathing about the get-thin-quick culture. “The more diet books there are, the fatter the population gets. They play the role of the arms dealer - they provide no solution, they just fuel the conflict. Or rather, they provide solutions that don’t work and create an appetite for more diet books.”

Surveys consistently show that around 20 per cent of us are on a diet at any one time. But in among the dozens of too-good-to-be-true new titles published this year, one slim volume proposes a third way between the feast or famine pattern followed by most serial dieters. On Eating, by Susie Orbach (Book of the week, Friday, January 4), features no recipes or calorie charts. In fact, its author - who first brought the issue of eating disorders into public consciousness with her 1978 book Fat is a Feminist Issue - calls for an end to diets. Instead, we should try to practise “calm eating”, following her five keys - eating when hungry, eating the food you are hungry for, finding out why you eat when you are not hungry, tasting every mouthful, and stopping eating when you are full - and reach “the size that is right for you”.

It’s an attitude shared by Slimming World. Its clubs currently help around 250,000 people lose weight. “Our philosophy is about adapting your eating in such a way that you can maintain it for life,” says Slimming World’s Alison Brentnall. “If you think ‘I’m going on a diet’, at some stage you have to come off it, which is not a successful way of maintaining weight loss long-term. We are about small changes, choosing lots of right foods, not going hungry, with emphasis on what’s good for you - not negative things such as don’t eat this, avoid that.”

Harry, a 51-year-old special needs teacher in Yorkshire, is 1.82m (5ft 10in) tall and tips the scales at just under 88kg (14 stone). With a body mass index of 27.5, he only has to gain a stone to be classified as obese. He admits that people who meet him for the first time might think he’s “a bit on the large side”, but there was a lot more of him 18 months ago, when he weighed 139kg (21st 10lb). After years of trying to lose weight, winning for a while, then putting it back on, he joined a Slimming World club and lost seven stones in a year through a regime that lets you eat as much as you like, as long as it is fresh fruit and veg, lean meat or fish.

He hated being so big. “I would be squeezing through doors and find my belly in the way. I got tired quickly and couldn’t give my all to the job. And that didn’t help my morale. When I was working with key stage 1 children, I often had to sit on little chairs or on the floor, and it was a struggle getting down and back up again.”

Now the health problems that prompted him to lose weight in the first place - heart palpitations, chronic indigestion, breathlessness and swollen ankles - have disappeared.

But these problems - and other conditions associated with obesity, such as type 2 diabetes, heart disease, osteoarthritis and high blood pressure - are not the only difficulties overweight people face. The psychological effects of being big can also hurt. A charity launched last month, Weight Concern, wants to counteract the social and occupational discrimination suffered by overweight people, by offering helplines and providing training for heath care specialists. “In a society where thinness is equated with attractiveness, overweight people often suffer unfairly from ridicule, humiliation and prejudice,” says the charity’s co-founder, psychologist Jane Wardle.

Commercial weight loss treatments are almost always unsuccessful in the long term, she argues, and psychological counselling to help people make changes in their lifestyle and feel better about themselves would be more effective.

Even if he is still officially overweight, Harry is not worried. “Losing the weight has changed my life. People are always telling me how well I look.” Whatever the heightweight charts say, he’s “extremely happy” with his weight, even if his wife doesn’t always agree. “Every time I put my arm around her she tells me how bony I am.”

Slimming World: www.slimming-world.co.ukWeight Concern: http:weightconcern.comOn Eating: change your eating to change your life by Susie Orbach, pound;4.99 (Penguin)