The first thing that catches my eye at check-in is a sporty-looking blonde woman, dressed in crisp white gear. She's standing at the side of a swimming pool, hollering to a bunch of bobbing heads.

"Now, has anyone got a hip replacement or pacemaker?" she yells, with cheerleader enthusiasm. "No? That's great!" she twangs. "Then let's go..." Abba's Dancing Queen blares out into the bright morning sunshine and the morning aquatics class begins.

It's day one at the Pritikin Longevity and Spa Centre in Miami, and I'm terrified. Aged 35 and with a girth that would serve as a good table tray on any airline flight, I have fled London after being told by my doctor that unless I take evasive action now, I am at high risk of developing diabetes, high blood pressure and, in the future, cancer.

Like 69 per cent of women in Britain over 30, I am overweight, though the details are gruesome to confess: I am 5ft 9in and, until recently, weighed 17 stone. Having avoided diets and fads because they always seemed to fail, I had somehow coasted along, in denial, eating when depressed, and wishing the problem would just go away, despite gentle and not so gentle prods from family and friends.

Of course, I've read the news stories and magazine articles. I know that more than eight million Britons are clinically obese, and that diabetes (which kills 33,000 people in Britain each year) and heart disease are on the increase as a result. I have read the Commons select committee warnings that by 2020, half of all British children will be obese. And I'm aware that the number of women with a body mass index of more than 40 - which indicates extreme obesity - has nearly tripled since 1993. But still I haven't done anything about my own situation.

Then, a couple of months ago, a friend mentioned that she had heard that quite a few Britons (more than 200 in the past year) were spending their hard-earned cash on a two-week residential weight-loss programme at the Pritikin Centre in Florida - and that it showed dramatic results. Over 70 per cent of type-2 diabetics on medication left the programme drug- and insulin-free. Pritikin helped others to lower their cholesterol levels by as much as 23 per cent. I made the call.

The Pritikin Centre does not offer the latest fad treatments or diet plans. It has been around for more than 30 years, is highly respected by the medical community, and has been used discreetly by thousands of people, including celebrities such as Mel Brooks, his late wife Anne Bancroft and, more recently, filmmaker Michael Moore.

As I settle into my vast room, complete with Jacuzzi, a mini-bar full of water and a view overlooking some seriously large yachts, I begin to realise that what had at first seemed like a luxury sunshine spa now feels more like a combination of boot camp, low-key rehab centre and Eighties cruise ship - complete with a rigorous timetable of events.

Every part of my day has been meticulously planned, from the moment I wake up for the 7am "sunrise stretch", followed by three hours of cardiovascular exercise, resistance and weight training. In the afternoons, there are lectures on everything from "the biology of weight control" to "dining out" and "exercise action plan".

And while food here is plentiful (three full meals a day, plus two snacks), there is no caffeine, no dairy, no sugar, no salt and - horror of horrors - no alcohol. Oatmeal and tofu are the order of the day, which hardly set my taste buds alight. My first meeting with my doctor - Robert Bauer - is not that cheering. My blood sugar is up at 125, instead of at the norm below 100, and he doesn't need to remind me that I am severely overweight. But like a new arrival at Alcoholics Anonymous, I am in denial, convinced that I really don't belong here.

By day four, I'm beginning to relax. Many guests I meet are not first-timers - people come back year after year for a booster dose of healthy eating, education and exercise. And they are happy to talk about how they ended up so heavy. Some say they had just worked so hard that they had never had time to take care of themselves. Rob Lilly, a 50-year-old businessman from California, who came to Pritikin in August weighing 23 stone (he's 5ft 7in), says he could hardly walk when he arrived.

"One day, my daughter asked me to go out with her and I said no, because I was too tired. I realised that, as a father, I had become unavailable to her because of what I was doing to myself with sloth and food." Others gained weight after a divorce, or had medical issues which prevented them exercising.

Many, like me, just realised they were going down a buttery slope and needed to stop. Vicky, a banker from New York, said she had taken a sabbatical from work so she could focus on her health. Emi, an Italian entrepreneur, says he came because there was nowhere else in the world like Pritikin. He's right. In Britain, we do have "fat farms", but few offer a philosophy for living that allows the weight to stay off.

Despite the celebrity fans, there's a lack of gloss and hype about the centre which stems from its founder, Nathan Pritikin. He was not a doctor, but a man on a mission after being diagnosed with heart disease aged 41, in 1958.

After researching populations that lived the longest, such as the Japanese and Chinese, he created an eating plan for himself, packed with vegetables, fruit and lean protein - and started exercising every day. After two years, his cholesterol levels plummeted, he lost weight and his coronary disease disappeared.

By the 1970s, he offered his plan to the public by opening a residential centre. His philosophy is simple: "daily exercise and an eating plan based on whole foods like fruit, vegetables, whole grains, seafood rich in omega-3s and limited lean meat". Now, the American government includes Pritikin advisers on its dietary public health advice board.

For me, proof that the Pritikin approach works is that I am just not hungry any more. By the end of day six, I have lost seven pounds - and have also stopped feeling tired and grumpy. I have discovered, too, that the Pritikin menu allows lobster, bison, crab cakes, chocolate mousse (OK, it's non-dairy) and apple cobbler. It is impossible to feel anything but full.

Jeff Novick, a leading nutritionist, who has lectured at Pritikin for eight years, says: "The reason most people fail at dieting is that they count calories and then feel deprived and hungry. Understanding the calorie density of food allows you to make better choices, to fill up, and not be deprived. You learn not what to take away, but what to add in."

Of course, the Pritikin Centre is not for everyone; it is expensive to fly to Miami, and a two-week stay costs between $4,000 and $8,000. And, yes, there is a whiff of earnestness and "rehab speak" among the staff which some cynical Brits might find hard to handle. But if you can swallow the positive thinking (including little cards on your pillow at night offering affirmations - ugh), what is on offer is sound advice.

As I head towards my last few days at Pritikin, something strange has happened. I am now a convert, and a little scared to step outside its safe cocoon. But the advice here is that even if you fall off your horse, you must get back on as fast as you can. There are certain guidelines to help you: if you binge one day, be careful the next two. Don't miss more than one day's exercise, even if you are travelling - find a staircase and walk up and down it a few times to get your heart racing.

The best news of all? I have lost one stone in three weeks (I gained an extra week's stay because of Hurricane Wilma), my blood sugar is now stable below 100, and I have gone down a dress size. And I have accepted that I have to continue this way of eating if I am to succeed in losing more weight - and I know I'll have to make a determined effort to plan my days to make sure I exercise and don't get hungry.

And without wishing to sound too gushy and earnest, I have realised that, aside from shedding the pounds, I have also lost the emotional anxiety that came with not doing anything about my weight. And so, unless I want to end up back at Pritikin in five or 10 years' time, the way ahead is clear.

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