In a central London basement, Shelina Permalloo, a former winner of Masterchef, cooks salmon for the 10 best Weight Watchers in the country. Permalloo herself is 4.5lb (2kg) off her 10% mark. The room combined has lost an incredible 720lb. Christina Edwards has lost the equivalent weight of her best friend and her best friend's two children.

What gets a person to their first Weight Watchers meeting? Jo, 30, had her name down for gastric band surgery two years ago. "The doctor said there was no way I would be able to lose the weight I needed to lose on my own. So I thought, I'll just lose a bit to make the surgery safer." Eighteen months later, she had lost 186 pounds – more than 13 stone. She's the person who, in a 1990s advert, would have been photographed holding out her old trousers to show how many people could now fit inside them. They don't really do that any more.

One day Sarah, 27, had a headache with stroke-like symptoms and for a short time considered the (thankfully unrealised) possibility that she might have had a stroke in her 20s. Rachel was diagnosed with diabetes and, eight-and-a-half stone later, no longer has it. Alison tried on the largest size in Long Tall Sally (this was in 2001 – I think their largest has got larger since then) and it didn't fit her. "I could feel curtains coming down over my eyes. I remember thinking, if I can just hold it together until I get to the street, I can start crying then." It could be anything, from a massive life event to a certain kind of look from a shop assistant.

It's so obvious, isn't it, dieting? Calories in versus calories out, it ought to be as easy as putting the right amount of petrol in your car. And yet this isn't how it works – it baffles everybody, from medics to epidemiologists. The more determined you are to lose weight, the more you put on. Some 95% of people are heavier five years after going on a diet .

Nutritionists look at the way food is manufactured, and particularly at sugar. Obesity specialists talk about emotional eating and the way obsessional behaviour around food is peculiarly related to trauma; the treatments for compulsive eating are often very similar to those for post-traumatic stress disorder. And the more obesity is discussed as a social problem, the more humiliation and judgment is attached to it. That's what I think, anyway. Sharon James does not agree. "I was a fat child. And I look at teenagers, now, and they're much bigger than I ever was.But because there are so many of them, they're not ostracised like I was."

Sandra Lewis concurs. "There's more big than slim, now. It's everywhere you look."

In this complicated, difficult, hostile terrain, there's Weight Watchers, popping up like a mirage, a Starbucks in the Sahara. People go because it works: if you follow the diet, go to meetings and use the website, you're eight times more likely to reach your target weight than if you DIY it. Most of the women watching the cookery demonstration, and then making Weight Watchers turkey patties of their own, have been at their target weight for months, many of them years. Some have become group leaders in their own right; they all still attend meetings. There is more going on here than calories in, calories out.

Donna started in 2008. She waited outside in the car for the first three sessions. She didn't tell her husband. "The first day I went to a meeting, I got home and he'd found out his hours had been cut. We've got four children. And I was spending £6 a week going to a meeting!"

"My hands were like jelly the first time I got on the scales," Rachel remembers. "I was just in total denial about how big I'd got."

More than that, Jo recalls, "I didn't realise how low my confidence was. I couldn't walk across a room without thinking people were watching me, laughing at me. I was really paranoid. The weight went on gradually, maybe a stone a year. And as your weight increases gradually, your confidence and happiness decrease gradually, and you really don't notice it."

Sandra Lewis says: "I went to the doctor about something, and he suggested I could do with losing a bit of weight, and I remember coming home to my husband and saying, 'how dare he!' But when I went to Weight Watchers, and reached my goal … I've got mirrored wardrobes, and I remember coming home and standing there, going 'welcome home, San.'"

So how do they do it? By not thinking of it as a diet, but as a different way of living; not thinking of restrictions, thinking of abundance. They're all foodies, they insist: nobody on Weight Watchers ever stopped enjoying food. I personally think it's something else: the energy and fellowship of women with a shared purpose, in a really good mood. They're like marines.

The turkey patties are so lean and so dry, it's like someone's unfurled a fire-resistant blanket in your mouth. But it's the marines so it doesn't seem to matter.