Newham primary schools are changing their school dinners to promote healthier eating.

What they used to eat

Chicken nuggets and chips
Cheesy pasta bake
Fishfingers
Coleslaw and potato salad
Peas, sweetcorn
Ready-made chocolate cake
Yoghurt

What they eat now

Chicken in a Mexican wrap
Tuna and pasta bake
Vegetable curry
Rice and pepper salad
Homemade apple crumble
Fresh fruit salad

Sara-Jane is giving it her all in the gym, concentrating hard as she jumps up and down on the aerobics step. Keeping up the pace leaves her sweaty and breathless but the coach shouts out encouragement to keep her and the rest of the class on the move. After steps, there is disco-dancing, some basketball followed by a spell on the running machines.

For this 16-year-old, turning her back on exercise is not an option. Her obesity has left her with a legacy of health problems, including sleep apnoea, a condition which causes her to wake up throughout the night as her breathing suddenly stops. The weight around her neck puts pres sure on her airway and means she has to sleep wearing a breathing device. This is a potentially fatal condition. If she doesn't lose some of her 20 stones in weight, this sweet-natured, rather shy girl faces the prospect of developing coronary heart disease or diabetes within the next decade.

Sara-Jane's mother, Hazel Perryman, watches her daughter sweating it out on the gym floor. 'It's strange, because my sister was as thin as a stick and now she's larger than me. Sara-Jane was a tiny baby - she weighed only five pounds - but look at her now.' She ponders over the cause. 'I don't think her diet is that unhealthy, but she started to put on a lot of weight when she was 10. We try not to make diet an issue, but we also try not to let her eat too much.'

Sara-Jane is not the only overweight child at the gym: every single one of them in the class today needs to lose weight for health reasons. They all come once a week to meet at Whitechapel's first 'fat club' for children in the borough of Tower Hamlets, in the East End of London, which has the highest number of poor households in the country. Eighty years ago, childhood infections and tuberculosis, bred by poor housing, were the biggest cause of premature death here. Now the single biggest health problem is obesity. Tower Hamlets has a higher rate than any other part of the UK, with 30 per cent of all under-16s identified as overweight, and one in 10 as obese. This should not be surprising, given that poverty and obesity are inextricably linked. But what is astonishing is that so little has been done, until now, to try and break the link.

It's an unpalatable fact that those with the least amount of money cannot afford to buy the healthy ranges of food for their families that doctors and dieticians now recommend. Around the corner from the gym is a large McDonald's, and on a rainy winter evening, it looks the warmest, most inviting place on Whitechapel High Road. And, of course, a visit is affordable to even the poorest of families.

There appears to be an idea prevalent in government circles that obesity will go away if poor people could simply be educated to make the right choices. But those choices are irrelevant when ultimately eating habits are based on availability and, crucially, cost.

We are only now finally waking up to the threat that fat poses. Next year, government reports will finally be published - one from the MPs' health select committee, one from the Chief Medical Officer, and one from the man at the Treasury. While Whitehall exercises its collective mind, communities are taking action themselves.

All over the country, clubs and children's programmes similar to the one in Whitechapel are springing up as the communities most affected by obesity attempt to prevent teenagers such as these from heading into an adulthood plagued by chronic illness. These children's lives have already been seriously affected by their extra weight. An affordable local fitness scheme may be the best chance, possibly the only chance, they have of changing their future.

A number of health strategies are now known to work. Jogging, cycling to school and going for a walk at the weekend all help children. So does targeting those most at risk by giving parents helpful advice on how to keep food diaries for their children, and how to introduce gradual changes to mealtimes. Five years ago, some professionals felt this was unfairly 'singling out' the most vulnerable. Now it is accepted by most that it is cruel not to intervene.

Susan O'Callaghan, Tower Hamlets' sports activity co-ordinator and coach, loves her Whitechapel club but wants to open it up to far more children. Already they have a waiting list; the phones ring constantly as GPs enquire whether they can enrol their young patients, but there is no certainty that the funding will be there for it next year. 'These children have had so many obstacles to overcome before they even arrive at this place,' she says. 'Some of the kids are on Prozac. They have very low self-esteem, usually because they've been badly teased. But they try so hard, and they find they can do things like running, which they had almost given up.'

All the parents I spoke to at the club had spent years trying to get the help they knew they needed for their children. Most have to travel miles across London to reach the club each Wednesday, some bringing younger children in tow. Joshua Ewers's mum, Ann, worries about her son. 'He's very conscious of his weight now,' she says. 'He doesn't like going swimming because people take the mick. That isn't the kids, that's the grown-ups. I ask you, how can an adult be so rude to a child? What does that tell you about our society?'

The children who come here are from many different ethnic groups, Asians, Caribbean, Turkish, Polish. Many families wonder whether they carry obesity genes that have singled out one child in such a cruel way from the rest of the brood. Susan Wright says: 'I've got five kids, and the boys can eat anything and not put on a pound. With Stephanie, the doctors said it was puppy fat and it would go when she hit puberty but it didn't. I can tell you, she's had her fair share of bullying at school.'

Mealtimes can be hard, she adds.' How on earth can I deny Stephanie a Kit-Kat when the others are having one? I tell her just to eat two fingers, not four.'

For adults over 25, many doctors regard it as too late to break their eating habits and make them fit and lean again. Patterns learnt in childhood are remarkably difficult to break. For children, there is still time, although catching them before puberty seems to be important. Lynn Porter has installed an exercise bike in the front room, so that she can check if her 15-year-old daughter, Terri, is using it in the evenings. 'It took me ages to persuade her to come here, but I think it's been worth it,' she says. 'Terri came home one day and told me she'd actually been running at school, which is quite a big thing for her.' Terri has a simple goal. 'I would like to fit into nicer clothes, and I'd like to get healthier, but I'm not going to set myself a particular weight to get to. I don't think that would work.

'The club here is good because you're with other kids like yourself and you make friends. I enjoyed doing the basketball, and the running is OK.'

The results of the Whitechapel programme will be studied by doctors at the Royal London Hospital, who want to see whether behaviour can be altered in such a radical way. Consultant paediatrician Dr Nigel Meadows says: 'We want to change their entire approach to life. By doing gentle exercise, we hope they'll see the benefits themselves, and look at what they need to eat.'

For Dr Meadows, there is really no alternative. 'We see children who are grossly obese. Many are pre-diabetic and are developing insulin resistance, and others have sleep apnoea. The truth is that they do risk a premature death if they maintain their weight.'

A few miles away, in the heart of London's Docklands, another health experiment is taking place, one which hopes to encourage children to form good eating habits at a very young age so that they do not grow up to be fat teenagers. School dinners are the target.

A former army chef has been drafted in to work with schools around the area, to see what can be done to make school dinners, traditionally that byword for stodge, more nutritious.

Ian Craine, a plain-speaking but good-hearted man whose job it is to train other school chefs in Newham, is working at the Britannia Village School. You have to walk through the smart new housing developments in the Royal Victoria Docks to reach this small primary school, which serves a fairly deprived, multi-racial community. Thirty languages are spoken here, and more than half the pupils qualify for a free school meal.

It's 11am, and in the kitchen Ian is standing over steaming pans, preparing the lunch. Every day, he has to offer a fish, meat and vegetarian choice, as well as a salad, sandwiches and two choices of pudding.

'What have we got today? Well, there's roast lamb, pasta with fresh vegetables and a cheese sauce, fishy sharks [the piscine equivalent of turkey dinosaurs] plus carrots and roast potatoes. Then for pud we have sponge covered with chocolate icing, fresh fruit salad, yoghurts, and cheese and biscuits.'

By training school kitchen staff, Ian is already achieving what Jamie Oliver aims to revolutionise in his next TV series. Oliver, who is a fierce critic of the bulky, fatty foods which pupils are usually served, will be visiting school kitchens around the country next year, in an attempt to show the staff how meals can be both appealing and healthy. Maybe Jamie has a few more tricks up his sleeve, but Ian has still not found a way of making the children want to eat fresh fish. 'We do serve breaded fish. If you offer the fresh stuff, they just won't eat it. But maybe as the meals develop, we can give it another go. I haven't given up hope entirely.' He's even tried to revive some interest in semolina, but confesses it was not met with universal popularity.

'For many kids, this is the only major meal they'll get, so it's got to be good. If they come back for seconds, fine. The way I see it is that if they can't eat good filling food at primary school, what hope is there for them once they get to secondary school, where they have more choice?'

He calls himself an ogre, although he's anything but that. 'If they come up and don't take any veg, I say, "What, don't you want to be like David Beckham?" You really have to encourage them to eat their vegetables, so we serve carrots and sweetcorn. I tell them they'll never see a rabbit wearing glasses.'

Ian has learnt a few devices to tempt them. 'Every child loves chocolate, but instead of making a fattening cake filled with it, I do a sponge and then mix some icing sugar with cocoa powder, to give it a nice topping.'

Can a school canteen, with less than 45p a day per head, really buy the fresh ingredients it needs to provide a fully nutritious meal? What about 31p? That's what some schools in Britain spend per head, half the cost of a prison meal. Ian shrugs his shoulders. The food suppliers are organised by the local education authority who have to contract out for it. ' You do what you can with the supplies you get.'

The problem with school meals, as he sees it, is not the budget but the fact that far too many kitchens have learnt to rely on burgers and pizzas which are bought in ready-made and are simply popped into an oven. 'I hate the "heat and eat" approach' he says. 'We have to get away from those days, where everything came with chips. You can introduce new foods. For example, we do Mexican wraps, where we take a fresh wrap and put in chicken, salad and a sauce. It's healthy and they love it.'

As the nine and 10-year-olds sit down with their lunch trays, they are keen to talk about what they like. Most of them have opted for the roast lamb and roast potatoes, and a surprising number have also gone for sponge and custard.

Patricia Namusoke, tucks in with gusto. 'I really like the lamb and the gravy. The meals are definitely better than they used to be.'

'I think they're rubbish,' her classmate Bobye Lawal interjects, with a certain amount of bravado. 'We want burgers, pizza and hot dogs, not this stuff. I hate all these vegetables, man.' But he still manages to clean his tray of food.

'For a while, we ran a cookery course for the older children,' says the head teacher, Leslie Church. 'We had one boy who was really quite disruptive, but when he joined this class, he totally changed. Food became a complete passion with him.'

He would like to offer more cookery classes as a way of teaching children about food, but he feels strongly that parents too must play their role in making sure their offspring eat properly. 'Sometimes I see a child walking into school with a bag of crisps, and I wonder, is that their breakfast today? Every parent in this country can afford to feed their child breakfast, and I think it's very debatable whether schools should be shouldering total responsibility for their nutrition. We can give them the best possible lunches packed with vitamins, but they still begin and end the day at home.'