There was a moment in the first episode of the new Jamie Oliver TV series last night when the Essex-lad-made-millionaire had an outburst worthy of a revolutionary. Emerging from a mealtime visit to Natasha, a single mother on benefits in Rotherham, he raged in his own inimitable language of protest: "Fucking hell ... it's fucking Great Britain. It's 2008. I've been to Soweto and I've seen Aids orphans eating better than that."

Natasha feeds her two children takeaways most nights. Aged five and two, they have never eaten a meal that has been properly cooked at home. Instead, they sit on the floor - no table, no cutlery - and eat shavings of doner kebabs or chips with processed cheese from polystyrene boxes with their fingers. Even instant noodles have to be negotiated without forks. The bottom drawers of Natasha's fridge are stuffed full of sweets and chocolate bars. "This is where all my money goes," she admitted. About £70 out of a weekly benefits cheque of just £80 on fast food and junk. Five-year-old Kiya has already had to go to the dentist twice to have rotten teeth removed. Natasha can see the life of obesity and illness ahead of them; it's not that she doesn't share the middle-class fantasy of sitting down to a cosy communal table each night, but despite her eight-hob gas cooker and the countless cookery shows on her flatscreen TV, she doesn't seem to know what to do.

Whenever Oliver leaves her, in this and subsequent episodes of his TV mission to teach the nation to cook, he is stirred to the same outrage, shouted from the barricade of his celebrity jeep. Natasha turns out not only to have a big cooker and TV but debts large enough to make her a pawn-shop regular, and depression deep enough to make her give up trying. When Oliver finds this out he confides to the camera in his car, "I don't blame her ... but I'm fucking angry. I'm fucking angry and I don't know who with or what with." He has just met poverty in all its 21st-century complexity - and it has a profound effect.

Miss this Ministry of Food series and you'll be missing some of the most powerful political documentary in years. In it, whether by intention or accident, the naked chef has entered the domestic life of a British town and captured a snapshot of the country's social health. The result is an indictment of the current political system as disturbing as any ideological tract. Food, and real people's experience of it, is still all about class.

The subtext is everything. Geoff, the 84-year-old Rotherham man who has never cooked in his life, seems keen and quick enough. Why he should suddenly be facing a crisis is never fully explained. But it turns out that his wife and lifelong helpmeet now has Alzheimer's disease and so everything is falling to him. He asked Citizens Advice where he could learn to cook. They had no answer. Does he have children to help? Yes, several, it turns out, but presumably distance and some of the longest working hours in Europe make it hard for them to give the support he needs. Clare, who is overweight and lives on 10 packs of crisps and a large bar of Galaxy chocolate a day, seems to struggle to read when given a recipe by Oliver. Is she one of those who fell through the net at school so that not only does she not know that liquids bubble when they boil but she stumbles when literacy and numeracy are required? This is a delicate but painful portrait of the socially excluded.

Middle-class people in the series eat junk, too. Time, or lack of it, is their problem. Women getting home from work can't find even half an hour to cook from scratch for the family, which Oliver can't understand. There are cultural barriers to eating well too. But what makes Oliver and the viewer angry, what shocks, is the deprivation.

Food has always been about class in Britain. "The nature of our diets has been entirely shaped by the class system of the 19th century and the white working-class experience of industrialisation," says Tim Lang, professor of food policy at City University in London. "From the first- when cheap white bread was sold to the poor as progress, because previously only the rich could afford it - what class you are from, and how much you have to spend has made a dramatic difference to what you eat, how you eat it and what you aspire to eat. Class imbues everything in food."

Which socio-economic class you are born into is still one of the most significant determinants of how healthy you will be. Natasha is simply conforming to the pattern. Her mother was a single parent on benefits too - she didn't pass on cooking skills to Natasha, and Natasha has none to give to her own children. The cycle of deprivation looks set to continue. Sharp inequalities can be clearly mapped, even short distances apart, according to Dr Tim Lobstein, director of the childhood research programme at the International Association for the Study of Obesity. Travel the eight stops on the Jubilee line tube from Central London's Westminster to Canning Town and you find a decrease in life expectancy of nearly one year for each station going east.

A child born in one deprived Glasgow suburb can expect a life 28 years shorter than another living only 13km away in a more affluent area, a three-year investigation for the World Health Organisation found in August. Commenting on one of the key factors, the report concluded: "Obesity is caused not by moral failure of individuals but by the excess availability of high-fat, high-sugar foods." The marketplace can generate wealth but it can "also generate negative conditions for health", which will only be resolved by tackling "the inequitable distribution of power, money and resources", it went on. In Rotherham, where areas around the town centre have deprivation levels that put them in the worst 6% of the country, one in 10 reception-age schoolchildren are obese. By year 6, 18% of them are obese, while 60% of adults are either overweight or obese.

Right across the country, those on low-incomes suffer higher incidence of a whole range of illnesses relating to poor diet. Lobstein catalogues them: higher rates of anaemia caused by lack of iron, especially in pregnancy. Mothers from low-income groups are also more likely to have children of low birthweight, who, in turn, are likely to suffer poor health and educational prospects as a result. Working-class families have more dental disease and more childhood eczema and asthma. They are more likely to suffer from obesity, both as children and as adults. They have higher rates of raised blood pressure, thanks to excess salt in their processed diets. They are more likely to suffer diabetes, heart disease, vascular disease and strokes. They suffer more cancers of the lung, stomach and oesophagus. They have more cataracts caused by poor nutrition than those in other classes. And the protective role of good diet is missing. A survey of men and women living on benefits found that a third ate no fruit at all during the week their diets were recorded.

It's not just diet, of course - alcohol and tobacco consumption are implicated, too, as poverty ties a Gordian knot of health problems. Over half of younger children in low-income groups are inactive almost all day as well, with two- to 10-year-olds getting less than 30 minutes of physical activity daily.

Elizabeth Dowler, professor of food and social policy at Warwick University who was recently involved in the government's Low Income Diet and Nutrition survey, says the class differences are stark but complicated. "If you live for more than six months on the minimum wage or on benefits there is growing evidence you cannot afford to buy the food you need for health. It is still to do with class but it's complex to unpick. Food is the flexible area that you cut back on when you are on a low income. Unlike council tax or utility bills, no one fines you if you don't spend on food and no one takes your children away, so that's what you cut, and you have a fag because that takes the hunger away."

When you are on a low income you buy the kind of food that fills you up most cheaply. What may seem ignorant choices to others are in fact quite rational. Lobstein has calculated the cost of 100 calories of food energy from different types of food. The cheapest way to get your 100 calories is to buy fats, processed starches and sugars. A hundred calories of broccoli costs 51p, but 100 calories of frozen chips only cost 2p. Good-quality sausages that are high in meat but low in fat cost 22p per 100 calories, but "value" fatty ones are only 4p per 100 calories. Poor quality-fish fingers are 12p per 100 calories compared with 29p for ones made with fish fillet that are higher in nutrients. Fresh orange juice costs 38p per 100 calories, while the same dose of energy from sugary orange squash costs 5p.

The FSA pointed out when it published its survey on the effect of low income on diet that middle-class people were eating increasingly high levels of junk, too. However, the same survey found that nearly a quarter of poor households skipped meals because they didn't have enough money. Nearly 40% worried that they would run out of food before more money came in. And yes, the working class smoke more and drink, as Oliver, falling over beer cans and fag ends outside a Rotherham house, notes.

It has been unfashionable recently to talk in class terms, but there's nothing here that hasn't been raised for decades, centuries even, as both Lang and Lobstein point out. When George Orwell wrote The Road to Wigan Pier in the middle of the 1930s depression, he set out to record the lives of the English working class in the industrial north. He was appalled by the quality of their diets. "A man dies and is buried and all his actions forgotten but the food he has eaten lives after him in the sound or rotten bones of his children." Orwell wrote down detailed accounts of how unemployed working-class people on welfare spent their money. He doubted it was even theoretically possible to live on their allowance. "The basis of their diet is white bread and margarine, corned beef, sugared tea and potatoes. Would it not be better if they spent more money of wholesome things like oranges and wholemeal bread?" Yes it would he answered, but "no ordinary human being is ever going to do such a thing. The ordinary human being would sooner starve than live on brown bread and raw carrots ... A millionaire may enjoy breakfasting off orange juice and Ryvita, an unemployed man doesn't ... When you are underfed, harassed, bored and miserable, you don't want to eat dull, wholesome food. You want something a little bit tasty. Let's have three pennorth of chips! Put the kettle on and we'll all have a nice cup of tea!"

To sit down to table to eat a family meal at leisure was always a luxury, according to food historian and consultant to the National Trust Sara Paston Williams. "In the slums of industrial towns in the 19th century everyone including the children was out working, and if you were poor you didn't have an oven to cook, so you didn't get together for a family meal." The royal family were in part responsible for the creation of the myth of the ordinary family meal, with father Prince Albert, mother Queen Victoria and children sitting down together, although aristocratic children were, in fact, largely fed in the nursery. Mrs Beeton spread the idealised habit to the burgeoning middle classes, who were keen to copy the rich and differentiate themselves in their new wealth from the poor. The pre-industrial and rural experience of food was different. Food might be short in bad years but generally you had better access to a wider variety of foods, including some milk and vegetables in the country. Families working on the land broke off together to eat in the fields. But the Victorian urban poor lived on takeaways, hot pies from the pie man often full of horrible stuff, and adulterated white bread and adulterated marrow jam.

To be hungry was deemed a natural state, and a necessary one since it goaded people who might otherwise be idle into working. James Vernon has traced changing attitudes to food and lack of it in Hunger: A Modern History. "Two hundred years ago hunger was considered a spur to effort. Adam Smith and Thomas Malthus debated whether the emerging market economy would eradicate hunger or depended on it, but both agreed that the market should be left to produce plenty or want without intervention from the state.

"It was only in the second half of the 19th century that this view was first challenged, when hunger was discovered as a humanitarian issue, that reflected the failure of the state to protect its citizens from economic downtowns over which they had no control."

The seminal work that changed attitudes was Friedrich Engels' The Condition of the Working Class in England in 1845. Drawing frequently on contemporary reports he read in the Manchester Guardian, he catalogued the misery of the poor quality, adulterated and inadequate diets of the poor. "The habitual food of the individual working man varies according to his wages ... descending gradually ... until on the lowest round of the ladder potatoes form the sole food. As an accompaniment weak tea with perhaps a little sugar, milk or spirits is universally drunk ... such a way of living unavoidably engenders a multitude of diseases."

Yet today the language of class has been almost removed from the political discourse. In Thatcherite and Blairite Britain, it has been framed instead in terms of "choice". There is talk of the need to give people opportunities, but after that, it's down to individual responsibility. It is no accident that the government's white paper on health and obesity was called "Choosing health" rather than the "social determinants of health".

Roger Stone, Labour leader of Rotherham metropolitan borough council, is reluctant to put food and poverty in its class context, too. "Jamie's programmes are not about us, it's not about the middle class, he's targeting people who are struggling," he told me. You mean the working class then? "No, goodness, in this day and age, it's not class."

You sense that Oliver used to think the same. Give people a chance, then it's up to them, and if, like some of the catering students in his previous programmes, they don't choose to take it, he's got no time for them.

But here he's on a journey. The series is set in Rotherham because it was there that a group of mothers helped their children rebel against the new school menus imposed during Oliver's last series. Julie Critchlow took orders for chips and crisps and shoved them through the school railings at lunchtime, earning herself a place in tabloid mythology. "Fat old scrubber" was what Oliver called Critchlow back then. He may speak estuary but he is firmly positioned with the middle classes who almost exclusively buy his books and watch his cooking shows. But Critchlow has her revenge. As phlegmatic as a tricoteuse at the guillotine, she tells him firmly in this series that he "lives in a bubble", he has no idea what it's like for ordinary people on benefits. And she and her elderly mother, who did teach her to cook, want to get a pair of choppers on his head. Oliver symbolically submits: he puts his designer hair mess into the hands of Critchlow's hairdresser daughter. A little while later he is found complaining to his producer: "Fuck. What's happened to my barnet?"

But it has done the trick, because now he talks differently. He's learned that you can't parachute in with change and waltz out again; tackling class inequality is a long, hard slog. This time he wants to get the whole town on board, to shift the tribe. He's not interested in training those who won't pass it on, who only want to cook for themselves as individuals. "This is about the home, it's about family, it's about neighbours." With impeccable timing, as the global markets go into meltdown, Oliver discovers social capital and declares: "They've got to own it. It's about ownership".

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