In a polished but well-used kitchen in west London, Anna is cooking supper. It's organic salmon, Asian-style, a variation on a Jamie Oliver recipe: grilled fillets of fresh pink fish served with beansprouts, cucumber, ribboned carrot, mango, coriander and a whizzy dressing, topped with lightly toasted cashew nuts and agave nectar.

And this, note, is an ordinary weekday evening. Anna and her boyfriend Karl are back after 7pm from a full day's work and this, they promise, is entirely representative of how they eat. Karl, who mostly does the weekend cooking, can't suppress a smile.

He is, he concedes, a lucky man. "Anna's obsessed by food," he says. "Constantly reading cookbooks, wanting to try things out. On me!" Before he knew her, he says, he was deluded enough to think he was eating healthily. "But I had no idea of the quantity of natural sugar in my diet, or the carbs." He has learned "a huge amount", he says, and feels "better. Really, so much better. Healthier. What we eat is just so important."

It's a truism, of course: our food matters. We have long understood this. Bad food does us no good. Back in 1875, in The Condition of the Working Class in England, Friedrich Engels asserted bluntly that a diet of potatoes and weak tea "unavoidably engenders a multitude of diseases".

So why is it that so many of us still don't get it? Why, amid the welter of TV chefs, cookery books and healthy-eating plans, of five-a-day campaigns and supermarket traffic-light labels, do so many of us still eat so poorly?

Partly, of course, it's about money: healthy food, in general, costs more than unhealthy food, sometimes a lot more. The quickest and cheapest way to get 100 calories of food energy into your body is through sugars, fats and processed starches. The most nutritious food – fruits, fresh vegetables, lean meats, fish, grains – are, for many, too expensive.

Three years ago Tim Lobstein, director of childhood research at the International Association for the Study of Obesity, looked at the cost of getting those 100 calories from various types of food. One hundred calories-worth of broccoli, he found, cost 51p, whereas 100 calories of frozen chips cost 2p. Good quality, lean, meat-filled sausages were 22p per 100 calories; cheap, fatty ones 4p. Fresh orange juice would yield 100 calories of food energy for 38p, while sugar-saturated squash would do the same for 5p.

Big food, our all-powerful western food industry – which long ago discovered, as the American writer and real-food campaigner Michael Pollan says, that "the more a food is processed, the more profitable it gets" – is at least partly to blame here. "Essentially," Pollan says succinctly, "the system we have evolved is based around wealthy farmers feeding the poor crap, cheap food, and poor farmers feeding the wealthy high-quality, expensive food."

Food poverty, as a result, has moved on from Victorian times, when all much of the population could afford to eat was bread and potatoes. These days, people on lower-than-average incomes suffer from poor access to healthy food; as Martin Caraher and Elizabeth Dowler of City and Warwick universities, co-authors of Challenging Food Inequalities, point out: "It's more often a problem of dietary imbalance than under-nourishment."

That's not just because healthier food is more expensive (it isn't, always), but also because it is harder to get. Big supermarkets offering a wide variety of quality foods at fair prices are not thick on the ground in deprived inner cities, favouring wealthier suburbs and out-of-town sites for car-owning customers. And shopping from convenience and corner stores can cost up to 13% more for a nutritionally adequate diet, the academics found.

The consequences? Illnesses related to poor diet – including obesity, anaemia, diabetes, raised blood pressure, heart and vascular disease, strokes and cancers of the stomach and oesophagus – all tend to be more prevalent in low-income groups.

But it's by no means always about money. A 2007 Food Standards Agency survey found that better-off people were eating ever greater quantities of junk, fast and processed food too: when we're tired or it's late or we just can't be bothered, restaurants, takeaways and ready meals are easier and quicker. Whatever Jamie says. (Plus, sugary, fatty and processed foods taste really good.)

And the flip side of that same expense coin is that it is perfectly possible, with skill and imagination, to eat healthily for not very much money. But you need energy and determination and to really understand and care about what you eat.

These days, of course, caring about what you eat is not just about ingesting food that will do you good (or at least, food that won't do you too much harm). It means – again, providing you have the means – thinking about where your food comes from and how it's grown: is it free-range, fairtrade, organic? Locally grown, minimum food miles?

Has it been sustainably caught, humanely raised, non-GM farmed? Is it from a small-scale, independent producer or a big-food multinational monolith? Will the grower get a fair return for his produce, or is an evil supermarket chain constantly driving down prices?

All of which means that real awareness of what we eat – food that is better for us, and for the environment – is essentially a middle-class preoccupation. Sociologists agree that most people who eat badly do so not because they are uninformed about nutrition, nor because they live too far from a decent supermarket, but because bad food costs less, and tastes good.

And so food has become, more than ever, one of the main indicators of social and class distinction in our society. If we're well-off, we're more likely to eat well: fresh, unprocessed, nutritious, locally produced, bought at a farmers' market or small independent supplier. (We may also, of course, choose not to eat well, for reasons of time and convenience.)

If we're not well off, we're more likely to eat badly: preserved, processed, high in sugar, fat and starch, mass-produced, bought from a convenience store or deep-discounter. (We may also, of course, be poor yet choose to make eating well our top priority).

"Tell me what you eat, and I'll tell you what you are," wrote the early 19th-century French writer and gastronome Jean Anthelme Brillat-Savarin. Here, five households do just that.

The single mother: Sheila Gumbi

"It shocks me that some parents don't take the time to cook," says Sheila Gumbi. "When I see the things they feed their children . . . It's not the way I was brought up."

Sheila, 32, was born in Malawi and now lives in a rundown flat on the South Acton estate, one of Europe's largest. She works part-time for a local charity, volunteers at the community centre and, with wages and benefits, gets by with her 13-year-old son Tyrone on £120 a week.

"I'm constantly having to juggle," she says, preparing a chicken and prawn curry, with vegetable rice and salad, in her clean if crumbling kitchen (she hopes to be rehoused soon). "I have been in a position where I've had to decide: do I buy food, or do I pay this bill? I hate it, but that does sometimes mean it's tinned food for me and my boy."

Generally, though, Sheila can buy pretty much the food she needs, spending maybe £50 a week. On Fridays, when she works late, there might be a takeaway or a frozen ready meal from Iceland, although to avoid that she'll often cook big the night before.

Otherwise, it's home-cooked and fresh every day: fufu (a thick African paste of maize meal), fish, curries (goat meat when she can), stir-fries, big thick dumpling soups, shepherd's pie, lasagne, macaroni cheese. Dessert is always fruit, some from a part-time stall Sheila runs on the estate. Tonight's sauce is from a jar; at weekends she makes her own.

Dinner is at 5.30pm sharp, eaten with Ty on the sofa. "There's no room here for a table," Sheila apologises. "It's really not good. Eating isn't just about food, it's about sitting down together, round a table."

Tyrone, no mean cook himself, is on free school meals. "I don't like to bring money into it," says Sheila. "But I know other mothers on benefits, with three children, they get more money than me, a lot more, but they're always struggling because they only eat convenience foods."

The very best food is, she shrugs, impossible. "Will I pay £12 for a chicken that will give just two servings?" she says. "No I won't. I could feed two of us for two days for £12. I'd like to buy free-range eggs. I'd love to have a big pot and put a lobster in it. I'll buy really good food if I have the money: I want to give my boy the best. But it's not always."

Every day Sheila blesses her parents, who "brought me up healthy: they gave me the knowledge of food, made me confident in the kitchen. There's no such thing as can't, you know. There are just different priorities."

The young couple: Anna King & Karl Walker

"I think the main problem," says Anna King, a nutritional therapist, "is that most people simply don't realise how important what we eat is. We wait till we're obese or very ill – with cancer, cardiovascular disease – before we change our diets."

Anna, 31, and her partner Karl Walker, 34, a sustainability consultant, are unabashed foodies. These days, some of the cooking Anna does at home is professional: she is planning and testing menus for clients. But, she confesses, she has always "loved food. Just adored it. It's a huge part of my life".

The west-London couple spend £70-£75 a week on food shopping: a weekly online shop at Ocado, topped up with organic staples from the local wholefood store – hummus, oatcakes, bee pollen, agave. They eat little red meat, once or twice a week, and no takeaways. Fish is sustainably caught or organically farmed, if possible; fruit and vegetables are bought locally.

"I'm very conscious of carbon miles," says Karl. "We always go for English when we can, and organic. With things like mangoes, of course, that's not possible. It's hard sometimes to get what you want."

Breakfast is organic granola with natural yoghurt and fruit for Karl; a homemade shake of berries, yoghurt and assorted ultra-nutritious (but top-secret) ingredients for Anna, who also tries to make herself a home-whizzed juice (fruit and/or veg) every day. Lunches, on weekdays, are soups, salads and often leftovers from last night's supper.

Tonight's supper was that organic salmon, Asian style. Dessert was berries, strawberries, yoghurt, mint, crushed nuts and agave. It was utterly delicious. It's a myth, says Anna, that eating well is expensive. "You can eat healthily for very little: lentils, salads, sprouts. People say they never have the time, but it's not difficult at all. Tins of chickpeas, tomatoes: I do a very good stew with those plus kale and a tiny bit of chorizo."

They resent food-industry manipulation: "Even probiotic yoghurts are chock full of sugar. There's a lot of misrepresentation. It's all low-fat this, low-calorie that, and precious little of it is natural." Food, says Karl, "is political. It is a question of choices."

Anna's line is simple: "There are lots of things in this world we can't control. I find it really empowering that I can control my health and the way I feel through what I eat."

The pensioner: Maurice Nadeem

"The thing is," says Maurice Nadeem, "I don't actually cook. My wife was a very good cook, you see, she used to do all the cooking. And there was the mess. So now what I do is, I put things in the microwave, really."

Maurice will be 82 this year. He served in the Royal Pakistan Air Force and then for five years with the RAF ("I was told I was the first Pakistani to be commissioned," he says, proudly) before studying law at Leeds university in the 1960s, where Jack Straw was among his classmates. He practised as a London solicitor before retiring.

Maurice has never cooked for himself. He was catered for in his service days and used to adore his wife's curries, he says; he would still enjoy a really good proper home-cooked curry more than anything in the world if he could. Sadly, he finds them too strong for him these days.

So he shops at the supermarket nearest his central-London home, a Waitrose, where he buys essentially ready-cooked meals, often Indian, which he reheats. There's also bread, crackers, cheese, eggs, canned soups and plenty of fresh fruit. Understandably, this is not cheap: Maurice spends maybe £100 a week on food, sometimes as much as £120.

Breakfast is "quite filling": two slices of toast, a boiled egg, two cocktail sausages and two bananas. He generally skips lunch, or if he's feeling peckish may make himself a cheese sandwich or heat up a can of tomato soup. A dedicated committee man throughout his life, he remains sociable, going most Mondays to a local day centre for older people.

Maurice's main meal of the day is at 4pm. His regular is microwaved chicken biryani: a hint of curry, but mild enough for his digestion. He says his service background has instilled in him the importance of personal fitness: "I eat what I need and no more; I drink in strict moderation; I make sure always to have plenty of fruit. And I take a brisk three-mile walk every day. That's discipline. Old habits die hard."

Maurice thinks he eats pretty well, although he'd like to do a bit of cooking for himself. "Waitrose has cooking lessons," he says. "I might try. It's never too late, is it?"

The three-child family: The Booth-Farmers

"We used to buy organic when our first child was born, but it's just ridiculously expensive now," says Catherine Farmer. "In fact, we did an experiment a while back," says her husband, Giles Booth. "Everything we used to buy that was organic or from the supermarket premium range, we bought from the value range instead. To see what exactly we could live with."

It's expensive, life with three children, especially if you want to feed them well. And Catherine, 43, a radio producer, and Giles, 44, a studio manager, will have to be more careful in future: she is soon to lose her job.

Food is important to the Booth-Farmers and their children, Henry, 11, William, eight, and Tilly, five. "We do try to cook fresh every day," says Catherine, over chilli in the family kitchen in Lewisham. "Pasta, curries, that kind of thing. On Sundays we roast a chicken that will do for sandwiches on Monday, and curry or a pie. Then for stock. We try."

Buying no ready meals bar pizza for the kids, once a week, and just the occasional takeaway (fish and chips of a Friday night), the family spends £600 a month on food: "Stupid amounts, really," says Catherine. "It's definitely getting more expensive. And I don't think we're extravagant, or wasteful. Not many people still boil up a chicken stock."

They have an allotment in Greenwich for potatoes, tomatoes, carrots, lettuce, sweetcorn, cabbage and the rest – and freeze a lot. The Booth-Farmers' recent cost-cutting exercise showed them they can live with cheap, value-range beans, canned tomatoes and biscuits (though not smoked salmon, ice-cream or ketchup. Nor, says Giles, will he sacrifice his Maldon sea salt). If they have to cut back more, it'll probably mean Asda rather than Sainsbury's.

They are particularly conscious of food for their children's health: when he was born, doctors feared Henry might go on to develop diabetes, so babyfoods were homemade from the start. Catherine campaigned hard a few years ago to improve the children's "really appalling" school dinners, and they now go off with lunchboxes of (for example) brown-bread ham sandwiches and grapes, or last night's supper in a food flask.

"We really like our food, and we both really like cooking," says Giles, who, in his spare time, runs a delightful (and useful) blog (suppertime.co.uk) of short, simple everyday family recipes. "We're not foodies. But good food is a real priority."

The flatsharers: Kieren Eyles and Mike McGrath

"OK," says Kieren Eyles. "Here's a new house rule: if you use a chopping board, you have to clean it up after." Undeterred, his flatmate Mike McGrath responds: "And I think you'd find it would be better, mate, if you used the old parmesan first."

Kieren, 32, who used to work in the pub business but is waiting on a PhD application, and Mike, 31, a fundraiser for a microfinance charity, share a flat in Wandsworth. They cook and eat together most nights if they're in; tonight, it's spaghetti carbonara with fresh steamed spinach and peas.

Both like their food. They do a communal online shop for £90-£100 every three weeks or so ("Sunday afternoon," says Mike, "you should see us"), topped up with maybe £50 a week between them on fresh produce: probably £40 a week each on food, in all. "I do have to be careful," says Mike. "Food's a big chunk of my budget."

Attitudes to eating among twenty- and thirtysomethings have changed a lot over the past 10-15 years, Mike reckons: "Generally, I think people are much more aware. One friend used to eat nothing but stir-fried sausages and supernoodles, and now cooks amazing stuff, three different kinds of naan bread. And another house I lived in, they only ate junk food. They've stopped that now; a conscious decision."

Mike, whose dad is "sports mad" and whose mum was a cookery teacher, makes a big effort to eat healthily, although he confesses to being "a bit partial to the naughty stuff. You know, chocolate and all that. And I'm from Birkenhead. So I like my pies and my pasties." Kieren says they always aim for five-a-day "or more", and "we're very good about fruit".

Their favourite dishes? Mike says he's "big into curries and stews, and stir-fries". Kieren says he makes a mean chocolate fondant with berry compote. Dinner parties, they modestly agree, can be "quite awesome". That's all quite embarrassing, says Mike: "You'd better not say it was me who talked about berry compote."