Many of the foods we now think of as "junk" started life with a more positive image. Sweets often evolved from 19th-century throat treatments, and, in their early marketing, manufacturers of fizzy drinks such as Coca-Cola, Pepsi and Lucozade also emphasised the medicinal benefits of their products. In the days when people generally consumed less sugar and caffeine, the pick-me-up qualities were a big selling point. Lucozade, it was claimed, "aids recovery", while Pepsi, invented by an American pharmacist in 1893, was considered refreshing and stimulating.
The original American hamburger, one source for which has been traced to New Haven, Connecticut, around 1900, was made of lean beef, broiled rather than fried, and served in dry toast. No ketchup, mustard, mayo or cheese.
And workers in England's industrial towns were sustained on nourishing, good-value fish and chip meals from the 1860s, when the first shops opened.
In the 20th century, processed foods such as Spam were an important source of protein for many families during and after the war. Chocolate, too, was believed to provide much-needed milk for children; remember the "glass and a half in every bar"? It is only in recent years that an occasional treat has turned into a daily meal. Which is where the problems start.
What's in a name?
We all think we recognise junk food when we see it: a bag of crisps, a fizzy drink, chocolate. But what about a low-fat pre-packed sandwich? A takeaway pizza? A home-baked cake? No one seems to know where the term "junk food" originated, but it's generally agreed that it defines any food with little or no nutrients but an excess of fat and calories. Often this means heavily processed food, but it doesn't necessarily exclude meals at home. "It's not about eating in fast food restaurants every day. It's about the type of food," says Dr Susan Jebb of the Human Nutrition Research Centre at the University of Cambridge. "Just because you're eating it at home doesn't mean it's OK. Pre-packaged food from the supermarket can just as easily be junk food."
The Food Standards Agency, an independent watchdog set up by the Government, is currently working on defining junk food, using a model that rates the nutrient, fat, sugar and salt content. But some campaigners would like to see the criteria broadened.
"It should be more comprehensive, to include residues from agriculture and pesticides; additives and preservatives; and production techniques such as genetic modification," says Charlie Powell, project officer at Sustain, an alliance of campaigners for better food and farming. The problem with the current lack of definition is that without agreed terms of reference it is difficult to introduce guidelines on junk food promotion or to encourage manufacturers to introduce better labelling.
The appliance of science
It's no accident that junk food is so tempting. Manufacturers have spent many years, and millions of pounds, honing their products to make them as attractive as possible. Some ingredients are chosen for their practical application - saturated fat, for example, is cheap and can withstand cooking at very high temperatures - but most final recipes include plenty of added extras for sensory appeal. Artificial colourings, some naturally produced from plant extracts such as turmeric, but others brewed chemically, are routinely added to make dishes more attractive. Flavour enhancers such as monosodium glutamate either add smell to food that has lost it during processing or enhance existing smells. Emulsifiers make products such as ice cream deliciously smooth, while artificial sweeteners, such as aspartame or saccharin, give a more intense sweetness than sugar can alone. Is it any wonder we are unable to resist?
The junk food habit
Children are eating more junk food than ever, the equivalent of eight chocolate bars a day more than their grandparents. The Government's National Diet and Nutrition Survey in 2000 found that 70 per cent of primary school children regularly guzzled fizzy drinks, on average getting through 30 glasses a week. And 92 per cent of four to 18-year-olds ate more than the recommended adult levels of saturated fat.
More worryingly, healthier alternatives are being edged out. Figures from the Department of Health last year suggested that, despite all the healthy eating campaigns, only one in nine children eats five daily portions of fruit and vegetables, and many are still eating none. One reason why habits are proving so hard to change may be the addictive nature of junk food. At Oxford University, investigations by the food and behaviour research group have found that rats can become addicted to high-sugar diets and experience withdrawal symptoms if their sugar fix is reduced. At Wisconsin University in the United States, researchers found that a good helping of junk food releases chemicals called opioids into the brain: the same chemicals that play a major part in drug or alcohol addiction. Meanwhile, a survey of schoolchildren undertaken by the Consumers' Association in 2003 found that almost half were deficient in the mineral zinc: this reduces the ability to taste and smell and so prompts cravings for very sweet, salty or spicy foods.
Kicking the junk food habit is not easy. Loyalty marketing schemes encourage us to keep going back for more, while financial incentives prompt us to "super-size" to bigger portions. And the very nature of junk food makes it difficult for the body to know when enough is enough. "Junk food has a high 'energy density' - the amount of calories it contains in proportion to its weight - so it throws our appetite control systems into disarray," says Dr Jebb. "If you eat the amount your appetite is signalling you need, you will eat far more calories than your body intended." This is a particular problem for children whose energy needs are lower.
Dr Jebb's research, published in 2002, found that typical menus at fast food outlets contained 65 per cent more calories per bite than an average British meal, and more than twice that of a recommended healthy diet. Which means a child tucking into a cheap hamburger and fries at lunchtime will eat almost twice as many calories as one eating the same weight of pasta and salad.
Getting the message across
While Jamie Oliver's recent demand for healthier school meals may have got results from the Government, it has also boosted certain foods. Sales of Turkey Twizzlers rose by 32 per cent after they were featured in the high-profile campaign, even though their low nutritional value attracted particular criticism.
So if there's no such thing as bad publicity, how much more effective is a carefully planned marketing drive? Many children see more than 5,000 television commercials for junk food every year, many timed for early evenings and Saturday mornings. A 2003 report by the Food Commission found that food advertising accounted for half of all advertisements aired during children's programmes, with three-quarters promoting high-calorie, low-nutrient foods. And manufacturers have plenty of money to spend. The food industry's global advertising budget is $40 billion, more than the gross domestic product of 70 per cent of the world's nations and 500 times more than the World Health Organisation is able to spend on preventing the diseases that junk food causes.
If you can't beat them, join them
Advertisers argue that blaming the junk food boom on advertising is too simplistic when other factors, such as family eating habits, have such an influence. And while campaigners would like to see a restriction on television advertisements, they also accept that the promotion of junk food is ingrained in almost everything we do. From celebrity endorsements to sponsorship of sport and entertainment events; from enticing displays at the supermarket to glossy features in magazines, the marketing of junk food is probably here to stay.
The answer is finding ways to make healthier eating equally appealing. The Health Education Trust, for example, has been swapping junk food vending machines for a wholesome, but equally profitable, alternative. "The vending infrastructure is designed for cans and bottles, not packs for milks, fruit juices and yoghurt drinks," says the trust's director, Joe Harvey. "But we have trialled new machines that can vend healthy alternatives at high capacity and can compete in terms of commercial application."
An international perspective
Think of junk food and you think of the United States: a study in 2000 found that a third of the average American diet consisted of junk food, while recent publicity for the wildlife department in Colorado had to explain the dangers of leaving out hamburgers for wild mountain lions.
But things are changing. The state of Connecticut, home of the hamburger, recently voted to ban junk food in schools in an effort to tackle obesity in children. The measures, described as the toughest in the US, will ban most fizzy drinks from school cafeterias and vending machines. Foods that the state's education department has classified as unhealthy will also be banned. Despite such moves, junk food is a growing problem almost everywhere. It is estimated that a quarter of all EU children are now obese, and even countries that, until recently, had traditionally provided healthy home-cooked meals are turning to quicker, more highly processed alternatives.
The International Obesity Taskforce recently found that over a third of Italian nine-year-olds and 27 per cent of Spanish children were overweight.
In France, the government is so worried about the spread of the junk food habit that it has banned vending machines in schools from this September, and introduced health warnings on advertising for certain foods.
A body of evidence
In the UK, the rapid rise in rates of childhood obesity has also been making the headlines. According to the Medical Research Council, the number of obese six-year-olds has doubled over the past 10 years, with numbers trebling for 15-year-olds. One in five children is now considered obese.
And while most children are still not fat, changing eating habits mean nearly all are at risk of becoming fat: indulging in just 200g (7oz) of junk food, or a couple of burgers, twice a week means an extra 59,808 calories a year, which is enough to put on almost 8kg (17.6lb). Even if a junk food diet doesn't lead directly to obesity, it can sow the early seeds for a whole range of conditions from heart disease to asthma or cancer.
Type two diabetes, usually only associated with adults and caused by poor diet, has become evident in children for the first time during the past few years.
Ior all in the mind?
Less well publicised than the physical effects of a junk food diet, however, are the links between nutrition and mental health. The artificial high caused by the sugar content of fizzy drinks plays havoc with blood-sugar levels, which in turn affects concentration and mood. Anecdotal evidence from experienced teachers that children's behaviour can deteriorate after break-time snacking is beginning to be explored by researchers keen to see whether the chemically produced hydrogenated fats in crisps and biscuits can change the way the brain works.
All of which means that anything from tearful tantrums to hyperactivity could be linked to poor nutrition. At New College secondary school in Leicester, cutting out morning break and replacing it with three five-minute "comfort breaks" has had a noticeable effect since it was introduced last year. "Behaviour has improved immeasurably," says the school's strategic director, Alan Maddox. "There are probably other elements involved, but cutting out time for pupils to fill up on pop and crisps is definitely a factor. It's a much calmer place now."
Making the right choice
Encouraging children to eat healthily is not as easy as simply banning the burgers, however. The first step is getting them to understand the choices they make. "It's not just the food on the plate. It's about making connections between what's picked off the supermarket shelf, what's taught in the classroom and what's served up in the dining room," says Jeanette Orrey, catering manager at St Peter's primary school in East Bridgford, Nottinghamshire, and school meals policy adviser to the Soil Association.
It's a slow process: St Peter's, for example, has been pushing the healthy message for six years and Ms Orrey admits "we're still learning".
Whatever is done in school, many children choose to head to the takeaway rather than the dinner table as soon as the bell rings. "Children don't learn to enjoy food at home as a social event. And we have lost two generations who don't know how to cook," says Ms Orrey. "Some children have never sat at the table; some don't know how to use a knife and fork. So they become more and more reliant on processed foods; junk food habits become self-perpetuating."
Jeanette Orrey believes that younger children can show the way. "I've had children go up to secondary school and hate the junk food on offer. They're just not used to it," she says. "If you don't get the message across at nursery or primary level, you may have lost your chance."
This can be particularly true when it comes to tempting teenagers to change their ways: while many find habits just too hard to break, some rebel against nutritional advice.
"Choosing junk food becomes a form of protest, like smoking," says Dr Peter Marsh, director of the Social Issues Research Centre in Oxford, which has been looking at young people's attitudes to food. "Teenagers know what they are eating is considered 'bad' and choose deliberately to fly in the face of the healthy eating mantra. It has a certain kind of shock value."
The Government has now set up a school meals review panel to help it decide whether to restrict school menus to low-fat options and what minimum nutritional standards should be mandatory in schools by September next year. The panel is headed by Suzi Leather, former deputy chair of the Food Standards Agency, and does not include any star names or celebrity chefs, seen by some as a snub to Jamie Oliver.
In the balance
Ultimately, it may be attitudes to junk food, rather than the food itself, which need to change. "When we want to treat our children do we take them to a fast food restaurant or do we take them swimming?" asks Dr Jebb. "It's easy to set up positive associations about junk food that are hard to break."
But the occasional Saturday afternoon treat is unlikely to do much harm and just shows how tricky it can be to draw the line between good food and bad food: it may not be what we eat that's the problem, so much as the quantities we eat it in. "Food is no longer something to enjoy, it's something to be frightened of, and that's very sad," says Dr Marsh. "The best thing is to encourage children to eat lots of different things. That way they get a balanced diet by default. There is no such thing as bad food, only a bad diet."
Main text: Jacqueline Yallop
Photographs: Getty; Stone
Additional research: Sarah Jenkins
Next week: This year's winners of Write Away, The TES's children's literary awards