Standing in the school playground with my son, I wonder where all the fat kids are. There in the queue is Ismail, aged six, who is so slight of build it worries me. Then there is Luke, who is going through a growth spurt and looks lanky, almost gaunt. Maddie and Laura are doll-like and look like they will stay that way, emulating the ice-skating, bike-riding, dance-obsessed girls in Year 6 who are starting to look gangly even now. None of them has the girth or ponderous rolling gait of Jazlyn Bradley - the American schoolgirl whose family tried to sue McDonald's in 2002 for making their daughter fat.

Weaned on Happy Meals, Jazlyn ate at McDonald's every day on the way to school - and usually on the way home as well. Even before she reached her teens, she was overweight. By the age of 20, she weighed 19 stone and had the medical problems of a middle-aged woman: Type 2 diabetes, high blood pressure and raised cholesterol. Her sisters Shakima, 18, and Naisia, 14, were also obese. African-Americans raised in a run-down apartment in the Bronx, they came to symbolise the link between obesity and low socioeconomic status.

"Obesity is a disease of economics," says Dr Ian Campbell, a Nottingham GP and medical director of the UK charity Weight Concern. "For men, the incidence of obesity in the poorest parts of the population is about 28%, while in the most affluent parts it is 18%. There is a huge differential."

That is how most people view obesity. It is a "Them and Us" condition - Them being the unemployed, economically depressed whose budget stretches only to cheap white bread and a 99p burger with fries; Us being the more affluent, educated but time-poor middle class who feed our children well enough, given the constraints of work, shopping for food - and microwaving it.

As we smugly put out our single Brabantia bin liner for the refuse truck (the rest of our food waste is in the wormery), we marvel at the detritus of pizza boxes, chip wrappers and KFC Bargain Buckets spilling from the eight bulging Somerfield bags in the garden of the council flat next door. Their children aren't as obese as Jazlyn Bradley, but wait until they are teenagers. Our son was weaned on organic spinach purée, not Turkey Twizzlers - and he has been to McDonald's only once, with his nursery. His burger made him sick, which is why he is pretty much vegetarian. It's true, we don't always cook from scratch using fresh ingredients (how many working families do?) but I've yet to see a child grow fat on Jamie Oliver's pasta sauce or a carton of plum tomato and mascarpone soup from the New Covent Garden Food Company.

As a result of such prejudices, fewer than half of parents with an overweight child identify it as such, Dr Campbell says. "Because obesity is associated with - and I hate the term - working-class dietary habits, a lot of middle-class people assume their child can't be obese and it isn't their problem. They wouldn't take their child to McDonald's, they restrict their children's fizzy drinks - but the fact is there are significant numbers of middle-class children who are overweight and inactive."

Hence the government's £275m Change4Life campaign, aimed at tackling obesity in every section of society and launched in mid-January with TV advertisments made by Aardman Animations, creators of Morph and Wallace and Gromit. Of that £275m budget, £200m was provided by food companies such as Kellogg's and Pepsico, manufacturers of some of the least healthy foods for children. Their initiatives focus more on sport and activity than on diet. "We have adopted ideas from successful movements such as Make Poverty History and Comic Relief," said public health minister Dawn Primarolo at the launch. "We want families to engage with the campaign and understand that obesity is not someone else's problem."

The most striking promotion showed a group of 10 London schoolchildren standing in a playground then used "digital ageing" software to depict how they might look in 2050 if they did not adhere to strict lifestyle and eating habits. Like the children in my son's playground, none of them was overweight to begin with, but nine of them morphed into lardy adults. That outcome was based on statistics from a Foresight report entitled Tackling Obesities: Future Choices, which predicts that 90% of today's children will become obese or overweight in adulthood. Their children, too, will fall mostly into these groups.

By 2050, some 70% of girls and 55% of boys will be overweight or obese, increasing their risk of not just diabetes, heart disease and high blood pressure - but also cancer. Last month, Sir Michael Marmot, professor of epidemiology at University College London, warned that new cases are set to increase from 10 million now to 16 million globally by 2020, and obesity in industrialised countries plays a major part. In Britain 13,000 cancer cases a year can be directly attributed to weight gain and obesity, and one-third of cancers are known to be caused by poor diet and lack of exercise.

What the Foresight figures and the government's digital ageing advertisement suggests is a kind of inevitability about children getting fat - but why does a child who is rake-thin now develop into an overweight adult? Isn't it partly to do with genetics and metabolism: once a beanpole, always a beanpole? There is a genetic component, Dr Campbell agrees, but obesity is also "progressive" and therefore deceptive when you look around any primary, or even secondary, school. "The percentage of the population that is overweight increases with age - so among two-year-olds it might be only 3%, at 10 years old it might be 9%, but by the time they get to 15 years old it is about 17%. It keeps increasing year on year. Obesity starts in infancy - you could argue that it starts before birth. It is a continuum."

For girls, the turning point comes in their mid-teens, when they give up PE and netball and are "more interested in music and boys", Dr Campbell says. "They become overweight fairly rapidly." For boys, the crunch comes when they leave school and stop playing football, while both genders are vulnerable when they go to college or university, "have a bit of money in their pocket, a bit of freedom, and start to eat things their parents would have restricted".

Clearly, exercise plays a part, but what I want to know is this: are children being led into an obese future by a diet that is simply average rather than excessive? If they avoid junk food but eat cereal every day (as 87% of UK families do - having, on average, seven packets in the house at any one time), take a ham sandwich and a muesli bar to school in their lunchbox, eat penne and pasta sauce on some weekdays, vegetarian sausage and mash on others and share a meal at Pizza Hut at the weekend, will they get fat - and sick?

To find out, I scrutinised the typical diets of children I know (see boxes), then went to three of the country's top medical experts - a cardiovascular specialist, a cancer researcher and a neurologist - to have them analysed for impacts on health. We weren't just looking at obesity, but high blood pressure, cancer and other ailments associated with diet. Often what was left out (oily fish, whole grains, vegetables) was more damaging than what was put in, but the three main villains in the piece were salt, sugar and refined carbohydrates. Of these, the most invisible but pernicious is salt. Think of how Kellogg's Rice Krispies taste without milk or sugar, and it will come as no surprise that a single serving contains as much salt as a packet of Walker's ready-salted crisps. Three servings (100g) contain 1.6g of salt - half the daily intake recommended for a six-year-old. "I have yet to see a child beyond a year old be satisfied with one portion," Dr Campbell says. "I've seen it in my own children and I've asked other people - and they often have two or three."

Salt plays an important role in keeping body fluids balanced, transmitting messages in nerves and muscles, and enabling our cells to absorb nutrients. However, we need only 1.4g a day - an amount we could easily get just from eating fruit and unsalted vegetables - but the UK average is 8.6g. "The food industry has done a good job in reducing salt levels substantially," says Graham MacGregor, professor of cardiovascular medicine at St George's Hospital in London. "They have come down by about 30% in three or four years, but the food companies have not done enough and they need to do more."

Last year the charity Consensus Action on Salt and Health (CASH), which Professor MacGregor chairs, found that a single children's meal of baked beans and a burger contains 4.2g of salt - far more than the 3g daily maximum for a six-year-old. Vegetarian sausages and burgers (a "healthy" alternative to fatty meat) were found to be among the worst offenders, while a Pret a Manger All Day Breakfast sandwich (another weekend treat) contains as much salt as six packets of ready-salted crisps.

"Salt is a silent killer," Professor MacGregor explains. "When you eat more salt, you retain more fluid - an extra 1.5kg of liquid if you go from a low-salt to a high-salt diet. That's sloshing around inside you and putting your blood pressure up, then, oops, you've had your stroke or heart attack and you're dead."

For a child, the prospect of having a heart attack or stroke is meaningless, but the building blocks are already in place. "Blood pressure starts rising in early childhood," MacGregor says, "and continues throughout life - and it tracks. The higher your blood pressure during childhood, the higher it will be when you become an adult. If you don't eat salt, it won't be nearly as high."

One antidote is eating lots of fresh fruit and vegetables, as the government's Five-a-Day campaign advises. "They help lower blood pressure because of their potassium," Professor MacGregor says - but according to Carrots or Chemistry?, published by the Organix food company and based on detailed food diaries, fewer than one in five children meets the government target.

High blood pressure accelerates atheroma - the build-up of furry deposits in the arteries. Due to their high-salt diet, even very young children are set on an almost inevitable course. "In three- to four-year-old children in Britain who had died in accidents, researchers conducted post-mortems and nearly all of them had early signs of atheroma streaking," Professor MacGregor warns. "These children are already developing vascular disease at the age of four."

Nevertheless, parents can do something to halt the seemingly inexorable trend. Reducing a child's daily salt intake by 2.5g (by avoiding processed foods) cuts their risk of having a stroke or heart attack by a quarter - and that risk diminishes further with every gram of salt removed. "If you are one of those few people who cooks everything from scratch at home, who bakes their own bread," Professor MacGregor says, "you will be on a low-salt diet anyway."

There is also "an almost incestuous relationship" between high salt intake and obesity, he explains. "In many studies, sweetened soft drinks have been the tipping point that makes children obese. If you eat more salt, you get thirsty and drink more fluid, and some of that will be sweetened soft drinks. If salt intake is halved in children, it reduces consumption by up to three drinks a week - which is a lot of calories." In one US study, the increased energy consumption as a result of salt intake was estimated at 278 calories a day. An intake of 3,500 calories produces 1lb of body fat, so children on a high-salt diet would put on 1lb every 12 days unless they took more exercise to compensate.

Sweetened soft drinks are not just the obvious ones - Coke, Pepsi and 7Up (each with more than 13 teaspoons of sugar per 500ml bottle, according to a Which? report last summer). A 170ml serving of (diluted) orange squash contains 2 teaspoons of sugar, while the same amount of orange juice contains three - not added, but as fructose. "If you give kids two litres of apple juice a day," says Professor Martin Wiseman, medical and scientific adviser to the World Cancer Research Fund (WCRF), "it is not much different from giving them two litres of Coke from an obesity point of view." The WCRF's advice is to drink no more than one 150ml glass of juice a day - because the sugar in it promotes obesity, identified as a leading cause of some types of cancer and singled out by Sir Michael Marmot in his warning last month.

Even fresh fruit, Professor Wiseman warns, is a highly calorific food that should be treated with caution. "One consequence of the government's Five-a-Day campaign is that children are eating fruit rather than vegetables to meet their target," he says. "If you are consuming an extra five pieces of fruit a day and changing nothing else [such as exercise], it will give you more calories because fruit is very sugary. Eating half a cabbage and some carrots won't."

More controversial still are his views on processed meats - eaten on an almost daily basis by children but linked in a comprehensive 2007 study to cancer late in life. Eating as little as 50g a day (one sausage, or a couple of slices of ham or chorizo) increases the risk of bowel cancer by 21% - from a 1:18 chance over a man's lifetime to a 1:15 chance. For women, the figures are slightly lower - and for children, there was no available data.

"What these meats have in common is that they are cured with a salt of some kind," Wiseman explains. "This seems to change the chemistry of the meat so it either contains, or contains something that is turned into, nitrosamines - and nitrosamines are carcinogenic compounds."

In its report Food, Nutrition, Physical Activity and the Prevention of Cancer, the WCRF recommended that processed meats should be eliminated from the diet completely. "That needs further unpicking," Wiseman says now. "Is it bacon, ham, sausages, salami, pepperoni? We just don't know. Some of those will be harmless, some will be the ones that carry the risk - but if you eat two sausages and a ham sandwich every day from childhood onwards, for decades, it is likely to be increasing your risk of colorectal cancer."

However, it is obesity - not chorizo - that increases the risk of cancer most convincingly. The WCRF recommends that we all stay lean by doing 30 minutes of vigorous exercise every day, or 60 minutes of moderate activity. If that seems a lot, it reflects just how many calories we are putting into our bodies without noticing. In children, these come mostly from refined sugar.

"The World Health Organisation says no more than 10% of our calories should come from refined sugar," says Dr Alex Richardson, a research fellow at Oxford University and an expert on nutrition and the brain. "Quite frankly, it is 33%." Breakfast cereals are a major source of sugar, but some cereal bars are worse. Jordan's Special Muesli Bar - a "healthy" snack - contains 34.4g of sugar per 100g. The Food Standards Agency defines a "high-sugar" food as one with more than 10g of sugar per 100g.

However, Dr Richardson's concern is not sugar itself (which is easy to identify by its sweetness) but refined carbohydrates such as white bread and pasta, white rice and even mashed potato. "If a child eats mashed potato or chips, I'm sorry, it might as well be sugar," she says. "There is no fibre and there are no nutrients - unless you have left the skins on the potato. Bread and pasta are just the same. Unless it is wholegrain, it is a waste of time."

Elisabeth Weichselbaum of the British Nutrition Foundation agrees. "The big difference is the fibre content," she says, "and fibre has been associated with a lower risk of some cancers and heart disease." Wholemeal bread and pasta are "a healthier package" than the white, highly processed versions eaten by children on a daily basis. Most of the micronutrients get lost in the processing.

One of these is chromium, Dr Richardson says, "which is critical to blood-sugar regulation and the whole insulin system, linking it to the premature Type 2 diabetes that is hitting teenagers and young adults". Refined carbohydrates lack vitamin E, "an antioxidant extraordinaire", says Dr Richardson, "that sits in membranes with fatty acids and makes sure your fats don't go rancid".

In one recent study, children were found to have "desperately low levels" of omega-3 fatty acids compared to an adult control group, simply because their parents "pander to them" and don't feed them any oily fish, Dr Richardson maintains. "Most children don't get anywhere near the 500mg a day of EPA and DHA [the long-chain omega-3 fatty acids found in, say, sardines] that they need just for a healthy heart and circulation, never mind their immune systems, their joints, their mood and their attention span. If, in early life, these oils are lacking, health problems are likely to be cumulative over time."

It is never too late to change a child's diet - which is the whole thrust of the current Change4Life campaign. If children who are slim now eat healthily and continue to exercise, they will not morph into supersized fatties. "Any social-marketing campaign achieves a greater result by tackling those who are at risk, or mildly affected, rather than those who are a done deal," says Dr Ian Campbell. "You are far better to target those of normal weight, with the intention of keeping them there, than to target those who are already obese."

In an ideal world, Professor Graham MacGregor says, intervention should come earlier: "Children need to start on a healthy diet from the day they are born. Once they are on solid food, you need to start thinking about not giving them salt, not giving them saturated fat, giving them a more vegetarian diet. If we did that, we could abolish cardiovascular disease." At current rates, 60% of our children will suffer from heart disease in their lifetime and 40% will die of strokes, heart attacks and heart failure. This is everyone's problem.

Running on empty
Salt, sugar and fat facts

Breakfast cereals

Last summer, a Which? report revealed that Coco Pops, Ricicles and Frosties (all made by Kellogg's) are "at least one-third sugar". Coco Pops contain 34g per 100g - three times the amount used by the Food Standards Agency to define a "high-sugar" food. In a 2004 survey of eating habits, a quarter of foods consumed by children were found to be more than 30% sugar. Rice Krispies also contain 1.6g of salt per 100g - half the daily limit for a six-year-old, earning a red light under the FSA's traffic-light labelling scheme. 100g of Weetabix, a healthier option, contain 4.4g of sugar and 0.65g of salt.

Toast, butter and marmite

Two slices contain 2.1g of salt, two-thirds of a six-year-old's daily maximum.

Cereal bars

Often given to children as a "healthy" alternative to sweets and biscuits, some contain more sugar than the worst breakfast cereals (above). A 2006 Which? survey found that Jordan's Special Muesli Bar contained 38.9g of sugar per 100g (about 5g more than Coco Pops) and 12g of fat, while a Kellogg's Fruit 'n Fibre Bar contained more sugar per bar (10g) than a Penguin biscuit (9.7g) and 9g of fat per 100g. Both companies have since reduced the sugar in their bars - to 34.4g per 100g and 9g per bar, respectively. Nuts, dried fruit and chocolate chips help pile on the calories.

Penne and pasta sauces

White pasta - like processed white bread and rice - has had most of its fibre and micronutrients removed and provides "empty calories". Chromium (which helps regulate blood sugar and insulin, linked to diabetes) and vitamin E (an antioxidant) are stripped out, says Dr Alex Richardson. "White pasta still counts towards the starchy food that should be making up one-third of our diet," says Elisabeth Weichselbaum of the British Nutrition Foundation, "but wholemeal is definitely the healthier option."

A tomato-based sauce contains more lycopene (which helps combat cancer) than fresh tomatoes, but adding vegetables would make this a healthier meal. In 2008 it emerged that Jamie Oliver's spicy olive, garlic and tomato sauce contains 3g of salt per 100g - more than twice as much as the equivalent weight of Turkey Twizzlers, and enough to get it banned in UK primary schools. A four-year-old eating a quarter of a jar would exceed their recommended daily intake.

On the kids' menu

Processed meats

In 2007, the World Cancer Research Fund suggested that processed meats (such as ham, chorizo, salami and sausages, universally adored by children) should be cut from the diet because of their links with cancer. Eating just 50g a day (a sausage, or a couple of slices of ham) increases the risk of bowel cancer in adults by 21% - but there is no data for children. "If you have two sausages and a ham sandwich every day from childhood, for decades, you are probably increasing your risk of colorectal cancer," Professor Martin Wiseman confirms.

Orange squash

"In many studies, sweetened soft drinks have been the tipping point that make children obese," says Professor Graham MacGregor. A 170ml serving of (diluted) orange squash contains 2 tsp of sugar - which adds up if a child is drinking squash several times a day. No Added Sugar varieties contain a small amount of "naturally occurring sugars" but also preservatives and the sweeteners aspartame and saccharin. In her book They Are What You Feed Them (Harper Thorsons, £12.99), Dr Alex Richardson says some of them contain phosphates, which "cause calcium to leach from your bones".

Apple juice

Under Food Standards Agency guidelines, pure apple juice qualifies as a "high-sugar" drink, containing 10g of intrinsic fruit sugars per 100ml. Orange juice is about the same. The World Cancer Research Fund - determined to reduce obesity, because it increases the risk of some cancers - suggests children consume no more than one 150ml glass of juice per day. Drinking more adds calories but confers little benefit in terms of fibre and antioxidants.

Vegetarian sausages, mashed potato

Seen as a healthy option, vegetarian sausages and burgers can be horrendously salty. Last May, Consensus Action on Salt and Health (Cash) revealed that one Fry's Vegetarian Traditional Sausage contains 2.8g of salt, the daily maximum for a six-year-old. That is four times as much salt as Butcher's Choice pork sausage from Sainsbury's. Two Quorn Leek & Pork Style Sausages contain 2g of salt. As for the mashed potato, it "might as well be sugar", says Dr Alex Richardson - effectively a refined carbohydrate with "no fibre and no nutrients" unless the potato skin is left on.

Too many treats?

Pizza Hut meal

In a 2007 survey of restaurant meals, CASH found that a family of four sharing a Pizza Hut lunch (including a medium-pan Super Supreme pizza, garlic bread, chicken wings and four desserts) could eat 12.3g of salt each - twice the daily limit for an adult (6g) and almost 2.5 times the limit for a seven- to 10-year-old (5g). A six-year-old could easily be eating more than four times his or her daily limit (3g).

Bread and "morning goods"

A staggering 21% of a child's salt intake is likely to come from bread, crumpets, croissants and bagels - which also contain refined flour, sugar and fats that can make children obese. Less salty are tea cakes, hot cross buns and English muffins. Biscuits, cakes and pastries account for 4% of the salt we eat, and breakfast cereals for 5%. Bacon and ham are very salty, but we consume less of them: they account for 8% of intake.

Pret a Manger All Day Breakfast sandwich

Described by Pret as "not for the faint-hearted", this is the kind of thing a 10-year-old might choose as a treat after Saturday-morning football. Containing free-range egg mayo, sausage, bacon, ketchup, tomato and cress on healthy wholegrain bread, it contains as much salt as six bags of ready-salted crisps.

Dried fruit

Doled out to toddlers like sweets, dried fruit is simply fresh fruit with the water removed - a pure fruit sugar that is highly calorific. While a fresh apricot contains 12 calories, a dried apricot packs 15 - and because dried fruit is sweeter and easier to eat, children will want loads of it. Though high in fibre, these snacks are a poor source of vitamin C, because it is destroyed in the drying process.

• This article was amended on Thursday 9 April 2009. We should point out that mashed potato (boiled potato with skin removed, with milk, butter and seasoning added) has less fibre and fewer nutrients [than sugar] but it still contains vitamin A, calcium and iron (in small quantities) and vitamin C in quite significant quantities.