The research found that foods which are high in fat and sugar can cause significant changes in brain biochemistry similar to those from drugs such as heroin and cocaine. Once hooked, the researchers say, many people find it almost impossible to switch back to a healthy diet, often leading to obesity.

The evidence is to be taken up by lawyers preparing multi-million-pound claims from people who allege that their “addiction” to fast foods has damaged their health.

They say the new research undermines the claims of companies such as McDonald’s and KFC that the decision to eat their products is down to “personal responsibility”.

The addictive nature of fatty foods has been established by researchers at Rockefeller University in New York who found that regularly eating the products can quickly reconfigure the body’s hormonal system to want yet more fat.

They also found that early exposure to fatty food could influence children’s choices so that they would always seek a similar diet, increasing the likelihood of obesity in later life.

In another study, to be published shortly, Professor Ann Kelley, a neuroscientist, and Matthew Will, of the University of Wisconsin, traced the biochemical changes in the brains of rats fed different diets.

Those given a high-fat diet became hooked and if the fat was taken away, displayed symptoms similar to those of a drug addict deprived of his or her fix. Fat rats also suffered changes in brain development.

“The research suggests that a high-fat diet alters brain biochemistry with effects similar to those of powerful opiates such as morphine,” said Will.

The daily recommended intake of energy for an adult man is about 2,300 calories, of which no more than 35% should come from fat and 11% from added sugars. Women should eat about 1,800 calories a day.

However, a meal at a fast food outlet — burger, chips, drink and dessert — can deliver almost all of an adult man’s recommended daily calories in a single sitting. For example, a McDonald’s quarter pounder with cheese contains 516 calories. A large portion of french fries adds another 412, and an accompanying large milk shake another 500 calories — while a chocolate doughnut or dessert gives a further 379.

The total is more than 1,800 calories, most of it coming from fat and sugar. By comparison, a leg of chicken with boiled potatoes and peas plus an apple contain about 800 calories, with a relatively small proportion comprising fat or sugar.

Some nutritionists say that it is unfair to blame just fast food firms for surging obesity when 85% of people’s food comes from supermarkets. Such stores promote processed foods with high levels of fat and sugar, along with snacks such as crisps, ice cream and chocolate, because they offer high profit margins.

Scientists at the food companies Nestlé and Unilever have also been investigating how snack foods make people binge eat. “We have projects currently running to investi- gate this,” a spokesman for Nestle said.

A typical snack such as a standard 34 gram packet of crisps can comprise 33% fat, have high levels of salt and provide 180-200 calories, more than 10% of a child’s daily needs. In recent years firms have competed by offering ever larger packets.

The new research will be featured in a BBC2 television programme, Big Mac Under Attack, to be shown on Tuesday. It suggests that high-fat and high-sugar diets can disrupt the delicate hormonal balance that would normally prompt people to stop eating.

John F Banzhaf III, professor of law at the George Washington University law school and who led America’s anti- tobacco litigation, said that the findings left companies selling food high in fat and sugar “deeply vulnerable”. He has written to six of the world’s largest fast food companies warning them of litigation.

“Most of these companies sell this food without any nutritional information, labelling or warnings. A product that is both dangerous and addictive is very difficult to defend,” said Banzhaf.

Professor Gary Slapper, director of the Open University law programme, said he believed that the first British legal actions over obesity were imminent. “There is an obligation to make risks clear to consumers and failure to do so makes food companies liable,” he said.

In France, fears of a consumer backlash have prompted McDonald’s to publish nutritional advice that inactive children should not eat its meals more than once a week.

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