Paul Vallely: The definition of fat gets wider and wider

Chocolate Horlicks was the chief beverage of aid workers during the Ethiopian famine of 1984/5. This was because the proprietor of the Daily Mirror, one Robert Maxwell, had dispatched one of its reporters to the local supermarket to buy up its entire stock and take it personally, with much fanfare and photography, on a plane to the famine region. The trouble was, when it got there, the starving rural peasant farmers would not touch it. Their palates have not been corrupted by chocolate, one of the aid workers in the horror camp at Korem told me, offering me a cup. They were desperate to get rid of it somehow.

Corrupt palates is an interesting concept. Here in the West, we are all presumably victims of that – which is why more than 60 per cent of English adults are now overweight or obese, making us one of the fattest nations in Europe. The Health Secretary, Andrew Lansley, announced another initiative to combat that the other day. The problem was that it was the same as all the previous initiatives: eat and drink less, he told us; take smaller portions and exercise more. Little wonder that health experts dismissed the initiative as, in the words of the chef and food campaigner Jamie Oliver, "worthless, regurgitated, patronising rubbish".

But if we all know that, why are we still fat, and getting fatter? There is more to it than individual decisions about diet and exercise. There have been numerous attempts in the past to explain this. In the Eighties, Geoffrey Cannon's Dieting Makes You Fat hypothesised that our Stone Age ancestors, faced with periods of feast and famine, had developed the ability to store fat and then live off it in times of scarcity. The more periods of famine, the better the body had to become at getting fat. So slimming diets (the equivalent of scarcity periods for the affluent) actually improve your ability to gain weight when you come off them.

I'm not convinced. I think the problem is in the mind, not the genes or the stomach. In the Fifties, a psychologist called Leon Festinger came up with a theory he called cognitive dissonance. It sought to explain how people can hold two conflicting ideas in their head at once. He had been studying a cult that was expecting the end of the world. After the appointed day, when the world failed to dematerialise, the group did not fall apart, but grew.

This was because, instead of concluding that they had shown themselves to be fools in giving away their worldly goods, the members decided that the aliens who had been about to destroy the planet had decided to give humankind a second chance. This was news which needed even more zealous evangelisation than the previous message. Failure only increased their fervour.

The anxiety provoked by cognitive dissonance, Festinger concluded, could be reduced by a number of techniques – denial, transferring blame or self-justifying reinterpretation of events. It's what the fox does in Aesop's fable where, being unable to reach some delicious-looking grapes high on a vine, decides that they were probably sour anyway. It is what smokers do when they aver that some heavy smokers live on to a ripe old age. It is what fatties do when they bluster that just one bite won't do any harm.

So, given this hedonistic dissonance, there is no point in England's Chief Medical Officer, Professor Dame Sally Davies, calling on everyone to be "more honest with themselves about their eating and drinking habits". Something more subtle than a lack of honesty is going on.

According to recent research in the US, three in 10 overweight people feel they are normal. This deluded ontology is even greater among the obese: seven in 10 of them feel they are only overweight. Nothing is so deceitful as the human heart. That is why some slimmers put mirrors everywhere or have pictures of enormously fat naked people on the fridge door.

But self-image is also fixed by society. In Britain, the weight at which people consider themselves to be overweight is much higher now that it was only eight years ago. In 1999, 81 per cent of overweight people correctly identified themselves as overweight. But by 2007 that figure had fallen to 75 per cent – almost certainly because there is greater social consensus that extra body fat is normal.

Retailers conspire in this re-evaluation of what it is to be fat. In the US, they do it openly with what they call "vanity sizing". Men's trousers marked size 36 waist can be anywhere between 37 and 41 inches. It is not only clothing manufacturers who force new realities on to us. Supermarkets do it with their two-for-one offers. An ad from Pizza Express popped on to my screen, saying "Kids eat free" alongside a photograph of two slim children who clearly do not eat pizza as a dietary staple.

Governments seem determined not to intervene. Washington last week announced it has decided against a ban on colourful cartoon characters on cereal boxes, despite research by Johns Hopkins University showing that cartoons play a key role in getting children to nag their parents for fatty foods. Food with a cartoon character on the pack actually tastes better, many kids think; one study showed that carrots wrapped in McDonald's packaging were said to taste better than "ordinary" carrots. Food companies now practise what they call 360-degree marketing using video games, mobile phones, web games, cartoons and movies as well as television ads.

There is an alternative model. Denmark has introduced a tax on foods that contain more than 2.3 per cent saturated fat. In the UK, Andrew Lansley, by contrast, says a regulatory approach to unhealthy foods would be "burdensome". "We have always been clear that we don't want to impose costs on business and consumers," he said, provoking a broadside from Sir Roger Boyle, who retired last week as the government's national director of heart disease, saying that Mr Lansley threatened the NHS with his obsession with market forces.

Food companies make the right noises about voluntarily reducing the high-density calorie foods, saying they are changing recipes, reducing portion sizes and improving labelling. But a spokesman last week added: "Reducing the calories in a product doesn't work if people just eat something extra as well."

The even more free-market US does the same, but Americans get fatter and fatter. Nutrition guidelines have been widely available there for three decades but obesity has doubled. One in three American children are overweight – not from eating fats in butter, bacon, cheese or steak but from extra carbohydrates in fizzy drinks, bread, processed food and takeaways. Consumption of these is now rising in England, where 23 per cent of four- to five-year-olds are overweight. At this rate, more than 85 per cent of adults in the US will be overweight by 2030. Britons should take the warning. We have seen the future and it is fat.

Three Rocky bars, a banana and an apple were eaten during the writing of this piece.