Obesity costs the world nearly as much as war, armed violence and terrorism. Yet in dealing with obesity, we are like 16th-century navigators using maps with missing islands and misshapen continents.

So concluded a report published recently by the McKinsey Global Institute. While the report recommended widespread intervention by governments, food companies, retailers, restaurants, healthcare providers and the media, it admitted that we did not really know what we were doing.

“The science on obesity and research into how to reverse this growing health burden is by no means complete,” the McKinsey researchers said.

Some trends are clear. It is easier to overeat than ever. Food is much cheaper. In 1900, the average United States household spent 42 per cent of its income on food. By 1950, this had fallen to 30 per cent. By 2003, the figure was 13.5 per cent.

Physical activity showed a similar fall. In 1969, 40 per cent of US children walked or rode bicycles to school. In 2001, 13 per cent did.

Shouldn’t people just eat less and exercise more? The problem is that the first is hard and the second, while bringing many benefits, is not that good at reducing weight.

Mr David Kessler, former commissioner of the US Food and Drug Administration, has argued that the food industry has discovered a combination of sugar, fat and salt that gives us a hedonic kick similar to that provided by heroin.

In his book, Food: A Very Short Introduction, Mr John Krebs, former United Kingdom Food Standards Agency chairman, calls Mr Kessler’s view “speculative” but adds that evolution has programmed us to enjoy sugar, fat and salt. The first two are crucial sources of energy and we need salt to replace the bodily fluids we lose during the day.

But while our ancestors ate food with sugar, salt and fat whenever they could because they were not sure when their next meal would be, our next meal, overloaded with these ingredients, is waiting for us on every high street.

As for exercise, Mr Krebs says 20 minutes on the treadmill uses up the same number of calories you find in a bottle of soft drink.

WHICH FOOD IS GOOD OR BAD?

I believe there is another problem. While, as Mr Krebs points out, our knowledge about nutrition has advanced enormously since people thought scurvy was caused by foul air below decks or over-salted meat, there is much we do not know about the role of food, not only in causing obesity but other, sometimes related, conditions.

The standard advice on the saturated fats in butter, cream and red meat is to eat less of them and more of the unsaturated fats in fish and plants. But in March, an investigation published in the Annals Of Internal Medicine found no significant evidence that saturated fats increased the risk of heart disease.

And while milk has long been promoted as important for healthy bones, a Swedish study recently published in The BMJ (formerly the British Medical Journal) suggested that greater milk consumption could result in a higher level of bone fractures in women.

These studies come with the usual precautionary notes that they require further investigation. And there is no point complaining about flip-flopping scientists. This is how science develops, through the testing of new hypotheses and the discarding of old certainties.

But it makes it harder to persuade people to alter eating habits. If the experts cannot make up their minds what food is good for us, why listen to them about how to lose weight?

We are not entirely lost on obesity. As the McKinsey study says, 16th-century sailors still found those maps useful. The consultants recommend redesigning cities to encourage physical activity and working with the food industry to reduce portion sizes.

This could be profitable for companies because it would reduce the food they use. McKinsey argues it is important to ensure companies have incentives to cooperate, which they refused to do when Mr Michael Bloomberg, as New York’s Mayor, tried to ban supersized sugary drinks.

As McKinsey says, there is no simple answer. The campaigns against drink driving and smoking had easy-to-understand messages. Driving under the influence causes injury and death, can leave you with a criminal record and problems finding work. Smoking kills — and harms your children.

Obesity, harder to understand, will, like the fight against terrorism, be a longer and far more complicated battle. THE FINANCIAL TIMES

ABOUT THE AUTHOR:

Michael Skapinker is an assistant editor of The Financial Times.