Containing obesity: The last course MICHELLE OBAMA WOULD probably be happy if every child were like Precious Moore of Clinton, Mississippi. Precious has just turned nine and her favourite food is the hamburger. But she likes fruit, too. She and her mother take a quick walk together every day. "We always exercise, since I was four years old," she explains proudly. Her seven-year-old sister often asks to go to McDonald's, but Precious disapproves. "I don't really like their food at all," she says, scrunching her nose. Precious and her ilk may be one reason why obesity rates in Mississippi are dipping from their towering heights. In 2005 some 44% of schoolchildren aged 5-18 were overweight or obese; last year the figure was down to 41%. Across America, obesity rates in 2010 among both children and adults seemed to be levelling out. It is unclear why. Recent anti-obesity initiatives may have helped. Dr Leibel of Columbia University thinks that Americans may be reaching their biological limit for obesity in an environment where food is cheap and exercise discretionary: those predisposed to obesity may have become fat already. Whatever the explanation for the minor improvement, the broader problem is hardly wasting away. In Mississippi white children are slimming down but black children are as round as ever. American boys and men are still getting bigger, as are black and Mexican-American women. And in most of the rest of the world people still seem to be getting fatter faster. According to Dr Stevens of the WHO and Dr Ezzati of Imperial College, London, obesity rates worldwide nearly doubled from 1980 to 2008, to 12%. Half of the increase was concentrated in just eight years, from 2000 to 2008. The fastest rise for women was in Oceania and parts of Latin America and for men in North America and Australasia. Even if obesity rates were to remain at their current levels, which seems too much to hope for, the consequences would still remain grave not just for the individuals concerned but also for world food supplies, productivity and government finances. This demands action. Given how hard it is to lose weight, governments should concentrate their efforts on trying to prevent people from gaining it in the first place. Children are an obvious target. Chubby kids often become chubby adults, so it is worth catching them before they do. By the same logic developing countries, which mostly remain slim for now (though obesity rates are rising), still have the chance to nip the problem in the bud. The question is how. From an economist's perspective, the best way to dissuade people from eating junk would be to make it more expensive. Franco Sassi of the OECD has modelled the cost-effectiveness of different measures to prevent obesity and reckons that fiscal measures are the only ones that will save more than they cost. But demand for food and drink is relatively insensitive to changes in price, and if one type of junk food becomes more expensive consumers may simply buy another sort instead. Taxing fat is not the simple answer it appears to be. Some fatty foods also contain healthy protein, and a rush to low-fat foods in the 1980s did not bring a decline in obesity. Sugary drinks, which have no nutritional value, are a better target. A well-designed tax would be high enough to deter consumers and broad enough to keep them from switching to another junky drink. But that would raise political problems. A high tax would disproportionately affect the poor, who might be outraged. The unfortunate truth is that no single policy will bring down obesity rates on its own. Societies got fat for a variety of reasons, and individuals, companies and governments must come to grips with all of them to reverse the process. It is easy to argue that if fat people would only stick to their diets and take more exercise, the problem would disappear, but environmental, psychological and biological factors make it much harder to lose weight than it seems. Much will depend on whether food companies will continue to push junk food or speed up the shift to healthier products. They have no incentive to stop making junk foods until consumers stop buying them, but consumers will not renounce such foods until companies make healthy ones more attractive. Insurers and governments, for their part, will need to find new ways to nudge people into better behaviour. Policymakers now have a better understanding of why people make poor decisions about their health, and mobile technology can help them to make better ones. As such experiments continue, there will be more evidence of what works, and whether it can be made politically acceptable. Such ideas can then spread elsewhere. If things were to go on as they are, the implications would be devastating. Ageing is already increasing the rate of chronic disease. Obesity could make the burden much heavier, overwhelming health services, particularly in poor countries. And agricultural supplies are already stretched. If everyone became as heavy as the average American, according to one estimate, the world's human biomass would jump by 20%, the equivalent of adding about 1 billion normal-sized people. Fortunately obesity is that rare thing, an entirely preventable problem. Its rise has been quick and extreme. Now the world must act to reverse that rise, and fast.