The UN's food agency on Tuesday said obesity and poor nutrition weigh heavily on the global economy and told governments that investing in food health would bring big economic as well as social returns. Lost productivity and spiralling health care bills linked to malnutrition "could account for as much as five percent of global gross domestic product (GDP)," equivalent to $3.5 trillion (2.6 trillion euros) a year, the Food and Agricultural Organisation (FAO) said. Improving nutrition would boost earnings, "with a benefit-to-cost ratio of almost 13 to 1," it said. In its yearly report, the Rome-based agency said 12.5 percent of the world's population (some 868 million people) are undernourished in terms of energy intake, while 26 percent of children worldwide are stunted by malnutrition. Some two billion people suffer from micronutrient deficiencies and 1.4 billion people are overweight -- of which 500 million are obese. In low- and middle-income countries, a rapid rise in obesity is affecting associated costs, it said. FAO said rising urbanisation, sedentary lifestyles and the increased availability of packaged foods meant policy-makers faced significant challenges in reversing obesity. But "the returns are high," the UN agency said. "Investing in the reduction of micronutrient deficiencies, for example, would result in better health, fewer child deaths and increased future earnings," it said. Lack of micronutrients -- such as vitamins, iron and iodine -- are particularly common in developing countries, experts say. The costs of undernutrition are estimated at two-three percent of global GDP, equivalent to $1.4 to $2.1 trillion per year, the FAO said. Though no specific data on the economic costs of obesity exist, the cumulative cost of all non-infectious diseases, for which obesity is a leading risk factor, were estimated to be about $1.4 trillion in 2010, it said. The agency urged global leaders to enhance nutrition in food systems -- in part through agricultural policies -- and promote behaviour change through education. "Agricultural research and development priorities must be made more nutrition-sensitive, with a stronger focus on nutrient-dense foods such as fruits, vegetables, legumes and animal-source foods," it said. Proposals for positive interventions in the food system for better nutrition include changes in production "up to the farm gate", with micronutrient fertilisers, bio-fortified crops and crop and livestock diversification. Governments could also help consumers through subsidies, targeted food assistance for at-risk groups such as children and the elderly, and better product labelling to help shoppers choose more nutritious foodstuffs.