Because of war, drought, political instability, high food prices and poverty, hunger now affects one in six people, by the United Nations‘ estimate.

The financial meltdown has compounded the crisis in what the head of the U.N. Food and Agricultural Organization called a “devastating combination for the world’s most vulnerable.”

Compared with last year, there are 100 million more people who are hungry, meaning they consume fewer than 1,800 calories a day, the agency said.

“No part of the world is immune,” FAO’s Director-General Jacques Diouf said. “All world regions have been affected by the rise of food insecurity.”

The crisis is a humanitarian one, but also a political issue.

Officials presenting the new estimates in Rome sought to stress the link between hunger and instability, noting that soaring prices for staples, such as rice, triggered riots in the developing world last year.

Josette Sheeran of the World Food Program, another U.N. food agency based in Rome, said hungry people rioted in at least 30 countries last year. Most notably, soaring food prices led to deadly riots in Haiti and the overthrow of the prime minister.

“A hungry world is a dangerous world,” Sheeran said. “Without food, people have only three options: They riot, they emigrate or they die. None of these are acceptable options.”

Even though prices have retreated from their mid-2008 highs, they are still “stubbornly high” in some domestic markets, according to FAO. On average, food prices were 24% higher in real terms at the end of 2008 compared to 2006, it said.

“Malnutrition kills through the fact that it weakens the immune system of a child,” said Andrei Engstrand-Neacsu, a Nairobi, Kenya-based spokesman for the International Federation of Red Cross and Red Crescent Societies in East Africa. Some 22 million of the 1 billion hungry people counted by the United Nations are in the drought-stricken Horn of Africa, he said.

Engstrand-Neacsu said he had just returned from a corner of southern Ethiopia on the Kenyan border where the food situation is dire, and had been speaking to a family who lost a child to malaria in February. The parents said they were told he couldn’t be saved because he was malnourished.

Engstrand-Neacsu called on donors to act before “skeletal African children are shown on the television screen at dinnertime” in the West.

The number of hungry people is estimated to have reached 1.02 billion — up 11% from last year’s 915 million, FAO said. The agency said it based its estimate on analysis by the U.S. Department of Agriculture.

FAO said that the hunger rate is rising, too — that is, the number of hungry people is growing more quickly than the world population. Officials did not provide a rate but said the trend began two years ago.

Almost all the world’s undernourished live in developing countries. But all regions of the world have registered two-digit increases in hunger from last year.

The world’s most populous region, Asia and the Pacific, has the largest number of hungry people — 642 million, up 10.5% from last year. Sub-Saharan Africa registers 265 million undernourished, an 11.8% increase. Even in the developed world, undernourishment is a growing concern, with 15 million in all and a 15.4% increase, the sharpest rise around the world, FAO said.

The dire figures make it highly unlikely that a goal set by the wealthiest nations to cut hunger in the world in half by 2015 will be met, though officials vow to press world leaders at the Group of Eight summit gathering in Italy next month.

FAO said the calorie-limit it employs to declare a person hungry is on average 1,800, though it changes slightly from country to country.

Alice Lichtenstein, a professor of nutrition science and policy at Tufts University, said FAO’s hunger definition was reasonable, if a little conservative. She said the 1,800-calorie threshold represented the number of calories most adults need to maintain their body weight, but that the figure would vary depending on a person’s size and level of physical activity.

The number of calories for children varies even more. They need fewer calories because they are smaller, but also need increasing amounts as they get older to ensure they are growing.

World cereal production in 2009 was strong, but the global economic downturn resulted in lower incomes and higher unemployment rates — and therefore reduced access to food.

The crisis also affects the quality of nutrition, as families tend to buy cheaper, calorie-rich but nutrient-poor foods such as grains, at the expense of meat, dairy products and other expensive and high-protein foods.