Image caption The number of undernourished people, especially children, has increased in recent years

One billion people in the world were undernourished in 2009, according to a new report.

The 2010 Global Hunger Index shows that child malnutrition is the biggest cause of hunger worldwide, accounting for almost half of those affected.

Countries in sub-Saharan Africa and South Asia were shown to have the highest levels of hunger.

The report's authors called on nations to tackle child malnutrition in order to reduce global hunger.

The Global Hunger Index is produced by the International Food Policy Research Institute (IFPRI), Welthungerhilfe and Concern Worldwide.

The UN Food and Agriculture Organization (FAO) defines hunger as the consumption of fewer than 1,800 kilocalories a day - the minimum required to live a healthy and productive life.

Despite the number of undernourished people in the world falling between 1990 and 2006, the report's authors say in that number has crept up in recent years, with the data from 2009 showing more than one billion hungry people.

The most recent figures from 2010 suggest the number may again be falling but this data is not yet complete.

The Global Health Index (GHI) is calculated for 122 developing and transition countries.

Twenty-nine countries - mostly in sub-Saharan Africa and South Asia - have levels of hunger described as "extremely alarming" or "alarming".

After age two, the negative effects of undernutrition are largely irreversible Marie Ruel, Global Hunger Index co-author

The GHI shows hunger increasing in nine countries; North Korea and eight sub-Saharan African nations. The Democratic Republic of Congo saw the biggest increase; GHI rose by more than 65%.

The scores are based on the proportion of people who are calorie deficient, the proportion of children under five who are underweight and the child mortality rate.

The global food price crisis and the worldwide recession have contributed to the recent rise, says the report.

Children under the age of two are considered to be at most risk. Malnourishment at this stage harms physical and mental development and its effects are mostly irreversible causing life long damage.

In some sub-Saharan African countries, for example Burundi and Madagascar, about half the children have stunted growth because of they do not have access to an appropriate diet.

The authors argue that improving child nutrition would have the biggest effect on reducing global hunger.

They estimate that child malnutrition could be cut by about a third by providing improved health care and nutrition, not only to young children but also to mothers during pregnancy and breastfeeding.

Marie Ruel, director of IFPRI's Poverty, Health and Nutrition Division and co-author of the report, said many countries had to accelerate progress in reducing child malnutrition.

"Considerable research shows that the window of opportunity for improving nutrition spans from conception to age two," she observed.

"After age two, the negative effects of undernutrition are largely irreversible."

The report adds that reducing the numbers of hungry people will also significantly improve productivity and economic development.