The physical sensation of hunger is related to contractions of the stomach muscles. These contractions—sometimes called hunger pangs once they become severe—are believed to be triggered by high concentrations of the ghrelin hormone. The hormones Peptide YY and Leptin can have an opposite effect on the appetite, causing the sensation of being full.
Ghrelin can be released if blood sugar levels get low—a condition that can result from long periods without eating. Stomach contractions from hunger can be especially severe and painful in children and young adults.
Hunger pangs can be made worse by irregular meals. People who can't afford to eat more than once a day sometimes refuse one-off additional meals, because if they don't eat at around the same time on the next days, they may suffer extra severe hunger pangs.
Older people may feel less violent stomach contractions when they get hungry, but still suffer the secondary effects resulting from low food intake: these include weakness, irritability and decreased concentration.
Throughout history, portions of the world's population have often experienced sustained periods of hunger. In many cases, this resulted from food supply disruptions caused by war, plagues, or adverse weather. For the first few decades after World War II, technological progress and enhanced political cooperation suggested it might be possible to substantially reduce the number of people suffering from hunger.
While progress was uneven, by 2000 the threat of extreme hunger subsided for many of the world's people. According to the WFP some statistics are that, "Some 795 million people in the world do not have enough food to lead a healthy active life. That's about one in nine people on earth. The vast majority of the world's hungry people live in developing countries, where 12.9 percent of the population is undernourished."
Until 2006, the average international price of food had been largely stable for several decades. In the closing months of 2006, however, prices began to rise rapidly. By 2008, rice had tripled in price in some regions, and this severely affected developing countries.
Food prices fell in early 2009, but rose to another record high in 2011, and have since decreased slightly. The 2008 worldwide financial crisis further increased the number of people suffering from hunger, including dramatic increases even in advanced economies such as Great Britain, the Eurozone and the United States.