Hunger
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Hunger is a feeling experienced by animals when the glycogen level of the liver falls below a certain point, usually followed by a desire to eat. The usually unpleasant feeling originates in the hypothalamus and is released through receptors in the liver and stomach. An average nourished human can survive about 50 days without food intake. Hunger can also be applied metaphorically to cravings of other sorts.
The term is commonly used more broadly to refer to cases of widespread malnutrition or deprivation among populations, usually due to poverty, political conflicts or instability, or adverse agricultural conditions (famine). (See malnutrition for statistics and other information on hunger as a political and economic problem.)
Number of People Living in Hunger
According to The Borgen Project over 800 million people across the globe live in hunger. The condition is preventable and in September of 2000, the largest gathering of world leaders ever assembled met at a summit in New York City and agreed to a plan to end it by 2015. Known as the U.N. Millennium Goals, the plan to end world hunger has been agreed to by every nation on earth. An estimated $19 billion a year is needed to achieve the plan to end world hunger. To put the figure in perspective, Congress and the White House allocate $420 billion a year for defense spending.
Physiology
Hunger is mediated by several molecular signalling pathways in mammals. Hormones known to affect hunger include ghrelin, leptin, and Peptide YY3-36 [1].
Satiety
Satiety, or the feeling of fullness and disappearance of appetite after a meal, is a process mediated by the ventromedial nucleus in the hypothalamus. It is therefore the "satiety centre".
Various hormones, first of all cholecystokinin, have been implicated in conveying the feeling of satiety to the brain. Leptin increases on satiety, while ghrelin increases when the stomach is empty.
Therefore, satiety refers to the psychological feeling of "fullness" or satisfaction rather than to the physical feeling of being engorged, i.e. the feeling of physical fullness after eating a very large meal.
Satiety directly influences feelings of appetite that are generated in the limbic system, and hunger that is controlled by neurohormones, especially serotonin in the lateral hypothalamus.