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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.)
Hunger as a condition
The term hungry is commonly used to mean having an appetite for food or to be ready for a meal. After a long period without food, the mild sensation of hunger associated with being ready for a meal becomes progressively more severe, until it is acutely painful. As hunger grows, most living things will experience some internal effects. In humans and other animals, hunger can cause a gurgling sound with a bubbling feeling in the small intestine (many mistakenly think the stomach does this), and can shrink the stomach. Prolonged hunger will drive people to eat substances with no nutritional value (such as grass and soil) simply to fill their stomachs, but doing so actually has an adverse effect on energy balance as energy is still required to digest these substances.
Sometimes hunger is defined as the condition in which an organism can only use its protein tissue (e.g. muscles) as the source of energy, a state which sets in after all sugars and fats etc. are used up. [How to reference and link to summary or text]
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.
Neurobiology of hunger
Hunger is mediated by several molecular signalling pathways in mammals. Hormones known to affect hunger include ghrelin, leptin, and Peptide YY3-36 . The fluctuation of leptin and ghrelin hormone levels results in the motivation of an organism to consume food. When an organism eats, adipocytes trigger the release of leptin into the body. Increasing levels of leptin results in a reduction of one's motivation to eat. After hours of non-consumption, leptin levels drop significantly. These low levels of leptin cause the release of secondary hormone, ghrelin, which in turn reinitiates the feeling of hunger.
Some studies have suggested that an increased production of ghrelin may enhance appetite evoked by the sight of food, while an increase in stress may also influence the hormone's production. These findings may help to explain why hunger can prevail even in stressful situations.
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. 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.
Hunger appears to increase activity and movement in many animals - for example, an experiment on spiders showed increased activity and predation in starved spiders, resulting in larger weight gain. This pattern is seen in many animals, including humans while sleeping. It even occurs in rats with their cerebral cortex or stomachs completely removed. Increased activity on hamster wheels occurred when rats were deprived not only of food, but also water or B vitamins such as thiamine This response may increase the animal's chance of finding food, though it has also been speculated the reaction relieves pressure on the home population.
- Animal feeding behavior
- Eating disorder
- Food deprivation
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