Hunger is definitely one of the most powerful drives that humans have. For thousands of years we have worked as a means to get food. People have been starved to get them to do things. Food is the key to life so we are always trying to satiate our appetites. Food is also comforting. It is used as a coping mechanism and acts on the chemistry of our bodies the same way that an addictive drug does. This is why the world is currently in an obesity epidemic that only seems to be getting worse as time goes on.
Glucostatic Theory
In this theory, hunger is said to come from chemical imbalances which occur from low blood sugar levels. When a person does not eat anything their cells lack the glucose they need, which triggers chemicals to be released that tell the brain to eat. We get hunger pangs in the stomach, dizziness, headaches, etc. Once food has been digested and glucose has been sent to the cells of the body and brain, hunger will go away.
Hypothalamus
Scientists have studied areas of the brain on a quest to find the source of hunger. What research has shown is that hunger originates mostly from an area in the forebrain called the hypothalamus.  When a person’s stomach is empty their lateral hypothalamus is activated and the person gets a signal to search for food and eat. Once a person is full of food fat cells will release the hormone leptin which then stimulates the ventromedial nuclei, activating a signal to stop eating. The paraventricular hypothalamus tells the body what foods it needs and is the cause of ‘cravings’.
Hormones
Hormones are an important part of feeling hunger. The pancreas releases insulin into the bloodstream when a person has eaten something with sugar. After the sharp increase in blood-sugar levels, insulin is quickly released by the pancreas. The person feels relief from hunger but then might have an increase in hunger shortly afterward depending on the food they ate. This happens especially when people eat very sugary foods like candy bars. Foods high in sugar and starch will quickly raise the level of insulin in the body, helping metabolization of the sugar. Once the glucose has been metabolized, the body will crash back into hunger as the pancreas stops secreting insulin and the hormone disappears from the blood stream.

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