When you begin to eat, ingested food moves into the gastrointestinal tract where the volume and nutritive content are sensed by mechanical and chemosensory mechanisms. Carbohydrates and fats are the main nutrients that stimulate the release of these hormones. Food has pleasurable and rewarding qualities which drive appetite beyond metabolic needs. The multiple bioactive forms pertinent to feeding share a common carboxy-terminal octapeptide with an O-sulfated tyrosine. MCH and orexin neurons also communicate with noradrenergic neurons in the locus coeruleus, serotoninergic neurons in the dorsal and median raphe nuclei, and the histaminergic tuberomammillary nucleus.
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Without homeostasis, blood pressure, hunger, heart rate, sleep cycles, and immune responses would be completely out of whack. The brain uses neurotransmitters to regulate the functions of the body. The hormones created in the hypothalamus and pituitary gland are sent out into the body to regulate its functions. Neurotransmitters connect the entire nervous system to help with the regulation of all bodily functions.
One of the biggest tasks of the hypothalamus is hunger and appetite control. While leptin and ghrelin are hormones produced by the body to signal hunger as well as satiation, the hypothalamus has receptors for these hormones.
There are three regions within the hypothalamus itself that are associated with hunger and satiety. Motivation to eat comes from hunger.
The feeling of hunger comes from a lack of food as well as other factors. Leptin levels drop as hours pass with no food being consumed. At a certain point, the dropping levels of leptin are what begin to release ghrelin. When ghrelin is activated within the body, the hypothalamus will provide a trigger to the brain that the body needs fuel. Hunger regulation is one of the biggest problems for someone who is trying to lose weight.
When our bodies start to lose stored fat, the chemicals and transmitters in the body mount a defence because they believe it could be the start of a famine. Chemicals are released that boost appetite and try to conserve energy. This leads to a drop in metabolism as well as a boost in hunger or motivation to eat more. All of this is regulated by the hypothalamus, which is always watching and waiting to read hunger signals.
There are two mechanisms that trigger a feeling of satiation. One is in the brain, while the other is in the gastrointestinal tract. After we eat, the hypothalamus picks up the sensation of a distended stomach, other signals from the gut, and a rise in blood glucose levels. When it integrates these signals, you have the experience of feeling full. However, even though our brain sends us clear signals about hunger, most of us become adept at ignoring them and eat when we are not truly hungry.
It can take up to a half hour for the brain to catch up with the state of the stomach. This is the reason why we tend to prepare more than we can eat. When the stomach is full, it can take from 20 minutes to a half hour for the brain to know.
You can try this for yourself. Eat a modest amount for lunch when you are very hungry and when you have finished you will still feel hungry.
Wait for 20 to 30 minutes without eating anything, and see if you are as hungry as you were when you finished the modest lunch. It may quite surprise you. A variety of hormones and hormone-like substances signal the hypothalamus to cause us to feel hungry or full.
Hormones are chemical messengers that are secreted into the bloodstream by one of the many glands of the body. Hormones exert a regulatory effect on an organ. These glands release their secretions into the bloodstream in response to a signal. Examples of such signals include the falling or rising fuels within the blood, such as blood glucose, and chemical and nervous signals from the gut and the liver.
The levels of hormones in the blood then signal the hypothalamus to stimulate hunger or satiation. Examples of hormones and hormone-like substances that stimulate food intake include neuripeptide Y and galanin, while those that create feelings of satiety include leptin, cholecystokinin, and serotonin.
Foods containing protein have the highest satiety value. This means that a ham sandwich will cause us to feel full for a longer period of time than will a tossed salad and toast, even if both meals have exactly the same number of calories. Also, interestingly enough, high fat diets have a higher satiety value than carbohydrate diets. Another factor affecting hunger is how bulky the meal is, that is, how much fiber and water is within the food.
Bulky meals tend to stretch the stomach and small intestine, which sends signals back to the hypothalamus telling us that we are full so we stop eating. Beverages tend to be less satisfying that semisolid foods, and semisolid foods have a lower satiety value than solid foods.
For example, if you were to eat a bunch of grapes, you would feel a greater sense of fullness than if you drank a glass of grape juice.
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