system
Given the following post, respond as if you're commenting on Reddit:
human
What is the feeling of hunger? Is it a simple matter of needing something edible to fill up volume in your stomach? Is it a biologically driven signal that your body wants certain nutrients? Or something else?
gpt
If you think about it, hunger is an incredibly complicated system that takes into account stomach fullness (the physical stretchin you were talking about), nutrient in the intestines, nutrients in the blood, fat stores, and higher order functions like smell and taste. Not only that, but it integrates this into the pleasure of eating, the desire to seek out food, and satiety.


The field of "hunger research," or more approrpately Energy Homeostasis, began in the 70s when Jeff Friedman at Rockafeller University discovered a mouse in the world-famous Jackson Labs. A strain of mice got a spontaneous mutation that made them morbidly obese. It wasn't due to fat deposition, but due to the fact that the mice over-ate, essentially always feeling hungry. These mice were discovered to have mutations in the gene encoding the hormone leptin, the "thinness hormone."

I was then discovered that leptin was a hormone secreted by fat tissues, and it's receptor was discovered to e in the brain. This was one of the first instances where a body signal affected the CNS, and led to a huge field that exists today.
If we think about it today, it makes a lot of sense: hunger and food-finding in higher organisms such as humans and mice requires higher order tasks such as memory of how to find food, etc. also, every physiologic system requires an "integrator" to control it and maintain homeostasis. The kidney for calcium, the brainstem for blood pressure, and other system. Because these homeostac systems required multiple organs, the brain was usually the integrator.

As it turned out, it was previously known that the hypothalamus was important for body temperature regulation. It is the site of regulation of fever-inducing compounds, sweating centers, everything related to maintaining heat. You can imagine how it makes sense that body metabolism is related to energy intake though hunger pathways.


More recently, finer biomedical techniques allowed us to zoom in on other players in the hunger system. Leptin generally comes from the fat tissue, but it acts on a network of neurons that respond to a host of other hormones and chemicals as well. Common ones you might know are insulin for pancreatic function, in incretins like glucagon for liver energy states, blood glucose for...well...blood sugar, and even some fats and cholesterols.
From the leptin-sensitive ineuron system, other hormones have been discovered:CCK for stomach distension, orexin for the actual sensation of hunger, NPY and AGRP for the signal of starvation, melanocortins (another very interesting class or hormones) for satiety.

Well, These hormones acting in these neurons don't only stay in the hypothalamus. They act on seroton and dopamine systems in the brain to elicit the effects owe associate with eating.

As said before, this is a very exciting field that is being pursued by many institutions internationally. However, how exactly do all these parts work together, what other hormones and neuron networks are there, and how do these signals affect other regions of the brain as still unknown.