What triggers hunger?

Factual Questions
1

Over in the thread Before there was fire what did man eat? there was some discussion on the net energy derived from food. Basically, add up the total value of the nutrients absorbed by the body and subtract the energy required to actually process that food.

At some point (or so I gathered) it is actually possible to eat some foods that have a net negative energy input for your body. That is, your body will expend more energy processing and moving the food through your system than it will obtain from the food. Raw potatos and green bananas seemed to fall into this category.

My question is what actually triggers a hunger response in your body? Imagine stuffing yourself full of raw potatos and nothing else for several days (or weeks or whatever). Does merely having bulk passing through your system trick the body into thinking it is fed or is your body smarter than that? Will it say to itself, “I don’t care how much stuff is in me right now…I need some protein damnit!” and thus make you hungry?

In addition, if your body thinks it is lacking protein will it actually give you a craving for foods high in protein? I know mother nature built in a liking for salt since waaayyy back when salt was relatively hard to come by (before Morton came along). Nature wanted to encourage our intake of certain things and presumably animals that hated salt were selected out of the gene pool. It would seem remarkably smart if our bodies did this so I kinda doubt it but I don’t really know. (NOTE: I suggest cravings in only the broadest, general sense. I am not suggesting that our bodies are low in Folic Acid and somehow intuit the proper food to eat. Think in more sweeping terms…protein, sugar, salt, etc.).

2

I’m not really answering your question, but wanted to say that it is a very important one. A major area of research in metabolism has to do with the possible signals for hunger, satiety, cravings, etc.

Suffice it to say there is no single cause that gives rise to the feeling of hunger or its opposite, satiety. There is a role for mechanical effects (stomach distention), hormones (insulin, leptin, cholecystokinin, and many more), the brain (hypothalamus), and certainly psychologic factors as well. Given the profound importance of adequate feeding from an evolutionary standpoint, it is not surprising that it is a complex issue with much redundancy and feedback.

I have only glanced at this page, but it seems to give a sense of the type of things that may have a role to play.