Medical Hunger Hormones

I have been studying about the physiology of hunger. I have studied about the hormones that are involved in the process of hunger, like ghrelin, leptin, pancreatic polypeptide, cholecystokinin, cholecystokinin (CCK) and the neurons like AgRP, POMC and neuropeptide Y (NPY) etc.

I understand that ghrelin urges us to eat. Leptin gives us satiation.

But I am confused as to how hunger is triggered at first instance.
  1. What senses what, to trigger hunger?
  2. Is it ghrelin that initiates the whole process? Is energy level determines hunger/
  3. If so how ghrelin (or whatever initiates it) senses the energy need?
  4. Is energy need is synonymous (only) to the ATP level?
  5. How and at which state the hypothalamus (AgRP, POMC & NPY) is involved?
  6. What is the role of stomach, pancreas & intestines?
  7. How to distinguish hunger, appetite and satiety?
I have gone through many articles in ncbi articles. They look mostly convoluted (for me) and it is very difficult to distinguish the start from the end.

Can anyone throw some light on simple terms on the connection among the energy level, ATP, hunger hormones and hunger neurons so that I get clarity on complete physiology on hunger?
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But I am confused as to how hunger is triggered at first instance.
The question seems to seek a simple answer, but as your comments and other questions make clear, it's not really a simple question. Hunger is a name for an imprecisely defined set of emergent phenomena, the processes contributing to which are extremely complex.

The only responsible simple answers will include the proviso that it's not simple.

Here's an article that may or may not strike you similarly to others you've read:

It doesn't mention orexin/hypocretin, which is a mediator of appetite as well as of muscle tone. You can see the resemblance between the words "orexin" and "anorexia". It's a functional name.

Hunger, appetite, satiety -- it's complicated.
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Science Advisor
Chapter 4: Central Control of Feeding Behavior
Patrick Dougherty, Ph.D., Department of Anesthesiology and Pain Medicine, MD Anderson Cancer Center
Feeding by its nature is intermittent, yet the need for energy in tissues is constant. Thus, mechanisms have evolved for the ebb and flow of nutrients after feeding and during the post-absorptive period; and the maintenance of near-normal function during fasting. The remarkable stability of body weight in persons with access to adequate food supplies is testament to the precision by which metabolic needs are monitored and maintained. Aberrations in these controls can produce serious and even life-threatening conditions.

jim mcnamara

This discusses hormones, genetics in terms of obesity and hunger.

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