Controlling appetite isn't always an easy thing to do. For one thing, signals from the brain control how much you eat and when you feel full. It's actually your brain and not your stomach that lets you know you feel hungry. Another factor to consider is that appetite is not simply motivated by the need to satisfy hunger. A number of different hormones and neuron receptors play active roles.
Hypothalamus
The hypothalamus located in the center part of the brain controls appetite. It is where you find several clusters of neurons. One of these clusters -- the arcuate nucleus -- is the appetite center of the brain. Sensors in the arcuate nucleus measure hormone levels and regulate blood lipid and sugar levels. Various hormones act on different areas of the brain either to stimulate or reduce appetite.
Neurons
Neurons in the arcuate nucleus work either to stimulate appetite or suppress it. One type of neurons increases appetite by secreting neuropeptide Y -- known as NPY -- and agouti-related peptide -- or AgRP. Activating the NPY/AgRP releasing neurons increases appetite. The secretion of proopiomelanocortin -- referred to as POMC for short -- signals neurons to depress appetite.
Hormones
Hormones play a key role in controlling appetite. Some hormones, such as ghrelin, increase hunger, whereas PYY3-36 acts to decrease appetite. Ghrelin is a peptide hormone the stomach releases when it is empty. This activates NPY/AgRP releasing neurons telling you that you are hungry. The stomach stops secreting ghrelin once you start eating. Endocrine cells in the intestine release PYY3-36. This hormone inhibits neurons that produce NPY/AgRP, signaling that you are eating. Insulin is another hormone that decreases appetite by inhibiting NPY/AgRP-secreting neurons and activating neurons that release POMC.
Leptin
The hormone leptin also decreases appetite. Leptin is secreted by fat cells and may help to regulate ghrelin -- the hormone responsible for increasing appetite. However, the arcuate nucleus can become leptin-resistant, contributing to overeating and obesity. As a result, people with more body fat have more leptin in their blood. What happens is your brain doesn't get the message that your stomach is full and you keep eating even though your leptin level is high. You should only feel hungry if your leptin level is low. However, the brain becomes less sensitive to leptin when there is a lot of it. Consequently, your body has to release more of it before you feel full.
References
- Medbio.info; Central Control of Appetite and Body Weight; Robert Horn
- Mayo Clinic; Feast or Famine: Researchers Identify Leptin Receptor's Sidekick as a Target for Appetite Regulation; January 2011
- Evolutionary Psychiatry; Appetite Regulation and the Brain; Emily Deans; February 2011
- Endocrine Abstracts; How the Brain Controls Appetite; 2003
- Nutrition Wonderland; Leptin -- The Fullness Hormone; Christie Wilcox; June 2009
