Nutrition is a pretty important thing. It gives us the energy to sing Disney songs in the shower (and the energy to deny it later), jump the long jump at the track-and-field event, and stay awake while studying for the SAT. If we forgot to eat, we'd be goners. Luckily, our bodies have ways of telling us that we need energy by stimulating or suppressing hunger.
We have two major hormones that regulate how much food we eat and how energetic we are: leptin and ghrelin hormones try to keep us balanced and to maintain homeostasis.
When our energy stores are low, high levels of the hormone ghrelin are secreted in the stomach, circulate in the blood, and tell the brain, "Feed me!" We're not going to get into the anatomy of the brain and central nervous system in detail here, but ghrelin acts at a part of the brain called the hypothalamus. Having nothing to do with actual hippopotami, this is the part of the brain that regulates energy and energy use. When ghrelin gets to the hypothalamus, it stimulates certain brain cells, they become active, and you start thinking a double cheeseburger with curly fries might be nice right about now.
Once you've wiped all the grease off of your face and finished your meal, ghrelin levels decrease. No longer are those cells in the hypothalamus active, and your body gets the sense that it's full. Then you turn down the apple pie pocket dessert. We here at Shmoop are proud of your self-control.
A hormone to stimulate hunger is all well and good, what with the whole "no food and we'd die" thing, but there's got to be a limit. If our body had no way of telling us that we had enough food and we could stop eating, well, it's entirely possible we'd all be spherical in shape and pants with elastic waistbands would reign supreme. Fortunately, our body has thought of everything, and we've got a hormone to suppress hunger—leptin. This hormone is secreted from fat cells and circulates to the same hypothalamus. When leptin acts in the brain, it stimulates the production of other appetite suppressing compounds and blocks activity of certain brain cells that activate the hunger response. With high levels of leptin circulating in your body, just thinking about that second cheeseburger makes you queasy.
Since leptin is made by the body's fat cells, the more fatty tissue a person has, the higher their level of leptin. Scientists also think that leptin is made by the chief cells in the stomach, so food in the stomach might also activate leptin release to tell your body "Enough is enough!" and you decide to take the rest of your meal home.
Be sure to thank your leptin and ghrelin hormones next Thanksgiving as you try and stuff pecan pie al a mode into your already stuffed belly.