One aspect of Chinese philosophy declares there are two forces in the universe yin and yang. Yin is said to be characterized as being a negative force and yang a positive force. At first glance, these forces seem to be diametrically opposed to each other.
But supposedly contrary forces often work together to achieve harmony. Forces in nature are interconnected and Interdependent (e.g. dark-light). So too are the natural forces within our bodies (e.g. hunger-satiety). These natural, bodily forces that strive to achieve and maintain both balance and stability are referred to as homeostasis.
Yin and Yang. Dark half is depicted as yin and the light half as yang
Hunger, of course, is the craving for food. Satiety is the opposite of hunger. It means a feeling of contentment, of satisfaction with no desire for more food. The adipose (fat) tissue and glycogen stores are filled to satisfaction. Hunger is satiated. Appetite and hunger are often used interchangeably but the former also denotes a preference for food without hunger pangs.
What happens when we feel hungry? What makes us say, I’ve had enough to eat? There are several factors that interact and influence hunger and satiety.
Ghrelin and Leptin
Ghrelin and leptin are two digestive hormones that seem to be at opposite ends of the pole. They help influence the way the body balances the signals to eat or not to eat and thus play an important role in regulating hunger and satiety. How this is done is the question. And many questions remain unanswered.
Ghrelin is a protein (a peptide) mainly produced and released by the stomach. This hormone increases your appetite. It’s a hunger signal sent to the brain. We want the brain to interpret these signals and tell us, “more food please”.
Ghrelin was discovered in 1999. It’s called the hunger hormone because it’s part of a system that excites your appetite, increases food intake and enhances fat storage. Its secretion is increased when the stomach is empty and reduced when the stomach is full of food. It’s high when you’re hungry and low after you’ve eaten.
Leptin, in contrast, works to suppress appetite and hunger. Discovered in 1994, this satiety hormone is made by white fat cells and is secreted into the blood stream. It relays information to the brain which the latter uses to tell us, “I’m satisfied. I couldn’t eat another thing” In other words, the set point — homeostasis (see below) has been reached. The brain sends us a message to stop eating.
Gut-Brain Connection — Hypothalamus
The gut-brain connection is the biochemical conversation (signaling) that takes place between the gastrointestinal tract and the brain.
Hidden in the center of the brain is a crucial structure about the size of an almond, the hypothalamus. A key function of the hypothalamus is to harmonize the opposing forces within the body and as a result achieve a beneficial, dynamic and stable equilibrium.
The hypothalamus has been likened to a command center that receives information, processes it, and acts to keep the body safe and operating smoothly at optimal efficiency.
This small, brainy structure influences the physiology of hunger, thirst, sleep, and emotions. It’s also involved with the release of and regulation of chemicals and hormones.
Although there is ongoing research with new discoveries regarding satiety and hunger, two chemicals within the hypothalamus remain at this time are of particular interest.
‘NYP and CART’
NYP (Neuropeptide Y) and CART are two chemicals produced by the brain (hypothalamus). They seem to oppose each other as if they’re in a yin and yang tug of war. NPY is the chemical that increases appetite and hunger. CART decreases appetite and hunger.
When we’re hungry, ghrelin is produced from the lining of the stomach, goes into the blood stream, crosses the blood-brain barrier, stimulates NPY in the hypothalamus and the brain tells us to eat more in order to satisfy our hunger.
When we’ve had enough to eat, leptin produced from fat cells in our body, travels in the blood stream, crosses the blood brain barrier, stimulates CART in the hypothalamus and the brain signals us to stop eating.
The brain significantly influences the body’s natural hunger-satiety connection. However, stress, sleep, processed foods and medications also significantly impact appetite and hunger.
Although your stomach may make noisy, grumbling sounds it’s your brain that ultimately tells us to eat more or signals us to stop eating.
Glossary
Set point — is the target value (hormone levels, temperature) that the body attempts to achieve and maintain within an acceptable range. The body, for example, in the midst of internal variables works to keep the blood glucose level within a specific range — the temperature at a certain value. The definition of homeostasis and set point are essentially the same.
References
Anna-Sophia Wisser et al; Interactions of Gastrointestinal Peptides; International J. of Peptides, vol 2010
Kuoo LE; Neuropeptide Y; Nature Medicnine, July 2007
Dhillo, WS et al; Hypothalamic Interactions; J of Neuroendocrinol; 9/14/2002
This article is intended solely as a learning experience. Please consult your physician for diagnostic and treatment options.