In the last post, I explained that eating behavior is determined by a variety of factors, including hunger and a number of others that I'll gradually explore as we make our way through the series. These factors are recognized by specialized brain 'modules' and forwarded to a central action selection system in the midbrain (the reward system), which determines if they are collectively sufficient cause for action. If so, they're forwarded to brain systems that directly drive the physical movements involved in seeking and consuming food (motor systems).
The term 'homeostasis' is important in biology. Homeostasis is a process that attempts to keep a particular factor within a certain stable range. The thermostat in your house is an example of a homeostatic system. It reacts to upward or downward changes in a manner that keeps temperature in a comfortable range. The human body also contains a thermostat that keeps internal temperature close to 98.6 F. Many things are homeostatically regulated by the body, and one of them is energy status (how much energy the body has available for use). Homeostasis of large-scale processes in the body is typically regulated by the brain.
We can divide the factors that determine feeding behavior into two categories, homeostatic and non-homeostatic. Homeostatic eating is when food intake is driven by a true energy need, as perceived by the brain. For the most part, this is eating in response to hunger. Non-homeostatic eating is when food intake is driven by factors other than energy need, such as palatability, habitual meal time, and food cues (e.g. you just walked by a vending machine full of Flamin' Hot Cheetos).
We can divide energy homeostasis into two sub-categories: 1) the system that regulates short-term, meal-to-meal calorie intake, and 2) the system that regulates fat mass, the long-term energy reserve of the human body. In this post, I'll give an overview of the process that regulates energy homeostasis on a short-term, meal-to-meal basis.
The Satiety System (Short-Term Energy Homeostasis)
The stomach of an adult human has a capacity of 2-4 liters. In practice, people rarely eat that volume of food. In fact, most of us feel completely stuffed long before we've reached full stomach capacity. Why?
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