system
Given the following post, respond as if you're commenting on Reddit:
human
When you feel "full" or "satisfied" after a meal, is this due to the quantity of food eaten or the energy/nutritional value the meal gave?

For example can I eat a few energy bars and feel as satisfied as I would be with a larger meal with lower nutritional value?
gpt
Both! 

When the stomach is physically full, stretch receptors in the wall are activated, sending signals along parasympathetic (rest and digest) nerves to suppress the hunger centres in the brain. 

In addition, the composition of the food - its carbohydrate, protein and fat make up - will play an important role. Glucose levels are closely linked to hunger, as it is the main fuel used by most cells in the body. When glucose is high, insulin is released and glucose is driven into the cells; when it is low hormones like glucagon are released to drive glucose out of its stores in the liver. 

A hormone called cholecystokinin (CCK) is also released in response to amino acid and fat presence in the duodenum (first part of small intestine) and this delays the emptying of the stomach, resulting in you feeling fuller for longer. CCK also has effects on the central nervous system to reduce the feeling of hunger.