system
Given the following post, respond as if you're commenting on Reddit:
human
Metabolically, what advantages are there to "Carbo-Loading" by loading up on rice/pastas the day before a big marathon or hike?
gpt
The most important thing about carb loading is the fact that it replenishes and maximizes your glycogen stores. When you carb load, your body digests those carbs into glucose, fructose, and galactose (mostly glucose). Fructose and galactose can be converted to glucose through a series of chemical reactions in the body. The thing is, you don't need all the energy at that moment. So, when you have such an excessive amount of glucose in the body, insulin is secreted which stimulates glyconeogenesis: glucose gets polymerized in a chain to form glycogen. Glycogen is almost entirely stored in muscles and the liver. 

The importance of this is that when the body needs energy, it can then break down the glycogen through a process known as glycogenolysis. This process is stimulated most directly by the epinephrine that is pulsating through your body during physical exercise (it can also be stimulated by glucagon, but during exercise it is primarily epinephrine doing the stimulation). 

By maximizing your glycogen stores, you maximize the energy your body has stored, therefore, maximizing your energy output and your success in any physical activity. That is why for practically any sport, you are supposed to carb load, not protein or fat load, the day before. This is also why everyone says you should eat the majority of your carbs a couple hours before you workout; you need your glycogen stores filled to push yourself.