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In the fed state, when the body has plenty of energy (especially blood glucose), there’s no immediate need to store energy or try to make blood sugar last longer for brain function. At this time, insulin levels increase because insulin helps take nutrients out of the blood and delivers them into cells for storage. This includes amino acids, fatty acids, and carbohydrates—basically, all the macronutrients from the food you eat are moved into cells.

Since high blood glucose is dangerous in the long term, the body prioritizes keeping it in check. Most cells rely on carb metabolism (glycolysis) to break down carbs into smaller components to produce ATP (the body's energy currency) in the mitochondria. During this state, the body mainly uses carbs to fuel ATP production.

If you have more carbs than needed for immediate energy, the excess glucose is stored as glycogen in the liver and muscles through a process called glycogenesis. If there is still excess glucose after glycogen stores are full, the liver converts some of it into fatty acids, which are released into the blood. These fatty acids are then taken up by fat cells (adipocytes) and stored as triglycerides. This process is called lipogenesis.

Since the body is focused on carb metabolism, most of the extra fat is stored rather than used for immediate energy. Additionally, protein synthesis ramps up during the fed state, as the body builds and stores proteins for future use. Protein is the least preferred source for energy, but it is important for preparing the body to handle future stress or activity.

Overall, insulin plays a central role in this process, driving all these anabolic reactions (storing energy, building proteins, and making fat) to ensure the body is prepared for any future energy needs

Transition to the Fasted State: Once you've been without food for a while, your body shifts from using nutrients that are actively being absorbed to relying on internal stores. This transition helps preserve blood glucose levels, which are crucial for the brain and central nervous system.
Energy Source Shift: As you enter a fasted state, the body starts to reduce its uptake of glucose into cells and increases reliance on fat metabolism for energy. The fat is mobilized by lipolysis, which is the breakdown of triglycerides (fat stored in fat cells) into fatty acids and glycerol.
Beta-Oxidation: The fatty acids released into the bloodstream undergo beta-oxidation, breaking them down into acetyl-CoA, which is used to fuel the Krebs cycle (or citric acid cycle) for ATP production.
Maintaining Blood Glucose: Despite switching to fat metabolism, your body still needs to maintain blood glucose levels for organs like the brain. This is where your liver plays a crucial role:
- Glycogenolysis: The liver breaks down stored glycogen (the carbohydrate form of energy storage) into glucose, which is released into the bloodstream to maintain blood glucose levels.
- Gluconeogenesis: If fasting is prolonged, the liver can also create glucose from non-carbohydrate sources, such as fatty acids and glycerol. This process is called gluconeogenesis.
Decreased Glycolysis and Glycogenesis: Since the body is relying more on fat for fuel, the breakdown of glucose via glycolysis decreases. Also, the storage of glucose (via glycogenesis) is reduced because there's less need for glucose storage when the body is using fat.
Protein Breakdown (Proteolysis): If the fast extends longer, the body may start to break down proteins (muscle tissue) into amino acids, which can be used to create glucose through gluconeogenesis. This is called proteolysis.

In summary, during the fasted state:

Lipolysis and beta-oxidation increase, providing fatty acids for energy.
The liver helps maintain blood glucose levels by breaking down glycogen and producing new glucose through gluconeogenesis.
Glycolysis and glycogenesis decrease because the body is prioritizing fat as the main energy source.
In prolonged fasting, protein breakdown can also contribute to glucose production.

This intricate process ensures that your brain and central nervous system are always provided with the necessary fuel, even when you're not eating

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