Carbohydrates are made of sugar molecules, and the most important one is the 6-carbon sugar, glucose. It’s important to keep a steady amount of glucose in the blood, because cells use it to produce energy - in the form of adenosine triphosphate, or ATP. One of the ways you can do this is by eating carbohydrate-rich foods like pasta. But in between meals, when fasting, the body maintains glucose levels using gluconeogenesis.
Gluconeogenesis is a metabolic pathway that uses enzymatic reactions to make glucose from other molecules, like amino acids, lactate, and glycerol. Gluconeogenesis primarily takes place in liver cells, but it can also happen in the epithelial cells of the kidney and the intestines. Specifically, it takes place in the cytoplasm, mitochondria, and endoplasmic reticulum of cells found in these tissues.
Okay, so let’s say you’re going hiking in the woods, and you fuel up on some pasta before you leave. Now, during the hike, you get lost and end up stranded with no food. Initially, the glucose in your pasta is broken down by a series of enzymatic reactions to make pyruvate, producing ATP in the process. This is called glycolysis, and it keeps you going for a couple of hours. Some of the extra glucose is stored in the liver cells in the form of glycogen, which is a bunch of glucose molecules stringed together. When you’re fasting, you still need glucose, in particular for your red blood cells and your brain. And you might need it to find your way out of the woods. So, it’s up to your liver to maintain adequate blood glucose levels while fasting. There are two pathways that can contribute glucose: glycogenolysis and gluconeogenesis. So with glycogenolysis, the liver breaks down glycogen into individual glucose molecules, but that only helps for 12 to 24 hours of fasting, because glycogen stores are finite. In contrast, gluconeogenesis makes glucose from scratch, so it can keep on going in the event that you fast for more than a day. Actually, by 12 hours of fasting, gluconeogenesis is the main provider of glucose in the bloodstream.