Carbohydrate Metabolism – 5 Metabolism Pathways
There are 5 main pathways for Carbohydrate Metabolism:
If the demand for glucose is low and if there is extra glucose from carbs you’ve ingested, insulin gets stimulated to transfer glucose into the muscles and the liver. If energy demand is high, or your body just needs to tap into your existing glycogen stores, adrenaline kicks in to stop your body from storing glycogen. When you’ve fasted, are low on calories, or are exercising, carb metabolic pathways shift to glycogenolysis.
This is the process of splitting glucose. During fasting or exercising, your body says, “Let’s break up all this glycogen in our muscles and use it.”
One thing to note is the brain and red blood cells can’t store their own glucose, so they rely on the glucose from your last meal. When that runs out, they tap into your liver for the glycogen there. Once glycogen gets stored in the muscle, it can only be used in that muscle. When a person is well fed and energy demand is low, glycogenesis is pretty much your main carb metabolism.
This comes from 2 sources: blood glucose and stored glycogen. Glycolysis is running in high gear if energy demand is high or you’re working very close to your anaerobic threshold, like 10-20 max rep sets.
The Krebs Cycle and Electron Transport Chain
These are during steady state cardio and basically any activity below your anaerobic threshold (like when you’re running or squatting 135lbs for a few warm up sets).
One of the reasons why people will feel sluggish on low carb diets, especially in the first few days, is that they are not regenerating enough ATP through the Krebs cycle. The liver doesn’t have enough stored glucose to ship out to the brain and red blood cells. This usually happens when you’re eating less than 100g of carbs a day. As long as you’re taking in enough fat and you’re consistent with low carbs for 1-2 weeks, you’ll produce enough ketone bodies to compensate. This is what they mean when someone becomes fat adapted.
For your CNS to function optimally, plasma levels of glucose must be sufficient to provide enough energy transfer in the brain. This is how the brain is protected when liver glycogen is low due to low carbs. This helps control the high levels of blood lactate that accompany high intensity activity.