JUST ASK CORI!
Extensive parts of our nervous system create their energy based on carbs alone. The nervous system is a major consumer of the body's carbs. This should not be a surprise as the nature of such a system would be to function at super speeds; neural impulses travel through the body in terms of milliseconds (a second divided by a thousand).
Since bigger molecules require more reactions, and take longer to catabolize, as well as take more time to transport to the cells to even be available, molecules bigger than simple sugars such as glucose would not be suitable as a fuel resource for such a super energized and fast biological system. Such a system would be slowed down dramatically if it ran on fat or proteins.
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Imagine you have been driving for five hours straight (ill-advised by the way), you are feeling tired and you want...sugar! - Not fat, not protein, sugar! sugar! sugar! and more sugar!
We need to distinct between feeling hungry, which would relate to the consumption of carbs, fat, and proteins as options, compared to feeling tired where we want sugar to energize the nervous system.
Analyzing the stores on the side of the road, excluding anything that is not food or beverages, a very high percentage would include sugar in it. This is not random at all, it is well thought of, since people are neurally tired, needy of those carbs. Yet, the body does not have stores "on the side of the road". This begs the question, what happens if carbs are running low for any reason, and there is no external source to replenish carb availability.
Time and time again, I suggest that everything be related to the rules of nature, and to the first rule of nature "survival first". Accordingly, a situation where our nerves do not have carbs to create energy means the possible loss of activation of organs, coordination, balance, cognition, analytical thinking, reaction time, central drive, the ability to tap into common sense.
In addition to the nerves relaying heavily on carbs, mature red blood cells do not have a mitochondrion, thus must relay on anaerobic metabolic production of energy...glucose...sugars! - Red blood cells carry within them the oxygen and CO2 carrier protein hemoglobin. Reduced RBC function immediately means less oxygen supply, and less CO2 disposal. Coupled together, these two processes are crucial to proper and healthy human function. Accordingly, a situation where our RBCs do not have carbs to create energy means having dysfunctional blood tissue. Bad idea if you ask me.
As reviewed in previous posts relating to metabolism, all three metabolic pathways dedicated to ATP production in our cells, always work together in changing dominancies according to changes circumstances. Since one of them is the breakdown of carbs without oxygen (anaerobic glycolysis), it means that the more skeletal muscles are used, the more the strain on the other two tissues types increases.
The ability to survive in nature, drives many living forms to have "backup" metabolic processes in case food is scares. Obviously, humans need such backup processes regarding the possible scarcity of crabs, especially glucose. Our solution is a process called gluconeogenesis. Gluco = sugar; neo = new; genesis = creation. Thus, gluconeogenesis represents the body's ability to chemically repurpose other molecules to create glucose. This ability must relay on existing resources within the human body, since it is a solution for the possibility of lack of external resources.
The key to understanding the body's ability to create new molecules from other existing molecules in the body, is knowing the one molecule of glucose requires a specific organization of two pyruvate molecules and four hydrogen atoms. Since the human body has plenty of hydrogen, it is truly the pyruvate that is needed. Pyruvate is a carb by definition and a common building block of multiple other molecules with a carbon based chemical skeleton.
Since glucose is consumed quickly, evermore so when reserves are depleted, thus, the body needs to be able to get the pyruvate as quickly as possible under the circumstances. fat an protein molecules do not fit these needs best, leaving us with lactate molecules as the best candidate. Lactate molecules include one pyruvate and two hydrogen atoms, which constitute for exactly half a glucose molecules. Hence, two molecules of lactate have everything needed to create one glucose molecule (2 lactate = 1 glucose).
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On a regular basis, lactate molecules are circulated through the blood stream to the liver organ, where they enter the Cori cycle of the liver. The Cori cycle has the ability to catabolize every two lactate molecules into pyruvate and hydrogens, and rearrange them in the form of glucose. The glucose is then circulated to the active and needing cells. The use of the Cori cycle increases the more carbs are depleted, and the more fats and proteins will be used as well.
Lipolysis and proteolysis take longer, making them available for the Cori cycle slower than lactate. Proteolysis is a problematic process to begin with since you have to breakdown your skeletal muscles, and the poisonous gas amonia is released. Proteolysis is the second fastest option of the three, since skeletal muscles have an amino acid called alanine, made up of pyruvate and amonia. We of course, want the pyruvate.
In theory (only), since pyruvate is aerobically catabolized in to acetyl-CoA and CO, the reaction could be reversed to create the needed pyruvate, yet this option requires vast extents of energy investment, and thus is never chosen by the cell. Cells tend to avoid as much as possible, and probably completely, any reactions that demand vast quantities of energy investment.
In average, we begin to breakdown proteins as the main fuel for continuous exercise after 240 minutes (4 hours), after carbs have been mostly depleted. On a regular basis, proteins are not catabolized extensively, and never account for more than 3% of the total demand of energy for the activity analyzed. For the most part, it is kept much lower than even those 3%.
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