Carbohydrates are fundamentally the fuel that is required to make living things survive. This is not a simple reaction of carbohydrates being burned like gas and wood to give off heat, carbon dioxide, and water. In living beings, the conversion of energy is quite different and occurs due to chemical processes of oxidation, hydrolysis, and enzyme activity. Carbohydrates have to first be broken down to simple sugars like glucose for this to occur, as glucose is a monosaccharide. Glucose is then converted into a substance called pyruvate and some ATP or adenosine triphosphate is created as a result. From here the pyruvate enters the mitochondria of cells into a process called the citric acid cycle. This is when acetyl CoA enters the cycle to create citrate with oxaloacetate that is oxidized to give off carbon dioxide and is broken in a ten step process to eventually recycle the oxaloacetate. The end products of this cycle are crucial GTP or guanosine triphosphates that are similar to ATP in creating energy.
What is really interesting about this process is that fat and amino acids can also be broken down to acetyl CoA and brought into the citric acid cycle ensuring that energy is always available to cells. This has some down sides to it because the conversion of fat into an energy source is called ketogenesis. This process will increase the blood acidity levels and cause a condition called acidosis that can destroy cells and also the nerves. Using amino acids for fuel will cause useful cellular proteins to be destroyed and used to maintain stasis – also not a very good situation.
The first sign of carbohydrate deficiency is a decrease in insulin levels because there is no glucose in the blood in the first place. This will then cause the liver to start lioplysis and ketogenesis. Inevitably, the energy levels are the first systems to be affected and fatigue will result. After this, the sign of acidosis will result if the starvation continues any further. This is a situation that is akin to a diabetic emergency. Eventually, the body will start to lose muscle tissue because of amino acids being brought into the citric acid cycle to maintain stasis. This is the reason why the whole body starts to look emaciated during starvation and the arms and legs end up looking like just bones. After a certain point, starvation will eventually cause death.