
Originally Posted by
Splifton
Alright, when you exceed your macronutrients a few things can happen depending upon what macro was in abundance. With carbohydrates they can be broken down into glucose. Glucose can be converted into glycogen for the intramuscular energy stores. Glucose can also be used as a reactant for cellular respiration which is how our body creates a form of cellular "currency" per say. In cellular respiration the usual substrate is pyruvate. Now a reactant is a substance that is present prior to the chemical reaction and the substrate is the substance that is being modified in the reaction. The chemical reaction occurs and it's called oxidative phosphorylation and it takes place in the mitochondria compartment. The sole job of this oxidation is to successfully cleave a phosphate group to ADP and thus return it to it's former self which is ATP. So these little reactions are solely involved with topping off immediate energy reserves and help contribute to energy homeostasis. When we are in equilibrium with our energy stores then our body sees no reason to alter any metabolic activities to anticipate and prevent famine.
So any macronutrient can actively contribute to this cellular pathway, but the carbohydrate requires the least amount of conversions and thus is the ideal choice if present in adequate amounts. Now with protein our body can utilize it for amino acids by stripping away it's nitrogenous base and undergoing proteolysis. Once the protein is broken down we have specific amino acids that are relevant to the specific type of protein. These amino acids cannot be stored and are either utilized for immediate biological requirements or they venture on and become excreted while some can actively convert into glucose and contribute to more cellular metabolic reactions. Now in the instance that your body is in a state of energy fulfillment with glycogen stores and intracellular energy requirements, it can take this glucose molecule and feed it to the TCA cycle. Eventually down the line we end up with Acetyl-CoA. Acetyl-CoA can undego lipogenesis and become a free fatty acid. Eventually our body wants to store away our fatty acids and it begins triglyceride esterification to create a chemical bond and allows the macromolecule to be stored within our adipocytes or fat.
Now, fat can just be immediately converted into it's appropriate fatty acid and further esterified to become a triglyceride or it can actively participate in the myraid of other activities that were formally mentioned. Just as a note though proteins can convert into glucose which can convert into fat, but glucose can't turn a protein. Unless you are a plant. Fat can't become a protein either.
A very simplified version and I skipped quite a bit of possible alternatives as well as ketone production.