Well I guess it could depend on your definition of "source". (Sorry, I my education is in biology!)
When I think of energy source, I think of the smallest form of stored energy in the body - ATP. Not calories, as they are simply a measurement of energy (like a watt). Not sugar, protein, or fat - as they in turn become energy, which is stored in ATP, for the most part.
Cells gain the energy they need for all their amazing processes via “cellular respiration". ATP is the product of CR, and the main energy source for the majority of cellular functions. This includes the synthesis of macromolecules, including DNA and RNA and proteins.
Nutrients, such as glucose, amino acids and fatty acids get oxidized (this is why O2 is SO important!) and in turn gets synthesized into ATP, which more or less BREAKS apart and releases energy! Like a bomb!
Ok that was an elaboration, but you get my drift!
But to answer your question on the grand scheme... perhaps glucose and fatty acids??
The liver has the capacity to convert all three molecules (fat, protein, carb) into each other as the body needs… but this is not necessarily a good thing!
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