Which respiratory substrate releases the greatest amount of energy per gram when oxidised?

Explanation

Fat yields roughly 9 kcal per gram against about 4 for carbohydrate and protein, because its carbon atoms are far more reduced and so give up more electrons to the transport chain. This energy density is why fat rather than glycogen is the body's long term store and why migrating birds and hibernating mammals rely on it. Nucleic acids are informational molecules and are not used as fuel.