Well, fat digestion seems to be effective even comparing to hydrocarbons. I think longer hydrocarbons including sugar have to be broken into fructose and gluco

Well, fat digestion seems to be effective even comparing to hydrocarbons. I think longer hydrocarbons including sugar have to be broken into fructose and glucose during digestion. Fructose cannot be immidiately consumed by cells, so it's converted into glucose in liver. On top of that, there are about 5g of glucose total in bloodstream at any given moment, so exess have to be accumulated by liver/muscle cells that have to store it in a long-chain hydrocarbon form (glycogen). Each conversion requires some energy, so 5-15% isn't that much...