The aliens in my story have biochemistry that uses amino acids and sugars with chirality opposite to that of terrestrial life.
The aliens also have nanites that are capable of changing the chirality of amino acids and sugars found on Earth to that of their own biochemistry and back again, so that the aliens can digest terrestrial food and not poison terrestrial life-forms that might encounter the aliens' bodily wastes.
That then begs the question: what is the minimum net energy cost to perform this conversion of amino acids and sugars to molecules of the opposite chirality?
The nanites will be working on the things that the aliens eat before they are digested, converting food with our chirality to their chirality. Anything that remains undigested has its chirality changed back before it is defecated, so that opposite chirality alien feces won't poison local terrestrial life-forms. The nanites are not attempting to change the chirality of anything that is part of anything alive.