The antonyms for the word "more sustaining" would be words that describe something that is less nourishing or less able to provide support over a longer period of time. Some possible antonyms for "more sustaining" might include "less substantial," "less nourishing," "less satisfying," or "less enduring." These words suggest that the thing in question is not able to provide the same level of sustenance or support as something that is more sustaining. In other words, antonyms for "more sustaining" might be used to describe things that are not as filling, not as nourishing, or not as able to provide lasting energy or support.