This article contains plot details from the latest book.
Appetite Suppression is a skill used to slow the digestive system and lengthen the time someone can go without food, according to Unlocked. The only trick is distraction, focusing on something else to distract yourself from your hunger. It is a skill that is practiced in Exillium, since you have to focus on skills instead of your abilities. In Neverseen, when Sophie and her friends are in Exillium, it is mentioned in the book that their first skill that was trained was appetite suppression and that they did not eat lunch that day. Tam and Linh are also seen in this scene when Linh starts swaying from hunger and Tam helps her put her head between her knees. It is also said that Sophie struggled a lot with the training.
Also, in Flashback, Keefe makes Sophie practice appetite suppression by making her go an hour without her stomach growling while he ate. This shows that Appetite Suppression can be taught multiple ways.