The thing that fills them is food. This can be stated explicitly. Alternate translation: "be filled with food" or "have enough to eat" (See: [[rc://en/ta/man/translate/figs-explicit]])
James uses a rhetorical question to teach his audience. Alternate translation: "how does that help them?" or "that does nothing good for them." or "your speech by itself does not help them." (See: [[rc://en/ta/man/translate/figs-rquestion]])