Here "bread" refers to food. It does not mean that John never ate food. It means he fasted often, and when he ate, he did not eat good, expensive food. Alternate translation: "frequently fasting and not drinking alcohol" or "not eating fancy food and not drinking wine" (See: [[rc://en/ta/man/translate/figs-synecdoche]] and [[rc://en/ta/man/translate/figs-explicit]])
This can be translated as an indirect quote. Alternate translation: "they say that he has a demon." or "they accuse him of having a demon." (See: [[rc://en/ta/man/translate/figs-quotations]])