She became an enthusiastic preacher of healthy eating after her recovery.

preacher 是名词，由动词 preach（"宣讲、布道"）加后缀 -er 构成，表示"从事宣讲的人"。原指宗教中的"传教士、布道者"，在这里引申为"倡导者、宣扬者"，指积极传播某种理念或生活方式的人。