Objective: To determine what, how, for whom, why, and in what circumstances educational interventions improve the delivery of nutrition care by doctors and other healthcare professionals work. Design: Realist synthesis following a published protocol and reported following Realist and Metanarrative Evidence Synthesis: Evolving Standards (RAMESES) guidelines. A multidisciplinary team searched MEDLINE, CINAHL, ERIC, EMBASE, PsyINFO, Sociological Abstracts, Web of Science, Google Scholar and Science Direct for published and unpublished (grey) literature. The team identified studies with varied designs; appraised their ability to answer the review question; identified relationships between contexts, mechanisms and outcomes (CMOs); and entered them into a spreadsheet configured for the purpose. The final synthesis identified commonalities across CMO configurations. Results: Over half of the 46 studies from which we extracted data originated from the USA. Interventions that improved the delivery of nutrition care improved skills and attitudes rather than just knowledge; provided opportunities for superiors to model nutrition care; removed barriers to nutrition care in health systems; provided participants with local, practically relevant tools and messages; and incorporated nontraditional, innovative teaching strategies. Operating in contexts where student and qualified healthcare professionals provided nutrition care in developed and developing countries, these interventions yielded health outcomes by triggering a range of mechanisms, which included feeling competent, feeling confident and comfortable, having greater self-efficacy, being less inhibited by barriers in healthcare systems and feeling that nutrition care was accepted and recognised. Conclusions: These findings show how important it is to move education for nutrition care beyond the simple acquisition of knowledge. They show how educational interventions embedded within systems of healthcare can improve patients' health by helping health students and professionals to appreciate the importance of delivering nutrition care and feel competent to deliver it.