Current treatment for schizophrenia-spectrum disorders focusses primarily on psychotropic medication to treat symptoms, though their efficacy varies between patients and psychotropic medication is often accompanied by severe side effects. Nutritional interventions to prevent and treat mental illness have received considerable attention over recent years. However, evidence for nutritional interventions in schizophrenia-spectrum disorders remains limited in quantity and quality. Pathways currently in focus include: i) nutritional deficits and impairments in glucose metabolism, ii) inflammation and immune dysregulation (also known from the mild encephalitis hypothesis), and iii) altered gut microbiota. All of which appear to be interconnected. Key limiting factors for advancing research in this field are research challenges associated with assessing and interpreting inflammatory profiles, microbiota and subjective nutritional assessments, which is further complicated by illness characteristics. This review describes the state of evidence for key hypotheses, including underlying mechanisms, implicated in schizophrenia-spectrum disorders, the challenges in nutritional psychiatry research and the current state of nutrition interventions in mental healthcare.