New clinical trial evidence suggests that dietary changes can be powerful tools in the treatment of psychiatric conditions—but which dietary changes are most worth making, and why? In the past decade, two relatively new subspecialties within the field of psychiatry have emerged: nutritional psychiatry, which focuses primarily on the therapeutic potential of the Mediterranean diet, and metabolic psychiatry, which focuses primarily on the therapeutic potential of the ketogenic diet. This presentation examines the existing clinical evidence in support of both dietary philosophies, and then measure these (and other) nutrition interventions against newly proposed brain-healthy diet criteria. We will consider findings from a recently published study conducted in France in which 31 people with serious, chronic, treatment-refractory mental illnesses were placed on a whole-foods ketogenic diet in a supervised inpatient setting, and then discuss how the improvements in metabolic and mental health observed in this study suggest that this approach is worth considering as an adjunctive strategy in the management of major mental illnesses, particularly in those with poor metabolic health.