Dietary Patterns and Nutritional Interventions for Cognitive Impairment in Parkinson’s Disease: A Critical Narrative Review

Background/Objectives: Cognitive impairment affects most people with Parkinson’s disease (PD); mild cognitive impairment is present in 25–40% of people with PD without dementia, and nutritional advice often exceeds the evidence. We reviewed the human evidence that dietary patterns or nutritional supplements improve cognition in established PD, separating objective cognitive outcomes from patient-rated and biomarker outcomes. Methods: Critical narrative review reported according to SANRA. Sources were PubMed, Embase, Web of Science and Cochrane CENTRAL (inception to 29 September 2026) and reference lists. Controlled intervention studies in established idiopathic PD with an objective cognitive outcome or a specifically rated cognitive symptom formed the evidence set, appraised on risk-of-bias domains and classified by an explicit rule. Results: Twenty-three randomized controlled studies met the criteria (five dietary patterns, 18 supplements). Favorable between-group differences were reported in six studies (Mediterranean diet, a ketogenic pilot, creatine–coenzyme Q10, a probiotic, saffron and ginkgo leaf extract), all with fewer than 100 participants and heterogeneous cognitive endpoint status. The three largest trials did not demonstrate cognitive benefit. Other reports showed nonsignificant comparisons or did not provide a between-group cognitive estimate; one used only a rated symptom. No study showed durable functional benefit or reduced progression to dementia. Conclusions: No nutritional intervention has independently replicated, durable cognitive efficacy in PD. Nutritional adequacy, correction of documented deficiency, dysphagia care and individualized levodopa–meal timing remain the clinical priorities; a Mediterranean-style pattern is a reasonable default, not a cognitive treatment.