Introduction Psychological disorders including depression and anxiety are significant public health concerns. A Mediterranean-style dietary pattern (MDP) has been associated with improved mental well-being in observational studies. Evidence of the acute (defined as postprandial to 1 week) effects of an MDP on brain function, mood, cognition and important modulators, including sleep and the gut microbiota is limited. The current intervention aims to examine whether an MDP, compared with a Western diet (WD), improves mood, cognition and anxiety symptoms, postprandially, at 24-hour and after 5 days in adults with mild to moderate anxiety and depression. Methods and analysis Twenty-five UK adults (aged 18 or over) with mild to moderate anxiety and/or depression and low adherence to an MDP were recruited to a cross-over randomised controlled trial. Each participant undergoes a 5 day MDP and a 5 day WD in a randomised order with all meals provided. The co-primary outcomes are mood and anxiety, with secondary outcomes including cognitive function, brain perfusion (as assessed by MRI), sleep quality, blood pressure, plasma glucose, insulin, lipids, C-reactive protein, cortisol, brain-derived neurotrophic factor, gut microbiota speciation and microbial metabolites including short chain fatty acids. A linear mixed model and/or paired analysis will be used to compare the effects of treatments over time. Ethics and dissemination The study has received a favourable ethics opinion from the National Health Service London Queen Square Research Ethics Committee (22/LO/0796). The results will be disseminated through scientific journals and conferences. Trial registration number NCT05927376 .

Background Although the long-term effects of a Mediterranean-style dietary pattern (MDP) on cognition and overall mental wellbeing have been consistently described, the short-term effects of the MDP on cognitive performance, mood and anxiety have not been as widely reviewed. Objectives The aims of this systematic review were to synthesise the evidence from randomised controlled trials (RCT) to examine whether a MDP can alter cognition and overall mental wellbeing in the short-term (up to 10 days). This will also be used to identify research gaps and to inform the design of future acute RCTs in the area. Methods Ovid EMBASE, Ovid MEDLINE and Web of Science Core Collection were searched from inception to 8/12/2020. The data was synthesised narratively with no quantitative synthesis. The detailed protocol is available on PROSPERO, with the registration number CRD42021221085. Results A total of 3002 studies were initially identified. After the deduplication and screening stages, 4 studies (3 articles and 1 conference proceeding) were eligible to be included. Despite the very limited data obtained, the literature suggests that a MDP can improve cognition and mood in the short-term. Specifically, improvements in attention, alertness and contentment were consistently reported. Conclusion A MDP appears a promising strategy to improve short-term cognitive and mental health. A limitation of this review is the small number of studies identified, therefore, future studies are required to confirm these initial novel findings, and to provide granularity as to which domains are most responsive and in which population subgroups.