Eating a Mediterranean diet is good for the mind, research has concluded.
The research, by the University of Exeter's Medical School, is the first systematic review of previous studies into the diet's benefits to the brain.
It comes after research last month showed the same diet could help counteract a genetic risk of strokes.
The team, supported by the National Institute for Health Research Collaboration for Leadership in Applied Health Research and Care in the South West Peninsula, analysed 12 eligible pieces of research, 11 observational studies and one randomised control trial.
In nine of the 12 studies, a higher adherence to a Mediterranean diet was associated with better cognitive function, lower rates of cognitive decline and a reduced risk of Alzheimer's disease.
However, results for mild cognitive impairment - the stage before Alzheimer's or dementia, when someone could be experiencing some cognitive difficulties - were inconsistent.
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Lead researcher Iliana Lourida said: "Mediterranean food is both delicious and nutritious, and our systematic review shows it may help to protect the ageing brain by reducing the risk of dementia.
"While the link between adherence to a Mediterranean diet and dementia risk is not new, ours is the first study to systematically analyse all existing evidence."
She added: "Our review also highlights inconsistencies in the literature and the need for further research. In particular research is needed to clarify the association with mild cognitive impairment and vascular dementia.
"It is also important to note that while observational studies provide suggestive evidence we now need randomised, controlled trials to confirm whether or not adherence to a Mediterranean diet protects against dementia."
The Alzheimer's Society welcomed the research.
A spokesman said: "We've known for some that time following a Mediterranean diet such as eating oily fish, green vegetables and avoiding saturated fats may help stave off memory problems in later life but this study pulls the evidence together to understand the whole picture.
"This research shows that the choices we make on lifestyle factors like diet can reduce your risk of developing dementia.
These results, combined with the knowledge that exercise can also reduce your risk, shows that there are things people can do.
"One in three people over the age of 65 will develop dementia so we need more research to bring forward effective treatments and ultimately a cure."