Experts observed this effect over more than a decade in people whose diet featured lots of vegetables, fruit, nuts, fish, beans, olive oil, and poultry, with a "moderate intake" of wine, while limiting red meat, fried foods, and sweets.

The diet was linked to less  over time, especially grey matter, and reduced "ventricular enlargement", a key marker of brain ageing.

Researchers noted these are "well-established markers of brain ageing", explaining that grey matter "plays a key role in memory, learning and decision making" while ventricular enlargement reflects tissue loss.

They concluded that "foods rich in antioxidants, such as berries, and high-quality protein sources like poultry, may reduce oxidative stress and mitigate neuronal damage."

Conversely, "fast fried foods, often high in unhealthy fats, trans fats and advanced glycation end-products, may contribute to inflammation and vascular damage," they wrote.

Published in the Journal of Neurology, Neurosurgery and Psychiatry, the study analysed data from 1,647 people averaging 60 years old. They examined adherence to the "Mind" diet (Mediterranean-Dash Intervention for Neurodegenerative Delay), designed to support brain health and reduce Alzheimer's risk.

This diet features many plant-based, antioxidant-rich foods while limiting saturated fats and added sugars.

Participants completed food frequency questionnaires and had at least two brain MRI scans.

Over an average 12-year monitoring period, while all aged individuals displayed some loss of brain matter, those adhering most closely to the diet experienced slower grey matter shrinkage and loss.

Each three-point increase in diet adherence was linked with slower loss, equivalent to 20 per cent less age-related decline and two-and-a-half years of delayed brain ageing.

Similarly, each three-point increase was associated with slower expansion of total ventricular volume, equivalent to 8 per cent less tissue loss and one year of delayed brain ageing.

Benefits were also greater in older, active, and non-overweight individuals, indicating combined lifestyle approaches may have a significant effect.

Reacting to the study, Dr Jacqui Hanley, head of research at Alzheimer's Research UK, said: "Taken together with earlier evidence, the findings add to growing indications that eating a balanced diet, staying active and taking other healthy steps may support our brain health as we age.

"More long-term studies in diverse groups are still needed to untangle the role of diet, genetics and other factors."

Professor Catey Bunce, statistical ambassador at the Royal Statistical Society, commented: "As with many observational studies, the results are interesting and may point to possible associations, but they should not be interpreted as definitive evidence that the diet directly prevents brain ageing."

Michelle Dyson, chief executive of the Alzheimer's Society, said: "The Lancet Commission, which was part funded by Alzheimer's Society, found nearly half of dementia cases globally are linked to modifiable risk factors - things that can be changed either on an individual or societal level - and could potentially be prevented or delayed.

"Continuing to grow our understanding of risk, and encouraging people to quit smoking, keep physically active, eat a healthy balanced diet, control blood pressure and drink less alcohol can all help reduce the risk of dementia."