The study suggests a diet consisting mainly of fruit and veg, fish and olive oil but little red meat or dairy products may be associated with losing fewer brain cells caused by ageing.
The findings, published online by the journal Neurology, show that people who follow a Mediterranean-like diet had a larger brain volume than people who do not.
Researchers say the difference between the two groups was about the same as five years of ageing.
Study author Doctor Yian Gu, of Columbia University in the United States, said: “These results are exciting, as they raise the possibility that people may potentially prevent brain shrinking and the effects of ageing on the brain simply by following a healthy diet.”
The Mediterranean-like diet in the study included a high intake of vegetables, pulses, fruits, cereals, fish and mono-unsaturated fatty acids such as olive oil.
It also included low intake of saturated fatty acids, dairy products, meat and poultry, and mild to moderate amounts of alcohol.
Dr Gu said eating more fish and less meat was associated with less brain shrinkage.
He added: “Eating at least three to five ounces of fish weekly or eating no more than 3½ ounces of meat daily may provide considerable protection against loss of brain cells equal to about three to four years of ageing.”
The study involved 674 people with an average age of 80 who did not have dementia.
They completed questionnaires about their diet over the past year and then had brain scans an average of seven months after the diet questionnaire was administered.
The participants were divided into two groups based on how closely their dietary habits followed the Mediterranean diet principles: those who followed the Mediterranean diet principles in at least five food components – either higher consumption of healthy foods, or lower consumption of unhealthy food – and those who did not.
The people who more closely followed a Mediterranean-like diet had total brain volume that was 13.11 millilitres larger than those who did not follow the diet. Their grey matter volume was 5ml larger, and their white matter volume was 6.41ml larger.
Dr Gu said: “The magnitude of the association with brain measures was relatively small, but when you consider that eating at least five of the recommended Mediterranean diet components has an association comparable to five years of ageing, that is substantial.”
Dr James Pickett, head of research at the Alzheimer’s Society, welcomed the findings.
He said: “There is an increasing amount of evidence that eating a healthy diet, rich in fish, vegetables, legumes [pulses] and nuts is good for your brain.
“This study delves further into the potential benefits that diet could have, but it does not prove that a Mediterranean-style diet can stop your brain from shrinking as you age.
“Other key ways to keep your brain healthy is to take as much physical exercise as you can, stop smoking and keep your blood pressure in check.”