NEW YORK (NYTIMES) - Walnuts can improve cognitive function. Blueberries can boost memory. Fish oil supplements can lower one's risk of Alzheimer's disease.

You may have noticed these buzzy "brain food" claims scattered across online health articles and social media feeds. But can certain foods or diets really stave off or prevent dementia?

Experts say that while nutrition studies are notoriously challenging to carry out, there is a compelling and growing body of research that does suggest that some foods and diets may offer real benefits to an ageing brain.

The New York Times spoke with two dozen researchers and pored over the research to better understand the links between diet and dementia.

Pillars of a 'brain-boosting' diet

Scientists do not yet know for certain what causes Alzheimer's disease, the most common form of dementia.

And there is no medication that can reverse it, said Dr Uma Naidoo, director of nutritional and metabolic psychiatry at Massachusetts General Hospital and author of This Is Your Brain On Food. But, she said, "we can impact how we eat".

Research shows that people with certain conditions such as heart disease, high blood pressure, obesity and diabetes are more likely than those without such conditions to experience age-related cognitive decline.

And the risks of developing those conditions can be increased by poor diet and a lack of exercise, suggesting there are things that can be done to lower the chances of developing dementia, Dr Naidoo said.

Two diets in particular, the Mediterranean diet and the Mind (Mediterranean-DASH Intervention for Neurodegenerative Delay) diet - both of which encourage fresh produce, legumes and nuts, fish, whole grains and olive oil - have been shown in scientific studies to offer strong protection against cognitive decline.

One study, published in 2017, analysed the diets and cognitive performance of more than 5,900 older adults in the United States.

Researchers found that those who most closely adhered to either the Mediterranean or Mind diet had a 30 to 35 per cent lower risk of cognitive impairment than those who adhered to these diets less closely.

"Pretty much anything that will help keep arteries healthy will reduce risk of dementia," said Dr Walter Willett, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health.

Dr Ronald Petersen, a neurologist and the director of the Mayo Clinic Alzheimer's Disease Research Center, agreed: "What's good for the heart is good for the brain."

Leafy greens

One big change you can make to your diet, Dr Naidoo said, is to "up your plant game". Leafy greens are packed with nutrients and fibre, and solid evidence has linked them with slower age-related cognitive decline.

In one randomised controlled trial performed in Israel and published in 2022, researchers took brain scans of more than 200 people who had been split into three diet groups.

They found that after 18 months, those who followed a "green" Mediterranean diet - one rich in Mankai (a nutrient-packed green plant), green tea and walnuts - had the slowest rate of age-related brain atrophy. Those who followed a traditional Mediterranean diet were close behind.

Those who followed regular healthy diet guidelines - which was less plant-based and allowed for more processed and red meats than the other two diets - had greater declines in brain volume.

These neuroprotective effects were especially pronounced in people aged 50 and older.

Colourful fruit and vegetables

The more colourful the produce on your plate, the better the food usually is for your brain, several experts said.

In one 2021 observational study, researchers followed more than 77,000 people for about 20 years. They found that those with diets high in flavonoids - natural substances found in colourful fruit and vegetables, chocolate and wine - were less likely than those who consumed fewer flavonoids to report signs of cognitive ageing.

The Mind diet specifically points to berries, which are good sources of fibre and antioxidants, as having cognitive benefits. One study published in 2012 looked at more than 16,000 people aged 70 and older for more than a dozen years. It concluded that older women who ate more blueberries and strawberries had delayed rates of cognitive decline, perhaps by up to 2.5 years.

Fish

Many types of seafood, in particular fatty fish, are good sources of omega-3 fatty acids, which have been long associated with better brain health and reduced risk of age-related dementia or cognitive decline.

"Fish is brain food," said Dr Mitchel Kling, director of the memory assessment programme at the New Jersey Institute for Successful Ageing at the Rowan University School of Osteopathic Medicine.

One specific omega-3 fatty acid - docosahexaenoic acid, or DHA - found in cold-water, fatty fish, like salmon, is "the most prevalent brain fat", said Dr Lisa Mosconi, director of the Alzheimer's Prevention Program at Weill Cornell Medicine.

Nuts, whole grains, legumes and olive oil

Nuts and seeds have been repeatedly linked to slower cognitive decline.

In a 2021 review of 22 studies on nut consumption involving nearly 44,000 people, researchers found that those at high risk of cognitive decline tended to have better outcomes if they ate more nuts - specifically walnuts. However, the authors acknowledged some inconsistency among the studies and inconclusive evidence.

Another study, published in 2014, looked at about 16,000 women aged 70 and up between 1995 and 2001. Researchers found that women who said they consumed at least five servings of nuts a week had better cognitive scores than those who did not eat nuts.

Whole grains, as well as legumes like lentils and soya beans, also appear to have benefits for heart health and cognitive function. In a 2017 study of more than 200 people in Italy aged 65 and older, researchers found an association between consuming three servings of legumes a week and higher cognitive performance.

And olive oil, a main component of both the Mediterranean and Mind diets, has strong links with healthy cognitive ageing. One 2022 study of more than 92,000 US adults found that higher intakes of olive oil were associated with a 29 per cent lower risk of dying from neurodegenerative disease - and 8 to 34 per cent lower risk of mortality overall - compared with those who never or rarely consumed olive oil.

According to the experts, there is little to no evidence that dietary supplements - including fatty acids and vitamins B or E - will reduce cognitive decline or dementia. "Supplements cannot replace a healthy diet," Dr Mosconi said.