Data from the long-running Rush Memory and Aging Project, involving 960 participants who completed a food frequency questionnaire from 2004 to 2013, found that those who ate one daily serving of green, leafy vegetables had a slower rate of cognitive decline than people who rarely or never ate them. Those who ate at least one serving of leafy green vegetables showed an equivalent of being 11 years younger cognitively.

Participants had an average age of 81 at the beginning of the study. Analysis took into account such factors as seafood and alcohol consumption, smoking, high blood pressure, obesity, education level and amount of physical and cognitive activities.

A study involving 99 healthy older adults found that levels of monounsaturated fatty acids were associated with cognitive performance and the organization of the brain's attention network.

MUFAs are found in olive oils, nuts and avocados, and are a component of the Mediterranean diet, which has been associated with slower cognitive decline and lower dementia risk.

The study found that general intelligence was associated with how efficiently the dorsal attention network was functionally organized used a measure called small-world propensity, which describes how well the neural network is connected within locally clustered regions as well as across globally integrated systems.

In turn, they found that those with higher levels of MUFAs in their blood had greater small-world propensity in their dorsal attention network, suggesting that this is part of the reason for the link between diet and cognition.

And in another study by the same researchers, involving 94 older adults (65-75), blood levels of three omega-3 fatty acids (ALA, stearidonic acid and ecosatrienoic acid) were associated with levels of fluid intelligence, and this was mediated by the size of the left frontoparietal cortex. A larger left frontoparietal cortex was associated with higher levels of the omega-3 acids and better performance on tests of fluid intelligence.

A second study found that the size of a brain region called the fornix was associated with a balance of omega-3 and omega-6 fatty acids in the blood, and a more robust fornix was linked to better memory. The fornix is one of the first brain regions to be compromised in Alzheimer's disease.

A largish Chinese study, involving 541 9-11-year-olds, has found that those who ate fish at least once a week slept better and had higher IQ scores, on average, than those who ate fish less frequently or not at all.

The study suggests that sleep may be a reason for the association previous research has found between the consumption of fish / omega-3 oils and better cognition.

Children who reported eating fish weekly scored 4.8 points higher on the IQ tests than those who said they “seldom” or “never” consumed fish. Those whose meals sometimes included fish scored 3.3 points higher.

A study looking at how 51 women performed on a test of their attention after they ate either a meal high in saturated fat or the same meal made with sunflower oil (high in unsaturated fat), found their performance was worse after eating the high-saturated-fat meal than after they ate the meal containing a healthier fat.

This is particularly remarkable when you consider that both meals with high in fat, being designed to resemble fast-food meals that you might get at McDonald’s or Burger King. So this is showing that even this slight improvement can have a noticeable effect.

Additionally, those who had signs of leaky gut performed worse no matter which meal they had eaten.

A study involving 110 lean and healthy students (aged 20-23) found that those randomly assigned to a high energy western-style diet for a week performed worse on a memory test compared to those who ate their normal healthy diet. Moreover, those on the unhealthy diet then showed a greater tendency to eat more sugary foods.

The students at the beginning and end of the study were given breakfast in the lab. Before and after the meal, they completed word memory tests and scored a range of high-sugar breakfast foods according to how much they wanted and then liked the foods on eating them.

The more desirable people found the sugary food when full, following the western-style diet, the more impaired they were on the cognitive test. It’s suggested that both reflect impairment in hippocampal function.