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The researchers at Harvard looked at data from 77,000 middle-aged men and women. They tracked the participants’ diets over 20 years, calculating their intake of six classes of flavonoids, which included beta-carotene in carrots, flavanones in oranges and anthocyanins in berries. They also tested participants’ levels of cognitive decline, with questions such as, ‘Do you have more trouble than usual remembering recent events?’, ‘Do you have more trouble than usual remembering things from one second to the next?’ and ‘Do you have more trouble than usual remembering a short list of items?’

The study found that the people who got the most flavonoids from fruit and vegetables were 19% less likely to report difficulties with memory and thinking than those who ate the least. Researchers took into account their intake of other nutrients, such as vitamins and omega-3s, as well as non-dietary factors including age, weight, physical activity, smoking and alcohol intake, which are known to affect cognition.

While it’s unclear how flavonoids aid brain health, researchers cite a few factors. They improve blood flow to the brain, reduce inflammation, reduce blood pressure, promote new brain cell growth and help brain cells strengthen their connections. They also act via the gut-brain axis, the communication network that connects your gut and brain. Flavonoids provide food for your gut microbes, helping them produce lots of beneficial short-chain fatty acids that decrease the inflammation associated with cognitive decline.