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It’s not difficult to consume 29 mg of anthocyanins each day, the median intake of study participants whose diets provided the most. You’ll find plenty more than that in one-half cup of each blueberries (122 mg), blackberries (75 mg) and raspberries (60 mg).

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Brightly coloured fruits and vegetables – think blueberries, carrots and spinach – owe their hue to phytochemicals, powerful plant compounds thought to help preserve memory as we age.

Numerous studies, for example, have linked a high intake of flavonoids, the largest group of phytochemicals, to a lower risk of cognitive decline and Alzheimer’s disease.

Now, new findings from the Harvard T.H. Chan School of Public Health and Brigham and Women’s Hospital in Boston suggest that eating a flavonoid-rich diet guards against early brain changes that could eventually lead to dementia.

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Flavonoids defined

Flavonoids are bioactive compounds in fruits, vegetables and other plant foods. They’re categorized into six subclasses, each one residing in certain foods and having specific health properties.

The flavonoids most commonly consumed include anthocyanins (berries, red grapes, cherries, red cabbage), flavan-3-ols (green tea, black tea, cocoa), flavonols (onions, kale, broccoli), flavanones (citrus fruit), flavones (peppers, celery, parsley) and isoflavones (soybeans, legumes).

The new research

The study, published on July 28 in the journal Neurology, followed 77,335 American men and women for over 20 years. Dietary information was collected at the beginning of the study, when participants were, on average, 50 years old, and every four years afterwards.

Subjective cognitive decline was assessed twice during the study by asking participants six “yes” or “no” questions on recent changes in memory and thinking skills (e.g., Do you have trouble remembering a short list of items, understanding or following spoken instructions, following a group conversation, finding your way around familiar streets?).

These questions capture early memory problems or confusion that are noticeable, but not necessarily enough to be detected on a screening test. Subjective cognitive function is considered a precursor to mild cognitive impairment, a risk factor for Alzheimer’s disease.

Participants whose diets contained the most flavonoids (versus the least) were 20 per cent less likely to self-report cognitive decline.

When researchers looked at individual flavonoids, flavones showed the strongest protective effects. A high intake was tied to a 38 per cent lower risk of subjective cognitive decline, the equivalent of being three to four years younger in age.

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Flavanones and anthocyanins were also strongly associated with better later-life subjective cognitive function.

To arrive at their findings, the researchers controlled for other risk factors for dementia such as age, education, physical activity, smoking status, alcohol intake, diabetes, high blood pressure and cardiovascular disease. The study was observational so it doesn’t prove cause and effect. As well, dietary data was self-reported and could, therefore, be prone to error. Even so, the study’s strong points – its long follow-up period, large sample size and repeated dietary measurements – add to the validity of the findings.

How flavonoids protect the brain

The brain changes that lead to dementia may develop for years, even decades, before subjective cognitive decline. These new long-term findings suggest that eating a flavonoid-rich diet when younger may protect later-life brain health.

Flavonoids are potent antioxidants and, as such, are thought to shield brain cells from free radical damage. Flavanones in citrus have also been shown to inhibit the harmful effects of beta-amyloid, sticky proteins that form plaques and destroy brain cells.

Flavonoids may also protect memory by suppressing inflammation in the brain.

How much is beneficial?

The median (middle) intake of flavones in high consumers was 3.5 mg per day, an amount found in one-half cup of chopped green pepper or two medium (eight inch) celery stalks. One tablespoon of chopped fresh parsley delivers 8 mg of flavones.

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For flavanones, the median daily intake was 74 mg, which you’ll get by eating one-half of a grapefruit or one-and-a-half medium-sized oranges. You’d need to drink 1.5 cups of orange or grapefruit juice to get 75 mg of flavanones.

It’s not difficult to consume 29 mg of anthocyanins each day, the median intake of study participants whose diets provided the most. You’ll find plenty more than that in one-half cup of each blueberries (122 mg), blackberries (75 mg) and raspberries (60 mg). One cup of red grapes has 72 mg of anthocyanins.

Include a variety of colourful fruit and vegetables in your daily diet. Besides flavonoids, these foods supply vitamins and other phytochemicals linked to brain health.

Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on Twitter @LeslieBeckRD

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