First of all, flavonoids have important functions in plants, such as attracting pollinating insects, combating environmental stresses, including microbial infections, damaging UV’s rays. Moreover, flavonoids regulate cell growth.
Through food consumption, they provide humans high biological activity and display antioxidant, anti-inflammatory and anti-allergic properties. Dietary flavonoids represent a wide range of polyphenolic compounds that can be found in many fruits, vegetables, grains, herbs, and beverages.
Numerous studies support the fact that flavonoids may protect humans from diseases such as cognitive function decline, Alzheimer’s and Parkinson disease, and specific cancers. 
Finally, another study suggested that Ginkgo biloba (GB) extract containing 22%-27% flavonoids (ginkgo-flavone glycosides), and 5%-7% terpenoids (ginkgolides and bilobalides) could activate cell proliferation and cytotoxicity in human liver carcinoma cells.
Therefore, GB extract or its constituents has been considered as an anti-cancer agent. 
Flavonoids are categorized into six main subclasses based on their chemical structure: flavonols, flavan-3-ols, flavanones, flavones, anthocyanins, and isoflavones.
Red, blue, and purple berries; red and purple grapes; red wine
Daidzein, Genistein, Glycitein, Biochanin A, Formononetin
Soybeans, soy foods, legumes
More Flavonol More Brain Control
Thomas Holland, MD, of Rush University in Chicago, and colleagues, in Neurology reported that people who consumed the highest dietary intake of flavonols were 48% less like to develop Alzheimer’s dementia than people with the least intake.
“The bioactives in foods – which from our research would be specifically flavonols found in kale, spinach, tomatoes, tea, olive oil, apples, pears, and over 20 other foods — have antioxidant and anti-inflammatory properties. Consequently, they have the potential to protect against cellular damage due to oxidative stress and sustained inflammation,”Dr. Holland said.
The Memory and Aging Project by Rush University, has been studying a group of people to better understand, treat and hopefully prevent these problems associated with abnormal aging.
In the study, 81 was the average age of the sample, and 75% were female. Each year, participants had annual neurologic evaluations and completed a Questionnaire, which asked about the frequency of eating or drinking 144 food items in the previous 12 months.
So far, over a mean follow-up of 6.1 years, among 921 participants, 220 of them developed Alzheimer’s dementia.
The researchers concentrated on the intake at four flavonols — kaempferol, quercetin, myricetin, and isorhamnetin — and total flavonol (the sum of the four individual flavonols).
The lowest intake consumed was 5.3 mg per day on average; the highest was 15.3 mg per day.
People with the highest intake of isorhamnetin had a 38% reduced rate of incident Alzheimer’s dementia. Similarly, highest intake of kaempferol showed a 51% lower rate, and highest intake of myricetin was tied to a 38% reduced rate. Quercetin was not associated with Alzheimer’s dementia. 
Since the outcome in this study was self-report, is it possible that people who developed Alzheimer’s remembered fewer specifics about their diet for the last year. So it is possible that they might have reported erroneous answers to the questionnaire. In conclusion, this study might have limitations, Dr. Holland and co-authors noted.