Part 2: Classifications and Sources
The intake of flavonoids through food sources could be the simplest and safest way to combat diseases as well as modulate activities. The subgroups under flavonoids are: flavones, flavonols, flavanones, flavanonols, flavanols or catechins, anthocyanins and chalcones
Flavones
Flavones are one of the important subgroups of flavonoids. Flavones are widely present in leaves, flowers and fruits as glucosides. Celery, parsley, red peppers, chamomile, mint and ginkgo biloba are among the major sources of flavones. Luteolin, apigenin and tangeritin belong to this subclass of flavonoids. The peels of citrus fruits are rich in the polymethoxylated flavones, tageretin, nobiletin and sinensetin.
Flavonols
Flavonols are flavonoids with a ketone group. They are building blocks of proanthocyanins. Flavonols occur abundantly in a variety of fruits and vegetables. The most studied flavonols are kaempferol, quercetin, myricetin and fisetin. Onions, kale, lettuce, tomatoes, apples, grapes and berries are rich sources of flavonols. Apart from fruits and vegetables, tea and red wine are also sources of flavonols. Intake of flavonols is found to be associated with a wide range of health benefits which includes antioxidant potential and reduced risk of vascular disease. They are perhaps the most common and largest subgroup of flavonoids in fruits and vegetables. For example, quercetin is present in many plant foods
Flavanones
Flavanones are another important class which is generally present in all citrus fruits such as oranges, lemons and grapes. Hesperitin, naringenin and eriodictyol are examples of this class of flavonoids. Flavonones are associated with a number of health benefits because of their free radical-scavenging properties. These compounds are responsible for the bitter taste of the juice and peel of citrus fruits. Citrus flavonoids exert interesting pharmacological effects as antioxidant, anti-inflammatory, blood lipid-lowering and cholesterol-lowering agents. Flavanones are also called dihydroflavones, Over the past 15 years, the number of flavanones has significantly increased.
Isoflavonoids
Isoflavonoids are a large and very distinctive subgroup of flavonoids. Isoflavonoids enjoy only a limited distribution in the plant kingdom and are predominantly found in soyabeans and other leguminous plants. Some isoflavonoids have also been reported to be present in microbes. They are also found to play an important role as precursors for the development of phytoalexins during plant microbe interactions. Isoflavonoids exhibit tremendous potential to fight a number of diseases. Isoflavones such as genistein and daidzein are commonly regarded to be phyto-oestrogens because of their oestrogenic activity in certain animal models. The effect of genistein inducing hormonal and metabolic changes, by virtue of which they can influence various disease pathways have been reviewed in some studies.
Neoflavonoids
Neoflavonoids are a class of polyphenolic compounds. The first neoflavone isolated from natural sources in 1951 was calophyllolide from Calophyllum inophyllum seeds. It is also found in the bark and timber of the Sri Lankan endemic plant Mesua thwaitesii.
Flavanols, flavan-3-ols or catechins
Flavanonols, also called dihydroflavonols or catechins, are the 3-hydroxy derivatives of flavanones. They are a highly diversified and multisubstituted subgroup. Flavanols are found abundantly in bananas, apples, blueberries, peaches and pears.
Anthocyanins
Anthocyanins are pigments responsible for colours in plants, flowers and fruits. Cyanidin, delphinidin, malvidin, pelargonidin and peonidin are the most commonly studied anthocyanins. They occur predominantly in the outer cell layers of various fruits such as cranberries, black currants, red grapes, merlot grapes, raspberries, strawberries, blueberries, bilberries and blackberries. Stability coupled with health benefits of these compounds facilitate them to be used in the food industry in a variety of applications.
Chalcones
Chalcones are a subclass of flavonoids. They can also be referred to as open-chain flavonoids. Major examples of chalcones include phloridzin, arbutin, phloretin and chalconaringenin. Chalcones occur in significant amounts in tomatoes, pears, strawberries, bearberries and certain wheat products. Chalcones and their derivatives have garnered considerable attention because of numerous nutritional and biological benefits.
Adapted from:
Panche, A. N., Diwan, A. D., & Chandra, S. R. (2016). Flavonoids: an overview. Journal of nutritional science, 5, e47. https://doi.org/10.1017/jns.2016.41