What are flavonoids?

Flavonoids: Natural Compounds with Significant Health Benefits

Flavonoids are a diverse group of plant-derived polyphenolic compounds found abundantly in fruits, vegetables, whole grains, tea, and other plant foods that have potential health-promoting effects including reducing inflammation and providing preventive effects against various chronic diseases. 1

Classification and Structure

• Flavonoids comprise one of the largest groups of phytochemicals with over 5,000 different compounds identified 2
• They are classified into several subgroups based on their chemical structure 3:
  • Flavonols (e.g., quercetin, kaempferol)
  • Flavones (e.g., apigenin, luteolin)
  • Flavanones (e.g., naringenin, hesperidin)
  • Flavanols/catechins (e.g., epicatechin, EGCG)
  • Anthocyanidins (e.g., cyanidin, pelargonidin)
  • Isoflavones (e.g., genistein, daidzein) 2, 4

Dietary Sources

• Flavonoids are extensively found in:

  • Fruits (especially berries and citrus)
  • Vegetables (particularly dark green leafy and cruciferous varieties)
  • Whole grains
  • Tea (especially green tea)
  • Red wine
  • Nuts and seeds
  • Spices 1, 3

• The average adult consumes approximately several hundred milligrams of flavonoids daily, with specific intake of flavonols and flavones estimated at about 23 mg/day in some populations 5

Health Benefits

Cardiovascular Health

• High intakes of flavonoids are consistently associated with reduced risk of cardiovascular disease (CVD) and decreased incidence of coronary heart disease (CHD) and all-cause mortality 1
• Mechanisms include:
  • Potent antioxidant effects
  • Anti-inflammatory properties
  • Anti-atherogenic actions
  • Antithrombotic effects 6
• Some studies suggest that daily intake of 100 mg of flavonoids may reduce the risk of coronary heart disease morbidity and mortality by approximately 10% 6

Metabolic Health

• Among flavonoids, anthocyanins have shown significant impact on glucose metabolism and can improve total and low-density lipoprotein cholesterol levels, particularly in people with hyperlipidemia 1
• Green tea catechins, especially epigallocatechin-3-gallate (EGCG), have demonstrated anti-obesity properties and can help prevent intracellular advanced glycation end product (AGE) formation 1

Anti-inflammatory and Antioxidant Effects

• Flavonoids inhibit the biosynthesis of advanced glycation end products (AGEs) through:
  • Direct antioxidant properties
  • Metal-chelating ability
  • Protein interaction
  • Methylglyoxal trapping
  • Blocking receptors for AGEs 1
• These effects help reduce chronic inflammation associated with many diseases 1

Potential Cancer Prevention

• Carotenoids and certain flavonoids show dose-response relationships with reduced risk of certain cancers, including lung cancer 1
• However, epidemiological evidence for cancer prevention is less consistent than for cardiovascular benefits 5

Visual and Cognitive Health

• Specific flavonoids like lutein are strongly correlated with improved visual performance, reduced risk of age-related macular degeneration, and enhanced cognition 1

Bioavailability and Metabolism

• Contrary to earlier beliefs that flavonoids were poorly absorbed, research shows that specific flavonoid glycosides (bound to sugars) can be well absorbed 5
• For example, quercetin glycosides from onions show 52% absorption compared to 24% for the pure aglycone form 5
• Many flavonoids are metabolized by gut microbiota in the large intestine, producing bioactive compounds that benefit host health 1
• Flavonoids can also positively alter gut microbiota composition, promoting beneficial bacteria like Lactobacillus and Bifidobacterium 1

Clinical Recommendations

• The American Heart Association recommends obtaining flavonoids and other antioxidants from food sources rather than supplements 1
• Clinical trials of isolated antioxidant supplements have not confirmed benefits seen in observational studies of dietary intake, and some trials have documented potential harm 1
• A diet rich in plant-derived foods such as fruits, vegetables, whole grains, and vegetable oils is recommended to ensure optimal consumption of flavonoids 1
• Specific attention should be given to consuming:
  • Cruciferous vegetables
  • Dark-green leafy vegetables
  • Citrus fruits
  • Dark-colored berries 1

Safety Considerations

• Consumption of flavonoids through diet appears to be safe 3
• However, high-dose supplements may have unpredictable effects and potential interactions with medications 2
• Some studies have shown potential harm from isolated antioxidant supplements, including increased risk of lung cancer from beta-carotene supplements in smokers 1

Research Limitations

• Despite promising evidence, more clinical and epidemiological trials are needed to fully establish the role of specific flavonoids in disease prevention 3
• The evidence regarding flavonoid supplements is inconsistent, with some studies showing benefits, others showing no effect, and some suggesting potential harm 2
• Measuring total flavonoid intake remains challenging due to limited data on food content 5