Polyphenols are a large and diverse group of chemical compounds found in plants, functioning as secondary metabolites involved in defense mechanisms. Their health-promoting properties, mainly as antioxidants, are well-established. The primary classification of what are polyphenols classified as is based on the number of phenol rings and the structural elements linking them. This structural approach divides them into several major classes.
Major Polyphenol Classes and Their Subgroups
The most common classification divides polyphenols into four main groups: flavonoids, phenolic acids, stilbenes, and lignans. Each class has further divisions based on chemical variations.
Flavonoids
Flavonoids are the most abundant class, providing color to many plants. They share a C15 skeleton with two aromatic rings linked by a three-carbon chain forming a heterocyclic ring. Subclasses include:
- Flavonols: Examples include Quercetin (onions, apples), Kaempferol (broccoli), and Myricetin (berries).
- Flavones: Examples include Apigenin (parsley) and Luteolin (celery).
- Flavanones: Examples include Hesperidin (oranges) and Naringenin (grapefruit).
- Flavanols: Found as monomers (Catechins in green tea) or polymers (Proanthocyanidins in grape seeds).
- Anthocyanins: Provide red, purple, and blue colors. Examples include Cyanidin (berries) and Malvidin (red wine).
- Isoflavones: Found primarily in legumes like soy. Examples include Genistein and Daidzein.
Phenolic Acids
This non-flavonoid class is divided into two subclasses based on their carbon framework:
- Hydroxybenzoic Acids: C6-C1 structure. Examples: Gallic acid (teas) and Protocatechuic acid (onions).
- Hydroxycinnamic Acids: C6-C3 structure. Examples: Caffeic acid (coffee), Ferulic acid (cereals), and p-Coumaric acid (grapes).
Stilbenes
A smaller non-flavonoid group with a C6-C2-C6 skeleton. They act as plant defense compounds.
- Example: Resveratrol, found in red grapes and wine.
Lignans
Non-flavonoid phytoestrogens with a C6-C3 skeleton.
- Example: Secoisolariciresinol, abundant in flaxseeds.
Comparison of Major Polyphenol Classes
| Class | Basic Chemical Structure | Key Features | Primary Dietary Sources | Example Compounds |
|---|---|---|---|---|
| Flavonoids | C6-C3-C6 | Largest group; provides plant color; multiple subclasses. | Fruits, vegetables, tea, cocoa, wine | Quercetin, Catechin, Naringenin, Cyanidin, Genistein |
| Phenolic Acids | C6-C1 or C6-C3 | Common in all plant material, often bound. | Coffee, berries, cereals, spices | Gallic acid, Caffeic acid, Ferulic acid |
| Stilbenes | C6-C2-C6 | Plant defense compounds. | Grapes, peanuts, berries, wine | Resveratrol |
| Lignans | Two C6-C3 units joined | Phytoestrogens; metabolized by gut microbiota. | Flaxseeds, sesame seeds, whole grains | Secoisolariciresinol, Matairesinol |
The Function and Importance of Polyphenol Classification
Classifying polyphenols is vital for understanding their specific biological roles and health effects.
Structure Dictates Function
Structural differences among classes affect bioavailability and interaction with the body. Features like hydroxyl groups influence antioxidant capacity. Larger polyphenols are metabolized by gut bacteria into bioactive compounds, while smaller ones are absorbed higher in the digestive tract.
Beyond the Basic Four
Other important groups include tannins (large polymers causing astringency) and compounds like curcuminoids and polyphenolic amides, sometimes included in broader classifications.
The Impact on Human Health Research
Classification helps scientists pinpoint which specific compounds are responsible for observed biological activities, guiding research into their health impacts. This understanding also affects food processing and storage. Further information on the chemistry and health impact of polyphenols can be found here.
Conclusion
Polyphenols are classified structurally into families—flavonoids, phenolic acids, stilbenes, and lignans—based on their phenolic rings and linkages. This system, with its further subclasses, covers thousands of plant compounds. The detailed classification is crucial for nutritional science, enabling precise understanding of their diverse health benefits, from antioxidant action to gut microbiota modulation. Consuming a varied diet rich in polyphenols ensures intake of these beneficial phytochemicals.
