Polyphenols: What They Are and the Main Classes
What polyphenols are and how scientists sort them into four main classes (phenolic acids, flavonoids, stilbenes and lignans), with examples and food sources.
Content last updated . This page has not been medically reviewed.
Quick answer
Polyphenols are plant compounds that plants make as secondary metabolites, often for defense against ultraviolet radiation or attack by germs. [1] More than 8,000 polyphenolic compounds have been identified in plants. [1] Scientists sort them into four main classes: phenolic acids, flavonoids, stilbenes and lignans. [1] This page explains the classes only. It makes no health claims; claims for specific compounds are checked on their own pages.
How the terms are used
- Polyphenol is a chemical family name, based on the structure of the molecule (the number of phenol rings and how they are joined). [1]
- Antioxidant describes a test-tube property. It is not a synonym for polyphenol and does not promise a health effect. The Linus Pauling Institute says the contribution of dietary flavonoids to antioxidant function in the body is likely to be very small or negligible. [2] For the wider idea see Antioxidants Explained.
The four main classes
| Class | What it is | Examples from the sources | Where it is found |
|---|---|---|---|
| Phenolic acids | Divided into benzoic acid and cinnamic acid derivatives. The cinnamic acid type is more common. [1] | p-coumaric, caffeic, ferulic and sinapic acids. [1] | Widespread in plant foods; one review says hydroxybenzoic acids are generally low in edible plants, with exceptions such as certain red fruits, black radish and onions. [1] |
| Flavonoids | The most studied group. [1] Counts differ by source: more than 4,000 varieties [1] or over 5,000 compounds. [2] | Quercetin, myricetin and catechins. [1] Six main subclasses; see flavonoids explained. [2] | Fruit, vegetables, tea, wine, cocoa and soy foods. [2][1] |
| Stilbenes | Two phenyl groups joined by a two-carbon bridge; low in the human diet. [1] | Resveratrol, found largely in grapes. [1] | Grapes and red wine. [1] |
| Lignans | Compounds made from two cinnamic acid units; several are considered phytoestrogens. [1] | Secoisolariciresinol. [1] | The richest dietary source is linseed (flaxseed). [1] |
Why the same food can vary
Foods contain complex mixtures of polyphenols rather than a single compound. [1] The same review notes that ripeness at harvest, environment, processing and storage affect how much a plant contains. [1] For that reason, a single "polyphenol content" number on a label or website should be read with care; see reading supplement labels.What research has and has not shown
Population studies (epidemiology) have looked at flavonoids and health, but a 2002 review called them inconclusive: some supported protection from heart disease and cancer, some showed no effect and a few suggested possible harm. [3] NCCIH adds that antioxidants taken as purified chemicals might act differently from those in foods. [4] So findings from fruit- and vegetable-rich diets cannot be assumed to apply to concentrated supplements. Evidence for specific compounds is on the pages for quercetin, resveratrol and the overview of polyphenol supplements.Official intake levels
No recommended intake, adequate intake or upper limit for polyphenols as a group is described in the sources we read. They are not classed as essential nutrients in those sources.Frequently asked questions
What are polyphenols?
Plant compounds with a shared chemical structure, grouped into phenolic acids, flavonoids, stilbenes and lignans. [1]
Are all polyphenols antioxidants?
Many show antioxidant activity in lab tests, but the Linus Pauling Institute says their antioxidant role in the body is likely very small. [2]
Is resveratrol a flavonoid?
No. It is a stilbene. [1]
Is quercetin a polyphenol?
Yes. It is a flavonoid, a class of polyphenol. [1]
Are polyphenol supplements the same as eating polyphenol-rich food?
Not necessarily. Foods contain complex mixtures, and purified compounds might act differently. [1][4]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Safety
Naming a compound a polyphenol says nothing about safety. The Linus Pauling Institute reports that no adverse effects have been associated with high dietary intakes of flavonoids from plant-based foods, but concentrated supplements are a different matter, and the same source describes possible interactions of supplemental flavonoids with medicines. [2] See medication interactions. Pregnancy, children, kidney and liver conditions and planned surgery are covered on the pages for each supplement; ask a clinician or pharmacist before using any concentrated extract.
References
- Pandey KB, Rizvi SI. Plant polyphenols as dietary antioxidants in human health and disease. Oxid Med Cell Longev 2009. Read 2026-10-23.
- Linus Pauling Institute, Oregon State University. Flavonoids (reviewed 2016). Read 2026-10-23.
- Ross JA, Kasum CM. Dietary flavonoids: bioavailability, metabolic effects, and safety. Annu Rev Nutr 2002. Read 2026-10-23.
- NCCIH. Antioxidants: In Depth (updated July 2023). Read 2026-10-23.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.