HPC › Nutrition › Glossary
Polyphenols are a structurally diverse class of plant-derived secondary metabolites defined by one or more aromatic rings bearing hydroxyl groups. Found in fruits, vegetables, tea, coffee, and wholegrains, the class encompasses over 8,000 known compounds organised into five primary subclasses: flavonoids, phenolic acids, stilbenes, lignans, and tannins. Their primary biological roles are antioxidant, anti-inflammatory, and gut-microbiome-modulating.
The term should not be used interchangeably with 'antioxidants': polyphenols is a structural designation, antioxidants a functional one.
How it works
At the molecular level, polyphenols donate hydrogen atoms from their hydroxyl groups to neutralise reactive oxygen species (ROS), interrupting the chain reactions that damage proteins, lipids, and DNA. They also chelate metal ions such as iron and copper that catalyse the Fenton reaction, a major source of the hydroxyl radical and one of the most destructive oxidants in biological tissue. 12
Bioavailability is the central challenge in translating polyphenol chemistry into human benefit. Roughly 90-95% of ingested polyphenols are not absorbed in the small intestine; they pass intact to the colon, where the gut microbiome transforms them into smaller phenolic metabolites. 1 Some of these metabolites cross the blood-brain barrier, which may partly explain the observed associations between polyphenol-rich diets and cognitive outcomes. The practical implication is that in vitro antioxidant scores, such as ORAC values, poorly predict in vivo effect; whole-diet patterns matter more than isolated compounds.
Plants synthesise polyphenols via the shikimate and acetate-malonate pathways as a defence against UV radiation, herbivores, and microbial pathogens. 2 Flavonoids, the largest subclass with over 6,000 individual compounds, include flavonols (quercetin, kaempferol), flavan-3-ols (catechins in green tea), and anthocyanins, the pigments responsible for the red and blue colours of berries. 1 This evolutionary origin means that a food's polyphenol content partly reflects the stress conditions under which it was grown.
Example
Consider an individual who regularly drinks two cups of green tea and adds a serving of mixed berries to their morning meal. They are consuming roughly 400-600 mg of mixed polyphenols, primarily catechins and anthocyanins. A portion of these reach the colon intact, where gut bacteria convert them into smaller phenolic acids that enter circulation and, in part, cross into brain tissue. Supplement equivalents of individual isolated compounds have not consistently replicated this systemic effect.
The food matrix and microbiome transformation are not incidental to polyphenol function; they are the mechanism.
Why it matters
A 2024 meta-analysis of prospective cohort data found that higher total polyphenol intake is associated with approximately 7% lower all-cause mortality risk. 4 A separate 2023 systematic review of 37 observational studies found that higher flavonoid intake is associated with significantly better cognitive function and lower odds of cognitive decline in adults. 3 Both findings are associational: dietary polyphenol intake tracks with overall diet quality, physical activity, and socioeconomic status, making precise causal attribution difficult.
The practical priority is dietary pattern rather than compound-level supplementation. Epidemiological and experimental evidence consistently associates polyphenol-rich diets with reduced cardiovascular risk and lower inflammatory markers, 24 but individual polyphenol supplements have not reliably replicated these effects in randomised trials. For performance-oriented individuals, the most defensible strategy is consistent consumption of a diverse range of colourful plant foods rather than pursuit of any single compound at high dose.
Which foods contain the most polyphenols?+
The richest dietary sources include dark berries (blueberries, blackberries), pomegranate, dark chocolate (70% cocoa or above), green and black tea, red wine, extra-virgin olive oil, and wholegrains. A 150 g serving of blueberries provides roughly 300-500 mg of mixed polyphenols, predominantly anthocyanins. {{cite:10.1093/ajcn/79.5.727}}
Do polyphenol supplements work as well as whole foods?+
The evidence consistently favours whole foods over isolated polyphenol supplements. The food matrix, fibre content, and accompanying micronutrients appear to influence how polyphenols are absorbed and metabolised. Most intervention trials using isolated compounds have produced smaller or less consistent effects than observational studies of high-polyphenol dietary patterns. {{cite:10.1093/ajcn/79.5.727}}{{cite:10.1080/1040869059096}}
Can polyphenols protect brain health?+
Higher flavonoid intake is associated with better cognitive function and lower rates of cognitive decline, based on a 2023 systematic review of 37 observational studies. {{cite:10.1002/mnfr.202300472}} Some polyphenol metabolites cross the blood-brain barrier, which may partly explain this relationship. The strongest evidence currently is at the dietary-pattern level rather than for any single compound.
What is the difference between polyphenols and antioxidants?+
Antioxidants is a functional category describing any molecule that neutralises oxidative stress; polyphenols is a structural category describing compounds with specific aromatic ring chemistry. All polyphenols have some antioxidant capacity, but not all antioxidants are polyphenols. Vitamins C and E are antioxidants but not polyphenols. {{cite:10.1080/1040869059096}}{{cite:10.1093/ajcn/79.5.727}}
