Polyphenols

1. What This Class of Substances Is

Polyphenols are secondary plant compounds containing at least one aromatic ring and multiple hydroxyl groups. They include flavonoids (e.g., anthocyanins, quercetin), phenolic acids, stilbenes (e.g., resveratrol), and lignans. They are found in foods such as berries, green tea, turmeric, olives, cocoa, grape seeds, and pine bark. Humic acid complexes are also included, as they derive from plant degradation products and exhibit polyphenol-like structures. Chemically, they act as radical scavengers, chelating agents, and modulators of signalling pathways (e.g., NF-κB, Nrf2).

The effects in humans are difficult to standardise, as polyphenols exhibit low bioavailability and are metabolised differently between individuals. Gut bacteria convert them into active metabolites, whose concentrations in the blood often remain low. Additionally, dosage, matrix (e.g., extract vs. food), and study design vary. Controlled studies are challenging, as placebo controls are difficult to implement with foods. Humic acid complexes are particularly hard to investigate, as their composition varies significantly depending on their origin.

2. Which Materials Are Included

All substances in the database for which this mechanism of action is described—ordered by the extent of human testing. The numbers represent studies per type of investigation.

3. How Robust This Evidence Is

Particularly well-documented are turmeric/curcumin, resveratrol, anthocyanins, green tea/EGCG, and cocoa flavanols, each with over 70 controlled human studies. Olive polyphenols, grape seed extract, and quercetin follow with moderate evidence. Substances such as humic acid complexes, sulforaphane, or pine bark extract show almost exclusively laboratory and animal findings, while controlled human studies are lacking or scarce. The evidence base for effects on the gut microbiome is also limited, although polyphenols are likely to play a central role here.

These substances share antioxidant and anti-inflammatory mechanisms, which often appear similar in vitro and in animal models. While extrapolation to humans is plausible, it is not automatically valid. For example, anthocyanins and cocoa flavanols demonstrate consistent effects on vascular function, whereas resveratrol, despite promising laboratory findings, shows inconsistent results in human studies. Humic acid complexes may exhibit similar mechanisms of action, but comparable human studies are lacking. Interaction with the gut microbiome is a common denominator, yet this remains insufficiently researched.

4. Key Human Studies

• 1 The Essential Medicinal Chemistry of Curcumin (2017) — Turmeric and curcumin
• 2 Antioxidant and anti-inflammatory effects of curcuminoid-piperine combination in subjects with metabolic syndrome: A randomized controlled trial and an (2015) — Turmeric and curcumin
• 3 Comparative absorption of curcumin formulations (2014) — Turmeric and curcumin
• 4 A randomized, double-blind, placebo-controlled trial of resveratrol for Alzheimer disease (2015) — Resveratrol
• 5 High-dose resveratrol supplementation in obese men: an investigator-initiated, randomized, placebo-controlled clinical trial of substrate metabolism, (2013) — Resveratrol
• 6 Resveratrol supplementation improves glycaemic control in type 2 diabetes mellitus (2012) — Resveratrol
• 7 Blueberries reduce cardiovascular risk factors in obese men and women with metabolic syndrome (2010) — Anthocyanins (berries)
• 8 Chronic blueberry supplementation enhances task-related brain activation and resting perfusion in healthy older adults (2017) — Anthocyanins (berries)
• 9 Effects of anthocyanins on cardiometabolic health: a systematic review and meta-analysis of randomised controlled trials (2017) — Anthocyanins (berries)
• 10 Antiobesity effects of green tea catechins: a mechanistic review (2011) — Green tea and EGCG
• 11 Potential role of green tea catechins in various disease therapies: progress and promise (2012) — Green tea and EGCG
• 12 Randomised, placebo-controlled trial of green tea catechins for prostate cancer prevention (2015) — Green tea and EGCG
• 13 Blood pressure is reduced and insulin sensitivity increased in glucose-intolerant, hypertensive subjects after 15 days of consuming high-polyphenol da (2008) — Cocoa flavanols
• 14 Effects of low habitual cocoa intake on blood pressure and bioactive nitric oxide: a randomised controlled trial (2007) — Cocoa flavanols
• 15 Flavonoid-rich dark chocolate improves endothelial function and increases plasma epicatechin concentrations in healthy adults (2004) — Cocoa flavanols
• 16 Olive (Olea europaea L.) leaf polyphenols improve insulin sensitivity in middle-aged overweight men: a randomised, placebo-controlled, crossover trial (2013) — Olive polyphenols (hydroxytyrosol)
• 17 Human absorption and metabolism of oleuropein and hydroxytyrosol ingested as olive (Olea europaea L.) leaf extract (2013) — Olive polyphenols (hydroxytyrosol)
• 18 Oleuropein, a bioactive compound from Olea europaea L., as a potential preventive and therapeutic agent in non-communicable diseases (2019) — Olive polyphenols (hydroxytyrosol)
• 19 Effects of grape seed extract in type 2 diabetic subjects at high cardiovascular risk: a double-blind randomised placebo-controlled trial examining me (2009) — Grape seed extract (OPC)
• 20 Effect of grape seed extract on blood pressure in subjects with metabolic syndrome (2009) — Grape seed extract (OPC)
• 21 Effects of grape seed proanthocyanidin extract on menopausal symptoms, body composition, and cardiovascular parameters in middle-aged women: a randomi (2014) — Grape seed extract (OPC)
• 22 Effect of antioxidants on the gut microbiome profile and brain function: a review of randomised controlled trial studies (2025) — Gut microbiome
• 23 Gut microbiota-targeted dietary supplementation with fermentable fibres and polyphenols prevents hypobaric hypoxia-induced increases in intestinal per (2025) — Gut microbiome
• 24 Bioconversion-based postbiotics enhance muscle strength and modulate gut microbiota in healthy individuals: a randomised, double-blind, placebo-controlled (2025) — Gut microbiome

5. Open Questions

• Bioavailability: How can polyphenols (e.g., humic acid complexes) be formulated to achieve reproducible effects in humans?
• Dose-response: Are there threshold levels for individual polyphenols beyond which effects become measurable—or is the matrix decisive?
• Long-term effects: Which polyphenol-rich materials demonstrate sustained effects in chronic inflammation or metabolic diseases?
• Microbiome interaction: How do polyphenols influence the gut flora—and vice versa—in controlled human studies?
• Combination effects: Do polyphenols act synergistically with other substances (e.g., omega-3 fatty acids) or do they counteract each other?
• Standardisation: How can humic acid complexes and other poorly definable polyphenols be characterised in a comparable manner for studies?

See also the gap list for all substances. This page is recalculated with each harvest.