All about antioxidants and polyphenols: how they work and in which foods you can find them
Articles
Antioxidants are substances that neutralise reactive oxygen molecules in the body, so-called free radicals. The broadest group of antioxidants in our diet is polyphenols, of which scientists have already described more than 8,000 types. We usually consume around 1 gram of them a day, mainly from coffee, tea, fruit or cocoa. How exactly do they work in the body, in which foods are they most abundant and when is it worth reaching for a quality dietary supplement?
What will you learn in this article?
- What are antioxidants and free radicals?
- What are polyphenols and how are they divided?
- Effects of antioxidants: what do they do in the body?
- Antioxidants and polyphenols in foods: the richest sources
- The strongest antioxidant does not exist: why the combination matters
- Antioxidants from diet, or from dietary supplements?
- Frequently asked questions about antioxidants and polyphenols
- Key takeaways
What are antioxidants and free radicals?
Antioxidants are substances that slow down or completely stop oxidation, that is, damage to cells by reactive oxygen. The body produces some of them itself, for example the enzymes superoxide dismutase or glutathione peroxidase [1]. We get another significant part from food: for example vitamins C and E, carotenoids and polyphenols.
Free radicals, on the other hand, are unstable molecules with an unpaired electron. They are formed naturally during the normal functioning of metabolism, mainly in the mitochondria during energy production. However, their excessive formation is driven by smoking, UV radiation, polluted air or high physical strain.
Oxidative stress: when the body runs out of balance
Free radicals are not just "villains". The body needs them to transmit cell signals and to defend against infections. The problem arises at the moment when more radicals are formed than the body is able to neutralise. We call this state oxidative stress.
Biochemist Helmut Sies emphasises in the journal Redox Biology (2015) that the main defence against oxidative stress is formed by enzymes that the body produces itself [1]. Diet and nutrients complement this system, they do not replace it.
What are polyphenols and how are they divided?
Polyphenols are plant compounds that serve plants as a natural defence against UV radiation, pests or moulds. According to a review by Pandey and Rizvi (2009), scientists have described more than 8,000 types of them [2]. Every day we take in approximately 1 gram of polyphenols from our diet, which is roughly ten times more compared with vitamin C [3].
Main groups of polyphenols
Flavonoids: They form the largest group of polyphenols. They include catechins (from green tea or cocoa), quercetin (from onions, apples) or anthocyanins (in blueberries and blackcurrants).
Phenolic acids: These include, for example, chlorogenic acid, which is the dominant polyphenol in coffee.
Stilbenes: The best-known representative is resveratrol from the skins of red grapes.
Lignans: They are abundantly represented mainly in flaxseed.
Resveratrol under the microscope
Resveratrol enjoys great popularity. However, it is important to distinguish between effects in a test tube and in a living body. A study by Walle and colleagues (2004) showed that the human body absorbs at least 70% of the resveratrol taken in, but only trace amounts appear in the blood in their original form [5]. This is because the liver and intestine quickly convert it into other metabolites.
In general, the most abundant polyphenols in food may not always be the ones the body can make the best use of [4].
Effects of antioxidants: what do they do in the body?
Antioxidants help maintain the balance between the formation and breakdown of reactive oxygen species. However, the action of individual substances differs and cannot be assessed across the board.
After an expert assessment by the European Food Safety Authority (EFSA), the European Union approved the claim "contributes to the protection of cells from oxidative stress" only for a specific group of substances [6]:
Vitamins: vitamin C, vitamin E and riboflavin (vitamin B2)
Minerals and trace elements: zinc, selenium, copper and manganese
These nutrients act either directly, or they are part of antioxidant enzymes that the body produces itself.
And how do polyphenols work? For the vast majority of them, research is still ongoing and mainly examines their role in cell signalling [2]. The EU has not approved a general health claim for polyphenols themselves. The exception is olive oil polyphenols: olive oil polyphenols contribute to the protection of blood lipids from oxidative stress [7]. The claim applies only to olive oil containing at least 5 mg of hydroxytyrosol and its derivatives per 20 g of oil. You achieve the beneficial effect with a daily intake of 20 g of oil from a quality source.
Antioxidants and polyphenols in foods: the richest sources
You will find antioxidants in foods mainly in plant sources. We find the highest concentrations of polyphenols in spices, dried herbs, cocoa, dark berries, nuts and olives. The data come from the expert database Phenol-Explorer, compiled by Pérez-Jiménez and colleagues (2010) [8].
Does coffee contain antioxidants?
Yes. For most people, coffee represents the very largest daily intake of antioxidants [9]. This does not automatically mean that coffee is the healthiest food in the world. We simply consume it regularly and in larger volumes. You can read more about caffeine and sleep in our article Coffee or tea? The effect of caffeine on sleep and circadian rhythm.
The strongest antioxidant does not exist: why the combination matters
Are you looking for the very strongest natural antioxidant? There is no universal winner. The label "strongest" most often comes from laboratory tests in test tubes, for example using the ORAC method. However, tests in a test tube often do not correspond to how a substance behaves in the living human body [10].
A substance with high values in the laboratory may not have the same effect in the body. Absorbability and the speed of conversion into other metabolites are decisive. In addition, each part of the cell needs a different type of antioxidant. Instead of searching for a single molecule, it therefore makes more sense to rely on their combination:
Water-soluble antioxidants (e.g. vitamin C) act in the blood plasma and in the aqueous environment inside cells.
Fat-soluble antioxidants (e.g. vitamin E or astaxanthin) act in the fatty cell membranes.
Why it is worth combining astaxanthin with full-spectrum vitamin E, we discuss in the article The real power of antioxidants: Why combine astaxanthin with vitamin E. We write about a lesser-known form of vitamin E in the article Do you know tocotrienols and their unique benefits?
Antioxidants from diet, or from dietary supplements?
The basis should always be a varied diet rich in plant foods. It provides hundreds of mutually complementary polyphenols, fibre and other nutrients that isolated high doses in tablets cannot replace.
The rule here is: more does not mean better. A large Cochrane review (78 studies, almost 297,000 people) did not show that antioxidant supplements preventively reduced mortality [11]. High doses of synthetic beta-carotene and synthetic vitamin E, on the contrary, slightly increased mortality [11]. The US institute NCCIH also warns that high doses of synthetically isolated all-trans-beta-carotene increase the risk of lung cancer in smokers [12].
When does a dietary supplement make sense?
It makes sense to consider dietary supplements in situations where a normal diet is not enough to cover the need for specific nutrients. It is advisable to choose reasonable doses and forms that the body knows from food:
Vitamin C: One of the best options is to get vitamin C directly from natural sources. We recommend, for example, freeze-dried organic acerola powder without added ascorbic acid. You can also learn more in the article Is it possible to get enough vitamin C from the diet? and Acerola: More than just vitamin C.
Vitamin E and astaxanthin: The combination of astaxanthin and full-spectrum vitamin E in the product ProLife contains 2 mg of astaxanthin from the microalga Haematococcus pluvialis, 19 mg of tocotrienols and 4.4 mg of vitamin E in one capsule. One of the effects of vitamin E is that it contributes to the protection of cells from oxidative stress [6].
Zinc and copper: Both minerals contribute to the protection of cells from oxidative stress [6]. It is important to supplement them in a suitable and balanced ratio. You can read more in another article Zinc and its functions.
Selenium: a trace element that contributes to the protection of cells from oxidative stress [6]. You will learn more about this mineral in the article Do you know why you need enough selenium?
Key takeaways
The body relies primarily on its own defence: the body produces the most important antioxidant enzymes itself [1]. Diet and nutrients complement this system, but do not replace it.
Free radicals are not only harmful: the body needs them for cell signalling and immune defence. The problem is only oxidative stress.
Polyphenols are a diverse group: scientists have described more than 8,000 types of them [2]. We take in approximately 1 gram of them a day, roughly ten times more than vitamin C [3].
The amount in food does not equal usability: the body does absorb resveratrol, but the liver and intestine quickly convert it into other metabolites [5].
Coffee is the main source of antioxidants in the diet of Czechs: it makes up 43% of intake in men and 55% in women [9]. However, spices and cocoa contain the most polyphenols per 100 g [8].
The combination is key: water-soluble antioxidants (vitamin C) act in the blood plasma and inside cells, fat-soluble ones (vitamin E, astaxanthin) in cell membranes.
More supplements does not mean better: high doses of beta-carotene or vitamin E may rather harm the body [11].
When supplementation makes sense: when the diet does not cover a specific nutrient. Choose reasonable doses and forms that the body knows from food, for example vitamin C from acerola, vitamin E with astaxanthin or zinc with copper.
Sources
[1] Sies H. Oxidative stress: a concept in redox biology and medicine. Redox Biology, 2015; 4: 180–183. https://pubmed.ncbi.nlm.nih.gov/25588755/
[2] Pandey KB, Rizvi SI. Plant polyphenols as dietary antioxidants in human health and disease. Oxidative Medicine and Cellular Longevity, 2009; 2(5): 270–278. https://pmc.ncbi.nlm.nih.gov/articles/PMC2835915/
[3] Scalbert A, Williamson G. Dietary intake and bioavailability of polyphenols. Journal of Nutrition, 2000; 130(8S): 2073S–2085S. https://pubmed.ncbi.nlm.nih.gov/10917926/
[4] Manach C, et al. Polyphenols: food sources and bioavailability. American Journal of Clinical Nutrition, 2004; 79(5): 727–747. https://pubmed.ncbi.nlm.nih.gov/15113710/
[5] Walle T, et al. High absorption but very low bioavailability of oral resveratrol in humans. Drug Metabolism and Disposition, 2004; 32(12): 1377–1382. https://pubmed.ncbi.nlm.nih.gov/15333514/
[6] Commission Regulation (EU) No 432/2012 establishing a list of permitted health claims. https://eur-lex.europa.eu/eli/reg/2012/432/oj
[7] EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on health claims related to polyphenols in olive. EFSA Journal, 2011; 9(4): 2033. https://www.efsa.europa.eu/en/efsajournal/pub/2033
[8] Pérez-Jiménez J, et al. Identification of the 100 richest dietary sources of polyphenols: Phenol-Explorer database. Eur J Clin Nutr, 2010. https://pubmed.ncbi.nlm.nih.gov/21045839/
[9] Joudalová K, Réblová Z. Dietary intake of antioxidants in the Czech Republic. Czech Journal of Food Sciences, 2012; 30(3): 268–275. https://cjfs.agriculturejournals.cz/artkey/cjf-201203-0010_dietary-intake-of-antioxidants-in-the-czech-republicdietary.php
[10] NutraIngredients. ORAC has 'ongoing value', says expert, as USDA removes online database. 2012. https://www.nutraingredients.com/Article/2012/06/13/ORAC-has-ongoing-value-says-expert-as-USDA-removes-online-database/
[11] Bjelakovic G, et al. Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database Syst Rev, CD007176.pub2. https://www.cochrane.org/evidence/CD007176_antioxidant-supplements-prevention-mortality-healthy-participants-and-patients-various-diseases
[12] National Center for Complementary and Integrative Health (NIH). Antioxidant Supplements: What You Need To Know. 2023. https://www.nccih.nih.gov/health/antioxidants-in-depth
