What are Polyphenols?

Overview

Have you ever wondered why it’s recommended to eat at least 5 servings of fruits and vegetables every day? Maybe you’ve heard that eating plant-based foods can help protect you against diseases. Well, polyphenols are partially responsible for this. 

Polyphenols are a type of chemical compound plants make to protect themselves against damage, such as ultraviolet (UV) radiation or dehydration.¹ They are often responsible for giving fruits and vegetables their distinct colour and taste.² 

As well as protecting plants, research has shown that when consumed, polyphenols also have a protective effect in humans, guarding against chronic diseases including cardiovascular disease, neurodegenerative diseases, and even cancer.³ 

This article gives an overview of the structure, types, and sources of polyphenols, along with their beneficial effects on our health. You’ll be surprised by how many polyphenols you’re already consuming without even knowing!

Chemical structure of polyphenols

Polyphenols all have a similar chemical structure that distinguishes them from other plant compounds. Specifically, they all have one or more aromatic rings and a minimum of one hydroxyl group attached.³ The image below shows a polyphenol called quercetin, which contains three aromatic rings and five hydroxyl (OH) groups.

If they just contain one aromatic ring, they can be considered phenolic acids, a subcategory of polyphenols. If they contain several of these ring structures, as the name implies (poly), they are considered polyphenols.⁴  This chemical structure is believed to be behind the antioxidant and anti-inflammatory properties of polyphenols - read on to find out how. 

Types of polyphenols

Polyphenols are organised into subcategories based on their chemical structure. The two main subcategories are flavonoids and non-flavonoids. Unlike flavonoids, non-flavonoids contain one aromatic ring, making them a type of phenolic acid. Over 4000 different polyphenols have been identified, and flavonoids are the most common type, accounting for approximately 60%.⁵ 

You can see the subgroups of flavonoids and non-flavonoids listed below, along with some dietary sources.

Flavonoids

Different types of flavonoids include:

• Flavanols
• Flavones
• Flavanones
• Anthocyanins
• Isoflavonoids

The table below lists some of the common flavonoids and where you might find them in your diet.⁵

Non flavonoids

Different types of non-flavonoids include:

• Stilbenes
• Lignans
• Hydroxybenzoic acid
• Hydroxycinnamic acid

The table below lists some examples of each of these non-flavonoids and their dietary sources.⁵

Health benefits of polyphenols

Antioxidant properties

The term ‘antioxidant’ is used to describe any substance that prevents damage caused by reactive oxygen species (ROS). ROS are a type of free radical containing an oxygen molecule that can damage other molecules in your cells. When there is an overload of ROS, it can lead to oxidative stress, and these free radicals can cause cell death and tissue damage.⁶

Polyphenols have been shown to have antioxidant properties, meaning they help prevent oxidative stress. Their chemical structure allows them to neutralise these free radicals and stop them causing damage.⁷ Polyphenols have also been shown to activate antioxidant enzymes in the body, which helps to amplify this effect.⁷

Oxidative stress is present in many chronic diseases such as cancer, diabetes, cardiovascular disease, and even neurodegenerative diseases. Many of the health benefits of consuming polyphenols may be attributed to their antioxidant properties.³

Anti inflammatory effects

Inflammation is a normal immune response to injury or infection. This is usually a short-term response. However, if something is off-balance and the inflammation is prolonged beyond what is necessary, it can cause damage to your body’s cells and tissues. This ‘chronic inflammation’ is linked with a number of chronic diseases including cancer, obesity, diabetes, arthritis, and neurodegenerative diseases.²

Polyphenols have been found to have an anti-inflammatory effect on the body. Studies have shown that consuming polyphenols is linked to a reduction in pro-inflammatory signalling molecules, the molecules that stimulate inflammation.² Some polyphenols even resemble anti-inflammatory drugs like ibuprofen in their chemical structure.²

There is also a lot of evidence that oxidative stress can trigger inflammation.⁸ This means the antioxidant activity of polyphenols can also contribute to their anti-inflammatory effect on the body.  

Cardiovascular health

Polyphenols have been associated with a lower incidence of cardiovascular disease in several different epidemiological studies.⁹ For example, a review paper covered a number of studies where a light-to-moderate intake of red wine was linked to a lower risk of developing coronary heart disease.¹⁰ Red wine is a rich source of flavonoids, including epicatechin and quercetin, and non-flavonoids such as resveratrol.

It is generally thought that polyphenols help prevent the main cause of heart disease, atherosclerosis (where fat and cholesterol build-up causes arteries to narrow, putting strain on the heart). This may be from their observed anti-hypertensive effect, i.e. they help to lower blood pressure.⁹ Due to their antioxidant effects, polyphenols may also help prevent the oxidation of low-density lipoproteins (aka the ‘bad’ cholesterol) which play a significant role in atherosclerosis.⁹

Cancer prevention

A significant amount of research has suggested that polyphenols have an anti-cancer effect,² with some researchers even suggesting that they should be considered as a potential drug therapy.⁵ It is thought that polyphenols may be able to interfere with several stages of cancer development.³ For example, both the antioxidant and anti-inflammatory effects of polyphenols are thought to help reduce the development of cancers.⁵ 

Many epidemiological studies show that consuming a Mediterranean diet, which is rich in polyphenol-containing foods, especially olive oil, is associated with a lower prevalence of cancers.³ These anti-cancer effects have been observed for prostate, gastrointestinal, lung, breast, ovarian and even bladder cancers.²  There is also some evidence that olive oil-based polyphenols may reduce cancer cell division (and thereby reduce the growth of tumours) and promote the death of cancer cells.³ 

In a more indirect way, polyphenols from several fruits and vegetables may prevent cancer cells forming by reducing oxidative damage to our cells’ DNA.³ Polyphenols may also interfere with the process of metastasis,^2,8 where cancer cells spread to other parts of the body and form new ‘secondary’ tumours. 

Neuroprotection

Polyphenols have a neuroprotective effect, which means they help to maintain normal cognitive function and prevent neurodegeneration (progressive nerve damage) and the onset of associated diseases such as dementia.  

Epidemiological studies have shown that daily consumption of polyphenol-rich foods is associated with a 50% reduced risk of developing dementia and may help to postpone the ageing process and the onset of Alzheimer’s disease.⁸ Other studies have suggested that polyphenols can reduce the prevalence of Parkinson’s disease.²

The neuroprotective effects of polyphenols occur in a number of ways. As with cancer and heart disease, their antioxidant and anti-inflammatory effects contribute to maintaining a healthy brain. Both oxidative stress and chronic inflammation in the brain are associated with nerve damage and impaired signalling, which leads to memory loss and impaired cognition.⁸ 

It has also been suggested that polyphenols can specifically prevent the build-up of proteins in the brain that accelerate the development of diseases like Alzheimer’s.² A study has even shown that resveratrol can increase immune system activity in the brain, helping prevent the onset of neurodegenerative diseases.¹¹

Other potential health benefits

The beneficial health effects of polyphenols are not limited to these conditions, and it is likely that there are many more benefits we have yet to find. Some of the other known effects are listed below:

• Antibacterial effect¹
• Protection against UV radiation¹
• Protection against obesity¹
• Protection against diabetes^{1, 10}
• Protection against COVID-19¹
• Promotes the growth of good bacteria in the gut microbiome¹²
• Protection against bone loss due to osteoporosis¹³

Food sources of polyphenols

Polyphenols can be found in a wide range of plant-based foods including fruits, vegetables, drinks, herbs and grains. Specific examples of these foods are listed below.^{2, 12}  

Fruits

• Berries (blackberries, blueberries, raspberries, strawberries, etc)
• Grapes
• Apples
• Oranges
• Lemons
• Grapefruit
• Plums
• Cherries
• Pomegranate
• Pears

 Vegetables

• Carrots
• Broccoli
• Eggplant
• Spinach
• Onions (yellow and red varieties)

 Drinks

• Black and green tea
• Cocoa
• Coffee
• Red wine

Herbs and spices

• Cinnamon
• Cumin
• Caraway
• Thyme
• Sage
• Parsley
• Basil
• Oregano

Grains

• Whole wheat
• Oats
• Rye

Nuts and seeds

• Walnuts
• Sesame seeds
• Flax seeds
• Peanuts
• Chestnuts
• Almonds

Legumes

• Soybeans and soy-based products
• Black beans
• White beans (like butter beans)

Other

• Extra virgin olive oil (refined olive oil contains lower amounts of polyphenols)³

Bioavailability and absorption of polyphenols

Bioavailability means the proportion of a nutrient that is absorbed into the bloodstream, and can therefore have an effect. Every polyphenol has a different bioavailability, due to its distinct chemical structure and whether it is modified by the gut bacteria during digestion.⁹ 

In general, polyphenols are thought to have a relatively low bioavailability.¹² Some, like flavanones, are considered more bioavailable than others, including flavonols and anthocyanins.¹² This means you’ll effectively get more polyphenols from citrus fruits than berries.

Polyphenols also differ in terms of where they are absorbed in the digestive system.² For example, quercetin (a type of flavonol) is well-absorbed in the small intestine, while most are better absorbed in the large intestine.¹⁴ Importantly, the amount of polyphenols you eat determines how much is absorbed, but not their bioavailability.⁹  

Potential risks and side effects of polyphenols

There are few risks and side effects reported from consuming whole-food sources of polyphenols. Your main concern should be determining the optimal intake of polyphenols you need to see health benefits.¹⁴ Regardless, the current dietary guidelines (consuming 5 servings of fruits and vegetables each day and following a Mediterranean-style diet) are supported by current research on the beneficial effects of polyphenols. 

You can also purchase polyphenol supplements. However, as these may have an altered bioavailability and contain significantly larger doses, it is recommended to consume whole-food versions.¹⁴

Summary

Polyphenols are a class of compounds produced in response to damage in plants. It follows that they can have a hugely beneficial effect on our bodies when we eat them. This is driven by their antioxidant, anti-inflammatory, and anti-cancer effects as well as the protection they offer against cardiovascular disease, neurodegenerative diseases, diabetes, and even COVID-19. 

You likely already consume polyphenols as part of your diet, as they are found in olive oil, red wine, cocoa, fruits, vegetables, grains, herbs, and spices, but with the extensive health benefits they provide, you might want to try adding even more sources of polyphenols into your diet!