What are Polyphenols?
Polyphenols are a large family of naturally occurring plant compounds, with around 8000 different types (1). They are found in a variety of plant foods including many fruits, vegetables, tea, and cocoa. Although they are utilised by the plant as protection against UV radiation from the sun and to prevent the invasion of harmful microorganisms (1), they have been widely studied due to their potential health benefits, particularly due to their anti-inflammatory and antioxidant properties. As a result, they play an important role in maintaining heart and brain health; and there is even evidence to suggest that if consumed regularly over a long period of time, they may reduce the risk of developing certain types of cancer (2).
Although they are present in lots of different plant foods, there are certain sources that are notably high in specific types.
• Cocoa, tea, and coffee are high in flavanols, one of the most extensively studied groups of polyphenols (1).
• Berries get their vibrant colour due to the presence of anthocyanins (3).
• The curcumin in turmeric is responsible for its anti-inflammatory properties.
• Red wine is known for high levels of resveratrol.
However, other top sources include plums, black grapes, apples, hazelnuts, almonds, red onion, spinach, turmeric, and extra virgin olive oil (3).
Each plant food contains a complex mixture of polyphenols, which all work together to confer their health benefits. This means that taking a supplement that contains high levels of a single polyphenol may not be as beneficial for good overall health, as variety is key (3).
Polyphenols and Gut Health
It has become increasingly evident that to have good overall health, we need to look after our gut, and nourish the trillions of microorganisms that perform essential roles in our bodies. This is particularly true when we consider polyphenols, as around 90% of the polyphenol content of plant foods remain undigested until they reach the large intestine, where they are digested by the resident microorganisms (2). As a result, the benefits gained from polyphenols is largely based on a person’s specific gut microbiota.
In fact, there is a two-way relationship between the gut microbiota and polyphenols. Not only do the bacteria break down the polyphenols into chemicals that can be absorbed and utilised by the body, but the polyphenols also act as a prebiotic (4). This means that the polyphenols act as a food source for the bacteria, which allows the number of beneficial bacteria to thrive. For instance, experiments have shown that flavanol rich foods, such as cocoa, green tea and blackcurrants, can increase the number of Lactobacillus, a type of beneficial bacteria, whilst simultaneously decreasing the number of potentially harmful Clostridium species. Also, it has been noted that the presence of polyphenols may help to enhance the effectiveness of certain probiotics (5). As a result, in order to achieve the maximum benefits from polyphenols, cultivating a diverse gut microbiota should also be considered. This can be achieved by consuming a wide variety of plant-based foods that are high in fibre, such as fruits, vegetables, wholegrains, beans and pulses, nuts, and seeds. It is recommended that 30 different plant sources should be included in the diet every week (6).
Polyphenols and Heart Health
Cardiovascular disease is globally one of the leading causes of chronic illness and death, yet regularly consuming foods high in polyphenols over a long period of time seems to have protective effects on the cardiovascular system (1). This is largely due to their anti-inflammatory properties, as they decrease the release of inflammatory chemicals known as cytokines, which play a key role in the development of cardiovascular disease (4). In particular, the polyphenol resveratrol, usually associated with red wine, may be important for long term heart health. This is because resveratrol has been shown to decrease blood pressure by relaxing the walls of blood vessels (7). In fact, the presence of resveratrol has been proposed as a factor in the “French Paradox”. This is the observation of a relatively low occurrence of cardiovascular disease amongst French people, despite a diet high in saturated fat. It is thought that this may be partly due to the moderate consumption of resveratrol rich red wine (1).
Polyphenols and Brain Health
There is evidence to suggest that polyphenols may be beneficial in protecting against both mental health conditions, such as depression, and neurodegenerative diseases, like Alzheimer’s Disease. Again, this can be attributed to their anti-inflammatory properties, as inflammation is one of the factors that can influence the development of these conditions (8). Additionally, polyphenols can act as antioxidants, meaning that they react with harmful free radicals to prevent them from damaging neurons, which are the cells in the brain. For instance, the prevalence of the polyphenol curcumin in turmeric helps to protect against neurodegeneration. As a result, there are lower levels of Alzheimer’s Disease in India compared to other countries, where turmeric is a staple ingredient in their diet (8).
Due to their various modes of action, polyphenols are an emerging therapeutic option (9). Due to their diversity in plants, they are easily accessible to all in many foods, particularly berries, apples, cocoa, green and black tea, and coffee. Even moderate amounts of red wine may be beneficial. Because each plant contains a unique combination of polyphenols that contribute to their positive health impacts, they should be consumed as the whole food rather than a supplement of a single polyphenol, to ensure the maximum benefit. Also, it is worth noting that the benefits gained from polyphenols are only dependent on the size and diversity of a person’s community of microbes in the gut, as they are responsible for digesting the polyphenols. As a result, nourishing the gut microbiome should be a priority by consuming a diet rich in a variety of plant-based foods. The key here is that no nutrient in isolation is revolutionary for health, it is all about variety and balance.
This post was written by Charlotte Green who is a second-year undergraduate at Durham University studying Biological Sciences. She is fascinated by the gut microbiome and the role of nutrition in health, so hopes to pursue a Master's degree in Dietetics when she graduates. Charlotte is also part of The Ugly Fruit Group, a Durham based project aiming to tackle food waste by donating to food banks and creating healthy snacks from surplus fruit and veg. You can also find her on Instagram @charlottevictoria23.
(1) Pandey, K.B. and Rizvi, S.I. (2009), ‘Plant polyphenols as dietary antioxidants in human health, Oxidative Medicine and Cellular Longevity, 2, 5, 270-278
(2) Rossi, M. (2020), Powerful polyphenols, The Gut Health Doctor, viewed 14/01/21, https://www.theguthealthdoctor.com/powerful-polyphenols/
(3) Rossi, M. (2019), Eat yourself healthy, Penguin Life
(4) Cardona, F., Andres-Lacueva, C., Tulipani, S., Tinahones, F.J (2013), ‘Benefits of polyphenols on gut microbiota and implications in human health, The Journal of Nutritional Biochemistry, 24, 8, 1415-1422
(5) Cory, H., Passarelli, S., Szeto, J., Tamez, M., Mattei, J. (2018), ‘The role of polyphenols in human health and food systems: a mini-review, Frontiers in Nutrition, 5, 87
(6) Cheng, Y.C., Sheen, J.M., Hu, W.L., Hung, Y.C. (2017), ‘Polyphenols and oxidative stress in atherosclerosis-related ischemic heart disease and stroke,’ Oxidative Medicine and Cellular Longevity
(7) Food for Thought (2018), Findings from the American Gut Project, Food for Thought UK, viewed 21/01/21, http://www.foodforthoughtuk.com/findings-from-the-american-gut-project/
(8) Gomez-Pinilla, F. and Nguyen, T.T.J. (2012), ‘Natural mood foods: the action of polyphenols against psychiatric and cognitive disorders, Nutritional Neuroscience, 15, 3, 127-133
(9) Westfall, S., Pasinetti, G.M. (2019), ‘The gut microbiota links dietary polyphenols with management of psychiatric mood disorders,’ Frontiers in Neuroscience
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