What Are Polyphenols? Top Sources and Benefits

Polyphenols are compounds naturally occurring in plants and found in many plant-based foods. Daily consumption of polyphenols has been proposed for various health benefits, such as protecting the cells from damage, promoting heart and brain health, and reducing inflammation in the body.

This article will review different groups of polyphenols, their functions, and potential health benefits for your body.

What are polyphenols?

Polyphenols are naturally occurring chemical components produced by plants. The word 'polyphenol' means multiple chemical ring structures with one or more hydroxyl groups joined together.

Polyphenols are synthesized to protect plants against internal free radicals (e.g., sun rays needed to produce sugars and oxygen can lead to damaging free radicals) or pathogens. They are also very important in the human diet. Nowadays, more and more scientific evidence confirms that polyphenols have antioxidant and anti-inflammatory properties and may even play a role in protecting humans from metabolic disorders and other chronic conditions.

Types of polyphenols

There are at least 10,000 different polyphenols in nature, and they are classified into several groups according to their chemical structure.

Phenolic acids

As encoded in the name, these compounds have a phenolic ring with a carboxylic acid group attached. Phenolic acids can be categorized into two main groups: one with lower content in foods and another more commonly found in fruits and vegetables, cereals, and beverages. The most popular examples in the latter group are caffeic, ferulic, and gallic acids.

• Caffeic acid is majorly found in coffee beans but can also appear in olives, berries, potatoes, carrots, and other vegetables and herbs.
• Ferulic acid can be found in vegetables (red beet, pepper, radish, etc.), fruits (blueberry, grapes), and especially in cereal bran. Studies show that regular consumption of whole grains (wheat, maize, barley, rice, oats, etc.) is associated with a lower risk of certain chronic diseases, likely due to the intake of phenolic acids.
• Gallic acid is found in grapes, tea leaves, and pomegranates. Current studies show the potential of gallic acid to be a new agent for the management of gastrointestinal diseases through modulation of the gut microbiome.

Flavonoids

Flavonoids are the most common polyphenols, which can be divided into seven groups. Found in over 4,000 plant species, they are responsible for giving plants their vibrant colors, including scarlet, blue, and orange in their leaves, flowers, and fruits.

Flavonoids are abundant in colored vegetables and fruit such as berries, apples, blueberries, grapes, oranges, plums, and strawberries. They're also present in some beverages and foods, including dark chocolate, nuts, tea, red wine, and soy. The most common flavonoid in the human diet is quercetin. Many foods, such as red onions and cranberries, contain a high amount of quercetin.

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Another significant example is kaempferol, commonly found in tea, beans, kale, broccoli, and spinach. Both kaempferol and quercetin are objects in many scientific research studies evaluating their potential anticancer properties.

Stilbenes

Stilbenes are a group of non-flavonoid polyphenols, and the most popular example of this group is resveratrol found in grapes, peanuts, berries, and red wine. Studies of resveratrol have shown a variety of health-promoting effects, such as anti-inflammatory, antioxidant, hepatoprotective, and neuroprotective activities.

Moreover, resveratrol has been proposed to play a role in helping prevent cancer. It is believed that the compound may affect how cells grow and develop, stop various harmful processes in the cells, and block the formation of blood vessels needed for tumor growth. However, these hypotheses are still under investigation and more large-scale human studies are necessary.

Lignans

Like stilbenes, lignans are non-flavonoid polyphenols found in oilseeds (sesame, flaxseed, linseed, and sunflower), whole grains (wheat, barley, and oats), nuts, legumes (soybeans), fruits, and vegetables (broccoli and kale). Sesamin and sesamolin are the main lignans found in the scientific literature, and the primary source of these lignans is sesame seeds. They have been proposed to hold various pharmacological effects, such as anti-inflammatory, antioxidant, and anti-carcinogenic. However, more research on their therapeutic potential is necessary.

Are there any health benefits of polyphenols?

Polyphenols have come to scientists' attention only recently. Nevertheless, many research studies have confirmed the role of a polyphenol-rich diet in reducing the risk of several human illnesses.

Gut microbiota regulation

Gut microbiota plays an essential role in our health. Polyphenols and their biologically active metabolites may affect the composition of the gut microflora by increasing the proliferation of healthy microbes while inhibiting the growth of harmful bacteria.

In addition, these compounds are believed to serve as prebiotics, improve gut barrier function, and in certain cases help reduce gut inflammation. For example, flavonoid isoorientin is proposed to prevent the development of intestinal inflammation caused by bacteria like Oscillibacter, Helicobacter, and Alistipes and thus may improve digestion and nutrient absorption.

Blood sugar control

Diabetes is a chronic disease characterized by elevated levels of blood sugar and is one of the world’s biggest public health problems. Certain polyphenols have been suggested to have anti-diabetic properties and are proposed to act by improving insulin sensitivity, inhibiting glucose absorption, reducing inflammation, or protecting cells from oxidative stress.

For example, studies have shown that resveratrol may play a role in diabetes by helping decrease insulin resistance and regulate fasting blood sugar. Quercetin is also believed to have anti-diabetic properties, and, according to studies, may shield individuals with diabetes from oxidative stress damage.

May help protect the skin

One of the most popular theories of explaining aging mechanisms is the hypothesis of free radical/oxidative stress damage. Polyphenols can potentially protect skin from damage caused by environmental factors, like ultraviolet radiation (UV) from the sun or other stressors like pollution, because of their antioxidant and anti-inflammatory properties. Additionally, polyphenols may stimulate collagen and elastin synthesis in the skin, which can help maintain skin elasticity, hydration, and firmness.

Lowers the risk of certain cancers

Polyphenols might help promote health by affecting the growth of cancer cells, which has been seen in experiments with cancer cells and lab animals. A lot of polyphenols, like quercetin, lignans, curcumin, different flavonoids, and resveratrol, have been tested and reported to have helpful properties against cancer.

For instance, resveratrol has been shown to be effective in inhibiting or preventing the growth and spread of cancer cells in lung, skin, breast, colon, prostate, and liver cancer cell studies. Clinical trials with pomegranate extract showed that pomegranate juice or extract is safe but did not significantly improve final outcomes in prostate cancer treatment. Nonetheless, in some prostate cancer cases, patients respond positively. Moreover, a polyphenol-rich diet has been shown to help prevent the onset and progression of colorectal cancer.

In scientific studies, polyphenols from our diet often show promise in combating cancer and chronic conditions. However, it's crucial to recognize that many studies use lab-grown cells or animal models, not human subjects. Because our bodies are incredibly complex, the effects observed in these studies may not translate directly to humans. Nonetheless, consuming a diet rich in polyphenols remains highly beneficial for our overall health.

May have anti-inflammatory effects

According to the results of many experimental research studies, polyphenols may decrease the production and activity of some proinflammatory molecules, as well as interfere with various signaling pathways involved in the inflammation process in the body.

Polyphenols in berries or fermented berry beverages have been suggested to play a role in reducing systemic inflammation and the amounts of inflammation modulators. Studies on the role of polyphenols in inflammation have shown promising results, but there are still many unanswered questions. For example, we need to understand more about appropriate polyphenol doses, their bioavailability, interactions with microbiota, and more.

Are there any side effects related to polyphenols?

If you are thinking of including more polyphenols in your diet, you should be aware of some side effects as well. Certain polyphenols can bind to minerals like calcium or iron. Iron deficiency is a common condition, and a polyphenol-rich diet may worsen it even more in susceptible individuals.

Some polyphenols can inhibit digestive enzymes. While in some cases inhibition of these enzymes has been proposed to be beneficial — such as in diabetes or obesity — suppression of digestive enzymes can have negative effects in healthy individuals. Additionally, if you're considering taking polyphenol supplements and you're already on other medications, it's important to consult your healthcare provider. Polyphenols have the potential to interact with certain drugs, so it's essential to discuss any potential risks or interactions with a medical professional.

In general, polyphenols are safe when consumed as a part of a diverse diet, so it is a good idea to include them in our diet. However, problems can arise when polyphenols are consumed in large quantities without moderation, leading to possible negative side effects. If you're thinking about taking polyphenol supplements, always consult with a healthcare specialist first.