Polyphenols, Phytonutrients and Flavonoids 101: A Category of Their Own (2024)

Besides nutrients, plant-based foods (legumes, vegetables, fruits, whole grains, nuts, seeds, and teas as well as herbs and spices) have another “crop” of naturally occurring compounds with potential health benefits. Collectively they’re called phytonutrients, meaning plant chemicals. “Phyto” means plant. Think fight for “phytos,” since they appear to promote health by sparking body processes that fight, or slow, the development of some diseases.

Phytonutrients comprise many different chemicals, including carotenoids, indoles, glucosinolates, organosulfur compounds, phytosterols, polyphenols, and saponins. 

Why have phytonutrients captured our attention? Because of their potential for health promotion! Today consumers are more interested in positive nutrition and self-care: adding (not avoiding) foods that may enhance fitness, boost immunity, slow aging, and prevent or slow the chance for chronic disease. Sound like you? Research on phytonutrients is the new frontier in nutrition, as exciting today as vitamin discoveries were a hundred years ago! 

Benefits of Phytonutrients

As public interest in phytonutrients soars, science is exploring their functional benefits. Phytonutrients are bioactive compounds in food that promote your health by helping to slow the aging process or reducing the risk for many diseases. Since the early 1980s, research has intensified in investigating how phytonutrients protect against some cancers, heart disease, stroke, high blood pressure, cataracts, osteoporosis, urinary tract infections, and other chronic health conditions.

These are among the ways that phytonutrients might work: serve as antioxidants, enhance immunity, change estrogen metabolism, enhance communication among body cells, cause cancer cells to die, detoxify carcinogens, and repair damage to DNA that’s caused by smoking and other toxins. Yet the benefits and actions of phytonutrients are still uncertain. Do they work independently, together, with nutrients and fiber, or do their actions add up? 

A wide range of biological functions such as antioxidant, antiviral, anti-inflammatory, and anticancer has been described for flavonoids (Xia et al., 2010; Khan et al., 2013; Gorlach et al., 2015; Iranshahi et al., 2015; Nabavi et al., 2015). The concern of the antiviral effects of flavonoids have been the subject material of various reports (Gescher et al., 2011; Roh and Jo, 2011; Chen et al., 2012) with reported antiviral activities through blockage of cellular receptors, blocking viral antigenic determinants, loss of enzymatic activities and inhibition of particle biosynthesis (Bae et al., 2000; Calzada et al., 2001; Chang et al., 2003; Savi et al., 2010). The natural product has been used during two earlier coronavirus outbreaks of SARS-CoV and MERS-CoV as a medicinal treatment (Li et al., 2005; Lin et al., 2005; Lau et al., 2008).

“Phytos”: In a Class of Their Own

Neither vitamins nor minerals, phytonutrients are substances that plants produce naturally to protect themselves against viruses, bacteria, and fungi, as well as insects, drought, and even the sun. Beyond that, they provide the color, aroma, and flavor that give food so much sensual appeal. Of the thousands of phytonutrients, more than two thousand are plant pigments that put a rainbow of colors on your plate! 

Like nutrients, phytonutrients are grouped accord- ing to their biochemical characteristics and probable protective functions. Only a few hundred have yet been studied. What we know today is merely the “appetizer.”

Research has revealed a few things. Most fruits and vegetables contain phytonutrients. Different plant- based foods supply different kinds and amounts; some have a remarkable variety. An orange, for example, has more than 170 different phytonutrients! In any fruit or vegetable, these substances appear to work together with nutrients and fiber for your good health.

For phytonutrients, food databases are limited to just a few hundred foods, with only a few key carotenoids and phytoestrogens. A database for flavonoids is being developed. No Dietary Reference Intakes exist for them yet. Here’s a quick look at several phytonutrient categories below.

4 Main Groups

With more than 8,000 identified polyphenols, they are broadly categorized into four main groups:

1. Flavonoids: The largest group, including anthocyanins, flavan-3-ols, flavones, flavanones, and flavanols, found in foods like berries, tea, and citrus fruits.
2. Phenolic acids: Divided into hydroxybenzoic acid and hydroxycinnamic acid, phenolic acids are present in grapes, berries, pomegranates, walnuts, chocolate, coffee, and green tea.
3. Polyphenolic amides: This category includes capsaicinoids (found in chili peppers) and avenanthramides (found in oats), known for their heat and cholesterol-lowering properties, respectively.
4. Other polyphenols: Smaller groups include resveratrol (unique to red grapes and red wine), ellagic acid (found in berries), curcumin (the yellow pigment in turmeric), and lignans (present in flax, sesame, and other grains).

Flavonoid Vs Polyphenol Vs Phytochemical

Phytochemicals cover all bioactive compounds discovered in plants, including  polyphenols like flavonoids as well as other non-polyphenolic substances. Polyphenols are a broader category of compounds that include flavonoids as well as other subgroups, whereas flavonoids are a specific type of polyphenol. 

While these terms have distinctions, they are often used interchangeably in casual language and scientific literature due to their close relationship in the context of plant-based compounds and their potential health benefits.

Here are the key differences and relationships between these terms:

a. Flavonoids

• Flavonoids are a subgroup of polyphenolic compounds.
• Flavonoids in food are generally responsible for colour, taste, prevention of fat oxidation, and protection of vitamins and enzymes.
• Flavonoids have antioxidant properties and are known for their potential health benefits. A study suggests that the risk of some chronic diseases may be lower at higher dietary flavonoid intakes.
• Common examples of flavonoids include quercetin, kaempferol, and anthocyanins.

b. Polyphenols

• Polyphenols are a larger and more general category of naturally occurring compounds found in plants.
• They are broadly dived in four classes; Phenolic acids, flavonoids, stilbenes and lignans.
• Polyphenols are known for their potential health benefits. Animal, human and epidemiologic studies show that various polyphenols have antioxidant and anti-inflammatory properties that could have preventive and/or therapeutic effects for cardiovascular disease, neurodegenerative disorders, cancer, and obesity. (R)

c. Phytochemicals

• Phytochemicals are a broad term that encompasses all bioactive compounds found in plants.
• This term is a more general one that encompasses a wide range of bioactive compounds found in plants, including flavonoids and polyphenols.
• Phytochemicals possess many biological activities and health benefits, such as antioxidant and free radical scavenging abilities, anti-inflammatory action, anticancer, anti-aging, and protective action for cardiovascular diseases, diabetes mellitus, obesity and neurodegenerative diseases. (R)

Terpenes

Terpenes may be classified by the number of terpene units in the molecule; a prefix in the name indicates the number of terpene units needed to assemble the molecule (Source).

Monoterpenes: Consist of two isoprene units. Example: iridoids. Iridoids are further sub-classified into 4 main groups i.e. seco-iridoids (oleuropein), cyclopentan, valepotriates and iridoids-alkaloids.

Tetraterpenes: Contain eight isoprene units. Biologically important tetraterpenes include the acyclic lycopene, the monocyclic γ-carotene, and the bicyclic α- and β- carotenes.

Carotenoids (beta carotene, lutein, lycopene, zeaxanthin, astaxanthin), limonoids and saponins are subgroups of terpenes, a large class of phytonutrients that work as potent antioxidants. The hundreds of carotenoids, often grouped by color, may decrease the risk of heart disease, stroke, blindness, and some cancers—and may help slow the aging process, improve respiratory function, and reduce problems associated with diabetes. 

Beta carotene 

• As an antioxidant, neutralize free radicals that may cause cell damage  
• May help slow the aging process  
• May reduce the risk of some cancers 
• May improve lung function  
• May reduce problems related to type 2 diabetes
• Foods that have beta carotene include yellow-orange fruits and vegetables such as apricots, cantaloupes, papayas, carrots, pumpkins, sweet potatoes, winter squash
• Green vegetables such as broccoli, spinach, kale

Lutein 

• Contribute to maintaining healthy vision
• May reduce the risk of cataracts and macular degeneration
• May reduce the risk of some cancers
• Foods that have lutein include green vegetables such as kale, spinach, collard greens, Swiss chard, Romaine lettuce, broccoli, brussels sprouts, Kiwifruit, Egg yolks  

Lycopene 

• Reduce risk of prostate cancer 
• May reduce risk for heart disease
• Most red fruits and vegetables such as tomatoes, tomato products, pink grapefruit, guava, watermelon (The red pigment in red peppers is from keto carotenoids, not lycopene.)

Zeaxanthin

• Contribute to maintaining healthy vision
• May help prevent macular degeneration 
• Corn, spinach, winter squash, green vegetables, citrus fruits. (Eggs have a small amount of zeaxanthin, too.) 

Astaxanthin

• Astaxanthin is a reddish pigment that belongs to a group of chemicals called carotenoids. It occurs naturally in certain algae and causes the pink or red color in salmon, trout, lobster, shrimp, and other seafood.
• Astaxanthin is taken by mouth for treating Alzheimer's disease, Parkinson's disease, stroke, high cholesterol, liver diseases, age-related macular degeneration (age-related vision loss), and preventing cancer. It is also used for metabolic syndrome, which is a group of conditions that increase the risk of heart disease, stroke and diabetes. It is also used for improving exercise performance, decreasing muscle damage after exercise, and decreasing muscle soreness after exercise. Also, astaxanthin is taken by mouth to prevent sunburn, to improve sleep, and for carpal tunnel syndrome, dyspepsia, male infertility, symptoms of menopause, and rheumatoid arthritis. (WebMD)

Polyphenols

This family group, which includes polyphenols (including flavonoids), offers protection from oxidative damage. Sometimes called bioflavonoids, more than eight hundred flavonoids work as antioxidants with many potentially protective health benefits; for example, some appear to promote cardiovascular health by helping to make blood cells less “sticky.” Isoflavones, anthocyanins, and catechins are sub-groups of flavonoids; some isoflavones are phytoestrogens—weak, nonsteroid estrogens. Lignans, other phenolic substances, are also phytoestrogens. 

Polyphenols can be broken down into four general categories — flavonoids, stilbenes, lignans and phenolic acids — with additional subgroupings based on the number of phenol rings they contain, and on the basis of structural elements that bind these rings to one another.

Types of polyphenols

More than 8,000 types of polyphenols have been identified. They can be further categorized into 4 main groups (Trusted Source, Trusted Source):

• Flavonoids. These account for around 60% of all polyphenols. Examples include quercetin, kaempferol, catechins, and anthocyanins, which are found in foods like apples, onions, dark chocolate, and red cabbage.
• Phenolic acids. This group accounts for around 30% of all polyphenols. Examples include stilbenes and lignans, which are mostly found in fruits, vegetables, whole grains, and seeds.
• Polyphenolic amides. This category includes capsaicinoids in chili peppers and avenanthramides in oats.
• Other polyphenols. This group includes resveratrol in red wine, ellagic acid in berries, curcumin in turmeric, and lignans in flax seeds, sesame seeds, and whole grains.

The amount and type of polyphenols in foods depend on the food, including its origin, ripeness, and how it was farmed, transported, stored, and prepared.

Polyphenol-containing supplements are available as well. However, they’re likely to be less beneficial than polyphenol-rich foods (Trusted Source).

Flavonoids - Anthocyanins (cyanidin, delphinidin, malvidin) 

• As an antioxidant, neutralize free radicals
• May help reduce cancer risk 
• May help prevent urinary tract infections
• Blueberries, blackberries, cranberries, cherries, strawberries, kiwifruit, plums, red grapes, red cabbage, egg plant (skin)

Flavonoids - Catechins (EGCG)

• As an antioxidant, neutralize free radicals
• May help reduce risk for cancers of the stomach, skin, and esophagus 
• May boost brain function (Stem Cell Reports)
• EGCG may help reduce the risk of COVID-19
• Tea (black, oolong or green), wine

Flavonoid - Quercetin

• Works as a potent antioxidant 
• May reduce the growth and spread of cancer cells
• May help reduce the risk of COVID-19
• Foods that have quercetin include onions, wine, tea, green tea and many vegetables including tomatoes

Pycnogenol

• French maritime pine bark extract rich in polyphenols and procyanidins
• Pycnogenol is the US registered trademark name for a product derived from the French maritime pine bark of a tree known as Pinus pinaster.
• The active ingredients in pycnogenol can also be extracted from other sources, including peanut skin, grape seed, and witch hazel bark.

Flavanones (hesperetin, naringenin)

• May help protect against heart disease 
• Citrus fruit

Isoflavones (daidzein, genestein, glycitein) 

• May reduce menopause symptoms such as hot flashes
• May offer protection from breast and prostate cancers 
• May protect bone health after menopause 
• Soybeans, soy-based foods 

Resveratrol

• As an antioxidant, may help reduce the risk of heart disease 
• Support normal cardiovascular health
• May help reduce the risk of cancer, blood clots, stroke
• May boost brain function (Stem Cell Reports)
• Red grapes, red grape juice, red wine
• Peanuts

Olive Leaf Extract

• Olive leaves contain several key polyphenols, such as oleacein and oleuropein. 
• Polyphenols occur naturally in plants, and research suggests that they may help protect against a range of conditions, such as heart disease and cancer.

Role of Polyphenols in Human Health

As noted in a 2010 scientific review in the journal Nutrients:

"Research in recent years strongly supports a role for polyphenols in the prevention of degenerative diseases, particularly cancers, cardiovascular diseases and neurodegenerative diseases … Recent studies have revealed that many of these diseases are related to oxidative stress from reactive oxygen and nitrogen species.

Phytochemicals, especially polyphenols, are the predominant contributor to the total antioxidant activities of fruits, rather than vitamin C.

Polyphenols have been found to be strong antioxidants that can neutralize free radicals by donating an electron or hydrogen atom … Polyphenols … complement and add to the functions of antioxidant vitamins and enzymes as a defense against oxidative stress caused by excess reactive oxygen species (ROS).

Although most of the evidence of the antioxidant activity of polyphenols is based on in vitro studies, increasing evidence indicates they may act in ways beyond the antioxidant functions in vivo. Modulation of cell signaling pathways by polyphenols may help significantly to explain the mechanisms of the actions of polyphenol-rich diets."

Phytosterols

Sometimes referred to as “plant cholesterol” because of their structural similarity, phytos- terols are fatlike substances in plants. Phytosterols offer protection from heart disease by helping to decrease the absorption of dietary cholesterol from animal-based foods.

Thiols (organosulfuric compounds)

Thiols are plant substances that contain sulfur naturally. Their food sources are easy to identify because of their pungent aroma. Some of these compounds you might hear about include glucosinolates, indoles, dithiolthiones, and isothiocyanates in cruciferous vegetables, and allyl sulfides in foods such as garlic and onions.

Organic Acids and Polysaccharides

These substances include organic acids such as oxalic acid, phytic acid, and tannins, which bind with iron and make it less available to your body. Yet they may have benefits, too; see the following chart. Caffeic and ferulic acids fit in this phytonutrient family, too. Oligosaccharides, made of many simple sugars, are yet another category of phytonutrients that work as prebiotics. 

Healthcare Experts’ Example Days of Adequate Polyphenol Intake

The following table provides sample daily meal plans from Sheikhha, Kimberlain, and Arnett, showcasing how to incorporate polyphenol-rich foods into your diet for optimal health and skin vitality.

Bottom line

Already there’s overwhelming evidence for the health benefits of plant-based foods: fruits; vegetables; legumes (including soy); nuts; seeds; and grains, especially whole grains. Research shows that you lower the odds for some cancers, heart disease, and other health problems by eating more fruits, vegetables, and grains.

Count on a variety of foods, not dietary supplements, to reap the benefits of the many phytonutrients from all kinds of plant-based foods. 

Adapted from:

• International Food Information Council
• American Dietetic Association 

Related: 

Best phytonutrients supplements

Vitamins and Supplements Resource List