We previously explored different types of polyphenols and their health benefits. Polyphenols control the formation of free radicals in the body through their antioxidant properties. They promote good health by protecting the body from cancer and other cardiovascular diseases. Polyphenols work to slow and control the effects of aging.
Polyphenols are usually found in large amounts of the skin of fruits. Sources of polyphenols include legumes, berries, tea, wine, olive oil, chocolate/cocoa, walnuts, peanuts, yerba mate, and other fruits and vegetables. They are efficient at treating obesity and promoting the production of anti-inflammatory agents.
Not all polyphenols are absorbed with equal efficacy. They are extensively metabolized by intestinal and hepatic enzymes and by the intestinal microflora. Below are some examples of sources of polyphenols in human food.
Which foods are high in polyphenols?
We can supplement our body’s need for polyphenols by eating foods that are rich in polyphenols. Some of the richest sources of polyphenols are:
Asparagus is a rich source of flavonols, containing 23 mg per 100 g of raw weight. It is also one of the few vegetables with lignans. Tannins – dimers, oligomers, and polymers of catechins – are also present.
It is very difficult to get hydroxybenzoic acid from edible plants. Berries are the exception, and this type of phenolic acid is found in many of these red fruits (cherries, strawberries, raspberries, blueberries, and blackberries). Coffee beans, which are the seeds of another type of berries, is the more common source of hydroxybenzoic acid.
Chlorogenic acid is found in blackberries and cherries.
Hydroxycinnamic acid is in most berries. The highest concentrations are seen in the skin of ripe fruit. While concentrations do decrease during the course of ripening, the total amount increases because the fruit also increasing in size.
Blueberries are known to be very rich in flavonols, which accumulate in the skin because their biosynthesis is stimulated by light.
Cyanidin, the most common form of anthocyanidin, is found in berries in proportion to color intensity. In some of the darker berries like blackberries, they can reach values up to 4 g per kg.
The Coffee Bean
Coffee is a special case of berries. It comes from a red fruit called known as the stone fruit. Despite being called a bean, the seed of this fruit has no relation to legumes.
Chlorogenic acid, which is formed from the caffeic and quinic acids found in coffee. A single cup of coffee may contain 70–350 mg of chlorogenic acid.
Hydroxycinnamic acids can also be found in coffee.
Brocolli is a rich source of flavonols. It also contains hydroxycinnamic acids, especially in the surface area exposed to the sun. The texture of this surface makes for a very efficient biosynthesis of these compounds with exposure to sunlight.
The solid parts of citrus fruit – the white spongy parts – have a very high flavanone content. In fact, a whole fruit can contain up to 5 times as much flavanones as one glass of orange juice. The skin of citrus fruits also contains large quantities of polymethoxylated flavones like tangeretin, nobiletin, and sinensetin.
Citrus fruits also contain aglycones: naringenin in grapefruit, hesperetin in oranges, and eriodictyol in lemons.
Hydroxycinnamic acids are found in abundant quantities in cinnamon, as the name would suggest.
One of the richest sources of catechins is cocoa. The bitterness of chocolate is attributed to tannins, which contain dimers, oligomers, and polymers of catechins.
Garlic is a source of lignans. Total flavonoid content is 3 mg per 100 g of weight, which is the main source of garlic’s antioxidant capacity.
Kale is a rich source of flavonols at 34 mg per 100 g of weight. This high content of flavonols is the reason it is considered a good source of antioxidants.
Leafy vegetables such as spinach, lettuce, and cabbage have a high glycoside concentration in their outer leaves. This is due to the higher flavone and flavonol content from exposure to light.
Flavones, which happen to be much less common than flavonols in fruits and vegetables, is abundant in some leafy vegetables like parsley.
Flavanones are found in aromatic plants like mint.
Anthocyanins are found in cabbage.
Isoflavones can be found in leguminous plants. Soya beans are well-known for their anti-aging properties. They contain 3 key molecules: genistein, daidzein, and glycitein. This source isoflavone content varies significantly according to geographic source, growing conditions, and amount of processing. For example, soybeans contain 580 to 3800 mg isoflavones per kg vs 30 to 175 mg for soymilk. The general rule in favor of less food processing holds.
Legumes have gallocatechin, epigallocatechin, and epigallocatechin gallate.
Lignans are also found in legumes like lentils.
Olives and Olive Oil
Olives and olive oil contain many polyphenols, but one of the main sources of antioxidant activity is hydroxytyrosol and its derivatives. It not only has a high potency, but it is also easily absorbed by humans.
Onions are one of the few vegetables that contain hydroxybenzoic acid. They are also a high source of flavonols. Onions also contain anthocyanins.
Peanuts contain stilbenes like resveratrol. This is the compound to which many studies attributed the anti-aging effects of red wine. Peanuts also contain a significant amount of phenolic acids and flavonoids, especially in their skins.
Pomegranates – in their seeds, juice, and peels – have long been known for their anti-aging benefits. These fruits are abundant in flavonoids. They also contain anthocyanins, punicic acid, and ellagitannins.
The polyphenol content of red wines is 10 times as high as that of white wines. Red wine with lower alcohol content is preferable because alcohol works through another channel to increase the rate of polyphenol depletion. One study found that excretion of catechin in the urine increased by 20% after intake of red wine vs dealcoholized red wine. The
Red wine contains up to 45 mg of flavonols per liter. Specifically, catechins are typically present in red wine in quantities up to 300 mg per liter. Red wine rich in tannins will also be rich in dimers, oligomers, and polymers of catechins. The artaric acid found in wine was shown to increase the absorption of catechin.
Anthocyanins are found in red wine, where the content can be 200-350 mg per liter. As wine ages, these anthocyanins are transformed into various complex structures.
Stilbenes like resveratrol are found in red wine. Resveratrol has been the subject of much funding for anti-aging research for its anticarcinogenic effects and role as a sirtuin activator.
Black tea is an important source of gallic acid, containing up to 4.5 g per kg of fresh tea leaves. Gallic acid acts as an antihistamine.
Black tea is a source of flavonols. Richer still is green tea. Green tea is one of the richest sources of catechins. A cup of green tea can contain up to 200 mg of catechins. Black tea loses much of its content of catechins due to fermentation of these flavonols to more complex forms of polyphenols known as theaflavins and thearubigins.
gallocatechin, epigallocatechin, and epigallocatechin gallate are also found in tea.
Epicatechins are also in black and green tea and happen to be stable when exposed to heat.
Tannins containing dimers, oligomers, and polymers of catechins are also in tea.
Contrary to common misconceptions, adding milk to black tea was shown to have no effect on the bioavailability of catechins, quercetin, or kaempferol in humans.
Cherrie tomatoes have a higher flavonol content than standard tomatoes because they have a higher proportion of skin, where the biosynthesis of flavonols takes place.
Food Processing and Bioavailability
It is important to understand that modern food processing affects polyphenol content in a significant way. Fruit juice is one example of this injury. Other examples include the dehulling of legume seeds and decortication and bolting of cereals.
In general, any grinding of plant tissues leads to oxidation that degrades the polyphenol content. This happens because plants contain these micronutrients in cellular compartments. Grinding and processing make contact between cytoplasmic polyphenol oxidase and other phenolic substrates in the vacuoles possible, which precipitates unwanted chemical reactions that consume the polyphenol content in food.
Modern food processing turns polyphenols into polymerized brown pigments. The process of making jam out of berries is one example. Fruit juices effectively remove certain flavonoids, and industrial food processing specifically targets the flavonoids responsible for discoloration and haze formation for removal. Therefore, manufactured fruit juices are not a good source of polyphenols, especially as fructose causes more oxidative damage than the polyphenols are supposed to guard against.
Baba, Seigo et al. “Continuous intake of polyphenolic compounds containing cocoa powder reduces LDL oxidative susceptibility and has beneficial effects on plasma HDL-cholesterol concentrations in humans.” American Society for Clinical Nutrition (2007): 85(3): 709-17.
Berrougui, Hicham, Souad Ikhlef, and Abdelouahed Khalil. “Extra Virgin Olive Oil Polyphenols Promote Cholesterol Efflux and Improve HDL Functionality.” Evidence-based Complementary and Alternative Medicine: eCAM 2015 (2015): 208062. PMC. Web. 13 Jan. 2018.
Brader, Lea et al. “Polyphenol-Rich Bilberry Ameliorates Total Cholesterol and LDL-Cholesterol When Implemented in the Diet of Zucker Diabetic Fatty Rats.” The Review of Diabetic Studies : RDS 10.4 (2013): 270–282. PMC. Web. 13 Jan. 2018.