Polyphenols are a group of plant chemicals that contribute to the vibrant colour, taste and aroma of many fruit, vegetables and flowers. They range in colour from emerald-green, golden-yellow and vivid orange to brilliant red, deep purple and snow-white. Some are totally colourless.
Polyphenols make flowers attractive to enhance pollination, and ensure fruit is aromatic and tasty so they are eaten and their seeds transported and scattered. They also discourage predators from eating leaves and roots by imparting astringent, bitter or hot, pungent flavours. They also provide antimicrobial protection against bacteria, viruses and fungi, have insecticidal effects, and act as a natural sunscreen against excess ultraviolet radiation. They also regulate the activity of plant hormones to control their growth and development.
In short, polyphenols are essential for plants to thrive and were initially thought to be essential for human health, too.
The rise and fall and rise again of vitamin P
Polyphenols were discovered in 1936 by Professor Albert von Szent-Györgyi, a scientist with a Nobel Prize for research into vitamin C. He noticed that crude extracts of vitamin C obtained from green peppers and lemon juice were more effective in alleviating scurvy than purified vitamin C alone. On further investigation, he isolated another component, which he called Vitamin P because it decreased the permeability of tiny blood vessels.
This substance seemed to fit the definition of a vitamin, in that it could not be synthesised in the body and offered health benefits, yet despite intensive research no one could identify any obvious symptoms or signs of deficiency when vitamin P was excluded from the diet.
By 1950, the United States downgraded vitamin P from its status as a vitamin. Instead, the term bioflavonoid was introduced – bio to reflect its biological actions, and flavonoid after the newly recognised classes of polyphenols known as flavonols and flavones.
The New York Times ran a scathing story denouncing vitamin P as ‘perfectly useless’ but scientists within Europe and Scandinavia kept the faith and the term vitamin P is still in common use.
Vitamin P is now entering a new and exciting phase with the recognition that bioflavonoids, and other polyphenols, play important roles within human cells.
What are polyphenols?
Polyphenols are a series of plant antioxidants, so-named because their chemical structure includes two or more phenol rings. The way these rings are bound together is used to further divide polyphenols into four main classes:
- Phenolic acids
It then gets a lot more complicated in the case of the flavonoids.
Flavonoids are polyphenols with a particular chemical structure (15 carbon atoms arranged in a C6-C3-C6 configuration) which makes them relatively small, light, and easily absorbed from the diet.
Over 6000 naturally occurring flavonoids have been identified and are further subdivided, based on their chemical structure, into six major families that are important for human health:
It doesn’t help that some of these names are easily confused!
Many flavonoids are unique to certain plants, while others are found in high concentration in particular sources.
The flavonoids that have biological actions in human cells are known as bioflavonoids, and are one of the key reasons why some fruit and vegetables have gained a reputation as ‘super’ foods.
Flavonols include quercetin, myricetin and kaempferol, which are found in onions, curly kale, leeks, broccoli, and blueberries.
Flavones include apigenin and luteolin, which are found in parsley and celery.
Flavanones include naringenin (which gives bitter notes to grapefruit), hesperidin (found in oranges) and eriodictyol (obtained from lemons).
Flavanols (or, strictly speaking, flavan-3-ols) exist as single units (monomers) or chains of two to ten or more units strung together, which are known as polymers (proanthocyanidins). Catechin and epicatechin are the main dietary flavanols and are found in fruits, dark chocolate, cocoa, red wine and tea.
Isoflavones have a weak, oestrogen-like action and include genistein, daidzein and glycitein. The main dietary sources are soy beans and their products.
Anthocyanidins are pigments found in abundance in coloured fruit and vegetables, including blackberries, blueberries, strawberries, cherries and aubergine/eggplant. These include cyanidin, pelargonidin, delphinidin and peonidin – names which reflect their role in contributing to the vibrant shades of many garden flowers.
The health benefits of bioflavonoids
Bio flavonoids are more than just powerful antioxidants. They act as natural, biological response modifiers that can modulate cell metabolism, and switch genes on or off to regulate cell growth and division, reduce inflammation, have anti-allergy and anti-viral actions, and are implicated in fat burning and weight loss.
In short, they optimise cell growth, renewal and regeneration.
As a result of these beneficial effects, bioflavonoids appear to protect against numerous age-related diseases and are once again under investigation as protective, vitamin-like substances that can’t be made in the body and must therefore come from the diet.
A growing body of evidence suggests that bioflavonoids:
- Boost fat burning to boost energy levels, aid weight loss and reduce obesity
- Reduce free radical attack to protect against inflammation and premature ageing
- Boost immune protection against infections and cancer
- Decrease blood pressure and blood stickiness to protect against heart attack and stroke
- Increase insulin sensitivity to protect against type 2 diabetes
- Protect brain cells during times of metabolic stress to guard against dementia
- Improve eye health, protecting against retinopathy and cataracts
- Have beneficial effects on bones and joints to protect against osteoarthritis and osteoporosis
As well as the bioflavonoids, other closely related polyphenols provide a host of health benefits, including:
Stilbenes such as resveratrol, which is found in red and black grapes and red wine
Lignans which are found in nuts, seeds, sweet potatoes, chickpeas and lentils
Phenolic acids which are divided into hydroxybenzoic and hydroxycinnamic acids, and are found in high concentrations in blackberries, raspberries, green and black teas, blueberries and coffee.
How to increase your intake of polyphenols
Different plant foods provide their own specific combination of polyphenols, some of which are unique to a particular fruit or vegetable.
In some fruit, the polyphenols are found in both the skin and flesh so that plums, cherries and raspberries are of uniform colour throughout. In other fruits, such as black grapes and apples, the polyphenols are found almost exclusively in the skin.
Some offer a different range of polyphenols in their skin to that in their flesh. The dark purple skin of the aubergine/eggplant, for example, provides as much as 5500mg polyphenols per 100 gram of peel, including flavonols, flavones and lignans. The whole fruit, including the pulp, provides between 2000mg and 4000mg flavonoid polyphenols per 100g – even white-skinned varieties provide more than most vegetables.
The level of polyphenols present in fruit and vegetables depends on the species, cultivars, growing conditions, ripeness, and how they are harvested, processed and stored.
If you follow a mixed, well-balanced, diet, your average intake of polyphenols is likely to come in at around 1000mg (1 gram) per day.
Around half of these polyphenols are eaten and absorbed in an active form, and half are obtained in an inactive form, bound to a sugar. Bacteria in the large bowel snip off the sugar, which releases and activates the polyphenol for absorption. Some people have a better balance of bowel bacteria to do this than others, and eating live Bio yogurt and/or taking a probiotic supplement can help this.
Typical daily intakes of polyphenols from different foods in an average diet are as follows:
Typical daily intakes
|Flavonols||Almonds, apples, apricots, beer, beetroot, blackberries, black and green grapes, black and green tea, black olives, broccoli, buckwheat, cabbages, cannellini beans, capers, lettuce, onions, oranges, oregano, parsnip, pears, plums, prunes, raisins, red wine, spinach, strawberries, tomatoes, watercress||13 mg/day
|Flavones||Beer, black and green olives, Brussels sprouts, buckwheat, celeriac, celery, chamomile tea, lettuce, olive oil, onions, parsley, pumpkin, red cabbage,||3 mg/day
0.6 mg/100ml beer
|Flavanones||Almonds, beer, grapefruit, onions, oranges, red wine, tomatoes||14 mg/day
|Almonds, apricots, apples, avocado, banana, beer, blackberries, blackcurrants, black and green grapes, black tea, broad (fava) beans, cannellini beans, cashew nuts, chestnuts, chocolate, cocoa, green tea, hazelnuts, kiwi, mango, peaches, pears, pecan nuts, pistachio nuts, plums, pomegranate, red wine, rhubarb, spinach, strawberries||150 mg/day
47 mg/100ml red wine
|Anthocyanins||Acai berries, apples, aubergine/eggplant, beetroot, bilberries, blackberries, black cherries, blackcurrants, black grapes, black olives, black soybeans, blueberries, blood oranges, cranberries, figs, plums, raspberries, red cabbage, red wine, strawberries||200 mg/day
22 mg/100ml red wine
|Isoflavones||Beer (hops), black beans, cannellini beans, chickpeas, kudzu, peanuts, soy bean products||5 mg/day (Europe) to 100 mg/day (Asia)|
|Stilbenes||Bilberries, black and green grapes, cranberries, dark chocolate, lingonberries, peanuts, pistachios, redcurrants, strawberries, Red wine, White wine, vinegar||
5 mg/100ml red wine
1 mg/100ml white wine
|Lignans||Almond, apricots, apples, arugula/rocket, asparagus, avocado, banana, basil, bilberry, blackberry, blackcurrant, brazil nuts, broad (fava) beans, broccoli, Brussels sprouts, cabbages, capers, cashew nuts, cauliflower, celeriac, chamomile tea, chickpeas, citrus, cloudberry, coffee, collard greens, dates, eggplant/aubergine, figs, fennel, flaxseed, garlic, grapes, hazelnuts, kale, kiwi fruit, lemons, lettuce, lingonberry, melon, olives, olive oil, oranges, papaya, peach, peanuts, pears, peas, pecan nuts, pine nuts, pineapple, pistachio nuts, plum, pomegranate, poppy-seed, pumpkin, pumpkin seeds, spinach, prunes, radishes, red wine, rhubarb, rye, strawberry, sesame seed, soy beans, sunflower seed, sweet potato, tea, tomatoes, walnuts, watercress,||
1 mg/100ml red wine
Some dietary sources mainly provide one class of polyphenols, such as figs (which contain lignans), while others are a rich source of many classes of polyphenol; red wine, for example, provides an extraordinary total polyphenol concentration of around 130 mg/100ml with those from some regions providing significantly more.
For more information, here’s a great post on 10 low-carb foods that are high in polyphenols from Michael Joseph.
Image credits: daxiao_productions/shutterstock