What are polyphenols and phytochemicals?
Polyphenols are natural plant-based chemicals found in healthy foods. There are more than 500 unique polyphenols. Collectively, these chemicals are known as phytochemicals. A full scientific review of polyphenols and cancer can be downloaded here.
This page provides:
- A classification of the more common types of phytochemicals
- A summary of their health benefits
- Tips on how to boost intake
Categorisation of Phytochemicals
- Flavonols: quercetin, kaempferol (onions, kale, leeks, broccoli, buckwheat, red grapes, apples)
- Flavones: apigenin, luteolin (celery, herbs, parsley, chamomile, rooibos tea, capsicum pepper)
- Isoflavones: genistein, daidzein, glycitein (soya, beans, chickpeas, alfalfa, peanuts)
- Flavanones: naringenin, hesperetin (citrus fruit)
- Anthocyanidins: (red grapes, blueberries, cherries, strawberries, blackberries, tea)
- Flavan-3-ols (tannins): catechins, epicatechin, epigallocatechin gallate (tea, chocolate,)
- Flavanolols: silymarin, silibinin, aromadedrin (milk thistle, red onions)
- Dihydrochalcones: phloridzin, aspalathin (apples, rooibos tea)
- Hydrobenzoic acids: gallic acid, ellagic acid, vanillic acid (rhubarb, grape seed, raspberries, blackberries, pomegranate, vanilla, tea)
- Hydroxycinnamic acids: ferulic acid, P-coumaric acid, caffeic acid, sinapic acid (wheat bran, cinnamon, coffee, kiwi fruit, plums, blueberries)
Other non-flavonoid polyphenols
- Other tannins: (cereals, fruits, berries, beans, nuts, wine, cocoa)
- Curcuminoids: curcumin (turmeric)
- Stilbenes: cinnamic acid, resveratrol (grapes, wine, blueberries, peanuts, raspberries)
- Lignans: secoisolariciresinol, enterolactone, sesamin (grains, flaxseed, sesame seeds)
- Alpha, beta and gamma carotene (sweet potato, carrots, pumpkin, kale)
- Lutein (corn, eggs, kale, spinach, red pepper, pumpkin, oranges, rhubarb, plum, mango, papaya)
- Zeaxanthin (corn, eggs, kale, spinach, red pepper, pumpkin, oranges)
- Lycopene (tomatoes watermelon, pink grapefruit, guava, papaya)
- Astaxanthin (salmon, shrimp, krill, crab)
- Saponins (chickpeas, soya beans)
- Limonene (the rind of citrus fruits)
- Perillyl Alcohol (cherries, caraway seeds, mint)
- Phytosterols: natural cholesterols, sitosterol, stigmasterol, campesterol (vegetable oils, cereal grains, nuts, shoots, seeds and their oils, whole grains, legumes)
- Ursolic acid (apples, cranberries, prunes, peppermint, oregano, thyme)
- Ginkgolide and bilobalide (Ginkgo biloba)
- Glucosinolates: isothiocyanates (sulforaphane) and dithiolthiones (cruciferous veg – broccoli, asparagus, Brussel sprouts, cauliflower, horseradish, radish, mustard)
- Allylic sulfides: allicin and S-allyl cysteine (garlic, leeks, onions)
- Indoles: Indole-3-carbinol (broccoli, Brussel sprouts)
- Betaines found in beetroot
- Chlorophylls found in green leafy vegetables
- Capsaicin found in chilli
- Peperine found in black peppers
Health benefits of polyphenols
The rich phytochemical contents of fruit, herbs and vegetables go a long way to explaining why these foods are so healthy. Regular intake of phytochemicals is linked to a lower risk of cancer, better cancer outcomes, and improved joint health and exercise performance. What’s more, phytochemical-rich foods are linked to a lower risk of chronic degenerative disease, so their regular intake may help reduce many of the conditions which are common after cancer including raised cholesterol, high blood pressure, macular degeneration, dementia, stroke and heart disease.
Improving gut health
Some polyphenols such as plant lignans found in nuts, resveratrol in red wine, ellagitannin found in tea as well as celery, pomegranate and turmeric, act as prebiotics which promote a healthy gut microbiome. More specifically, they impair the growth of pro-inflammatory firmicutes (bad bacteria) by preventing adhesion, thus providing more physical space for anti-inflammatory bacteroidetes. Polyphenols also preferentially feed healthy gut bacteria because the metabolism of polyphenols produces glycans such as butyrate, which are used as energy by the intestinal bacteria. Firmicutes have less of the enzyme required to digest glycans than bacteroidetes, so are less able to use them as food. Moreover, firmicutes are more repressed than bacteroidetes by the natural antibiotic properties of many polyphenols. Over time, diets low in polyphenols and high in sugar lead to firmicute overgrowth, causing inflammation, hyperplasia and dysplasia of gut cells, elevating the risk of cancer. They also damage the colon’s integrity, causing “leaky gut syndrome” and allowing toxins to pass into the bloodstream, triggering systemic inflammation and leading to a range of issues including low mood and arthritis. It goes without saying that the prebiotic benefits of polyphenols are enhanced by a healthy probiotic-rich diet (Kefir, miso soup, sauerkraut, kimchi etc) or a good quality probiotic supplement.
Reducing excess inflammation
Although an inflammatory response is an important part of a healthy immune system, persistent low-grade chronic inflammatory activity is associated with an increase in age-related diseases such as dementia, atherosclerosis, arthritis and cancer. There is a general consensus that excess inflammation is partly caused by the body overcompensating for an ailing immune system. The immune system responds to reduced T cells and natural killer cell potential (caused by ageing, obesity and diabetes) by increasing pro-inflammatory cytokines via modulation of NF‐κB. The price for these higher levels of pro-inflammatory cytokines is increased inflammatory exudates in tissues and the promotion of cancer via excess activation of COX–2 and prostaglandins. Phytochemicals, particularly the green tea polyphenol Epigallocatechin-3-Gallate (EGCG), quercetin, curcumin, caffeic acid, and caffeic acid phenethylester, have been shown to inhibit NF-kappa B signalling, while there is also evidence that they directly reduce prostaglandin and cox-2 pathways.
The anti-inflammatory mechanisms of phytochemicals go some way to explaining why foods such as turmeric, celery, tea, pomegranate, cordyceps and broccoli have reported anti-cancer properties. Furthermore, polyphenols protect us from environmental and ingested carcinogens by arming antioxidant enzymes, enhancing DNA repair pathways and directly influencing the biological processes that underlie the fundamental hallmarks of cancer progression and metastasis.
Reducing excess oxidative stress
Polyphenols can help regulate and reduce excess oxidative stress within tissues. Their antioxidant properties stem from an ability to facilitate activation of the NF‐E2 transcription factor, which enhances an appropriate antioxidant response to damaging reactive oxidative species. Reactive oxygen species (ROS) are generated at higher levels in obesity, after eating unhealthy foods such as burnt meat and processes sugar or smoking which explains why individuals with one or more of these lifestyle habits intake are particularly vulnerable to arthritis and cancer (Wang, Marseglia). Polyphenols also promote the natural adaptive response to ROS’ during exercise, yet do not affect the degradation of antioxidant enzymes after exercise, ensuring that the time cells spend with an optimal oxidative balance is greatly extended. Unlike direct anti-oxidant vitamins A & E, they do not over deplete ROS levels and cause anti-oxidative stress, but instead, improve antioxidant efficiency and capacity when needed.
Direct joint health properties
The ability of polyphenols to reduce systemic inflammation explains why consumption of these phytochemical-rich foods correlates with lower pain, stiffness and reduced mobility in people with OA affected joints. By inhibiting matrix metalloproteinase (MMPs) enzymes, overproduction of which is responsible for extracellular matrix (cartilage – collagen and proteoglycan aggrecan) degeneration, polyphenols also have some direct, joint protective properties. They also exert anti-apoptotic effects on chondrocytes in joints exposed to oxidative stress or cartilage degeneration.
An increasing body of evidence is demonstrating the impact of dietary polyphenols on preventing type two diabetes (T2D) and mitigating the adverse consequences of T2D among those who already have the disease. A combined analysis of the Nurses’ Health Study (NHS) and the Nurses’ Health II prospective study revealed that people with higher urinary excretion of flavanones, flavonols, phenolic acid and caffeic acid reported a lower incidence of T2D. The anti-diabetic effects of polyphenols may in part be related to the effects of the pulp and fibre often present in polyphenol-rich foods on slowing gastric emptying. Polyphenol-rich foods slow the glycaemic index of carbohydrates, help control body weight and improve insulin sensitivity, reducing the risks of metabolic syndrome and diabetes. Additionally, one laboratory study reported that glucose transport in gut cells was directly inhibited by flavonoid glycosides and non-glycosylated polyphenols such as EGCG. Other in vitro and animal studies have suggested that polyphenols exert their anti-diabetic effects via a number of mechanisms, including inhibition of the production of α-amylase and α-glucosidase, reduction of hepatic glucose output, stimulation of insulin secretion, and enhancement of insulin-dependent glucose uptake and activation.
Polyphenol-rich foods have been shown to help to improve muscle oxygenation, increase the time to fatigue and improve exercise performance. It is particularly important to have a high polyphenol intake if exercising because, at the start of an exercise session, reactive oxidative species (ROS) are generated as a by-product of normal energy-producing mechanisms (oxidative phosphorylation). In response to this transient increase in ROS, the adaptive up-regulation of antioxidant genes results in greater production of antioxidant enzymes via activation of Nrf-2. In the long term, provided individuals continue a sensible exercise regimen and have a healthy polyphenol-rich diet, exercise is an anti-oxidant. Polyphenols improve antioxidant efficiency and capacity, which means that the time cells spend in the zone of optimal oxidative balance is greatly extended. What’s more, when ROS levels drop after exercise and Nrf2 is degraded via binding and signalling from a protein called Keap 1, polyphenols have been shown to help the restoration process.
Tissue oxygenation and improved vascular health
Foods such as celery, pomegranate, beetroot and other leafy green vegetables are rich in nitrates which, in the presence of vitamin C and polyphenols, are converted to nitric oxide (NO). NO can increase tissue perfusion and oxygenation, lower blood pressure, improve mood and has been shown to improve muscle recovery and endurance as well as strength and stamina during resistance training. Intake has also been linked to a lower risk of hypertension and, consequently, heart disease.
Anti-viral potential of polyphenol-rich foods
Numerous lab experiments have demonstrated how phytochemical-rich plants have significant anti-viral properties, something which should of particular interest given the current Covid-19 pandemic. The following polyphenol-rich foods seem to have particular anti-viral properties.
Ellagic Acid and Epigallocatechin-3-gallate (Pomegranate, tea, red wine): Pomegranate is rich in anthocyanins, flavonoids, gallic acid, ellagic acid, quercetin and ellagitannins. In a laboratory study, the antiviral (HRV) effect of ellagic acid was evaluated by analysing the inhibitory effect of the acid on viral RNA replication. The acid’s 50% inhibitory effect on RNA replication was twice that of ribavirin.
Curcuminoids (Turmeric): By either competitively suppressing sterol regulatory element binding protein pathways or having a negative effect on cell penetration, Curcuminoids have been identified as a potential inhibitor of hepatitis C virus replication. Other studies have shown curcumin to inhibit proliferation of Human Papillomavirus, as well as parainfluenza virus type 3 (PIV-3), feline infectious peritonitis virus (FIPV), vesicular stomatitis virus (VSV), herpes simplex virus (HSV), flock house virus (FHV), and respiratory syncytial virus (RSV). Moreover, a study demonstrated considerable synergy between curcuminoids and ellagic acid when generating ROS mediated apoptosis in HPV infected HeLa cells.
Apigenin (Chamomile, parsley, celery, citrus fruits): Apigenin-rich foods have demonstrated many health benefits. In terms of viral effects, apigenin extracts have been shown to induce anti-HIV activity in T-cell lines transfected with HIV-I and HIV-1 (IIIB) infected MT-4 cells. Apigenin has been shown to slow viral replication by suppressing internal ribosomal entry site (IRES) mediated translational activity and by modulating cellular c-Jun N-terminal kinase (JNK) pathways.
Quercetin (Onions, pomegranate, citrus fruit): Quercetin displayed antiviral activity against different influenza virus strains, including H1N1and H3N2. The inhibitory effect of quercetin was particularly enhanced when the virus was pre-incubated with quercetin, or the cell was infected with the virus in the presence of quercetin. This suggested that quercetin could inhibit both virus infection and cell entry, reducing complications associated with the (H1N1) virus infection.
Hesperetin (Citrous bioflavonoids): Citrus fruits and some vegetables contain several naturally occurring dietary bioflavonoids – namely hesperetin, naringin, and catechin – which have demonstrated inhibitory effects on various viruses including herpes simplex virus type 1 (HSV-1), polio-virus type 1, parainfluenza virus type 3 (Pf-3), and respiratory syncytial virus (RSV). Hesperetin had no effect on infectivity but did reduce intracellular replication, while catechin inhibited the infectivity but not the replication.
Aloe Emodin (Aloe Vera): Aloe vera is an evergreen herbaceous plant widely grown in tropical and subtropical regions. It contains a wide range of bioactive ingredients, including vitamins, amino acids, polysaccharides and anthraquinones, and consequently has antibacterial, anti-inflammatory, antioxidant, wound healing-promoting, and immunity-enhancing properties. The anthraquinone derivative aloe-emodin reported anti-proliferative effects, particularly against the highly pathogenic H5N1 virus.
How to ensure adequate polyphenol intake
Asian and Mediterranean diets are typically abundant in polyphenol-rich vegetables, salads, herbs, spices, teas, nuts, fruits, seeds and legumes. Typical western diets, on the other hand, are dreadfully deficient in polyphenols, meaning we need to eat a lot more of them with every meal of the day. Great emphasis is often placed on exotic foods from distant sources, yet polyphenol-rich foods are all around us and readily available in most local supermarkets.
Techniques to boost polyphenol intake
It should be noted that when manipulating foods in order to boost their phytochemical or vitamin content, care has to be taken not to enhance the negative elements along with the good elements. As a rule, removing a chemical from food usually provides no benefits and could even do some harm. That is why good quality nutritional research in this field is so important, especially when it comes to boosting whole polyphenol-rich food intake. Here are some practical tips.
Juices and smoothies
Many of the fruit juices available on the market today aren’t actually ‘real’ fruit juices. They consist of water mixed with concentrate and extra sugar. Even real fruit juice has a high concentration of fructose as so many fruits are used to make them. There is also little chewing resistance to slow down consumption, making it very easy to drink a large amount of sugar in a short period of time. Juicing, which entails the whole fruit being put in the blender, is more effective at maintaining the pulp and fibre, yet still often involves a high fructose content. To overcome this, smoothie aficionados add avocado, vegetables such as kale or spices such as ginger, lowering the sugar content while improving the polyphenol intake.
Most polyphenols survive a degree of cooking, making soups an ideal way to guarantee an effective intake. Tomato soup significantly increases lycopene intake, making it perfect for those not keen on raw tomatoes. A vegetable broth flavoured with extra spices and herbs and consumed before a meal tends to fill the stomach, helping with weight loss regimens, while broccoli, onion and pea soup, with a sprinkle of turmeric and a generous twist of fresh ground pepper, constitutes the perfect superfood mix. To get the most out of soups, eat them with a fresh salad containing raw onions, lettuce or radish, all of which contain the enzyme myrosinase which is required to convert the sulforaphane in cooked cruciferous vegetables into the bioactive antioxidant enzyme glutathione. Also add pepper liberally, as the peperine it contains helps the bioavailability of polyphenols in both the vegetables and other spices.
Some more forward-thinking food outlets are offering healthy shots (around 50ml) of polyphenol-rich ingredients. The fact that they are not heated means they preserve their nutrient and polyphenol content. Common shot blends include ginger with apple, and turmeric and chilli with orange juice. These provide a quick boost but are usually not cheap. It is possible to make your own shot by grating fresh ginger into a small apple juice and adding a twist of lemon. If you have the time, it is also possible to make ginger shots with a high-powered blender, a technique which gets much more out of the root. Roughly chop ginger and add a few tablespoons of water or lemon juice to the blender. Blend until the ginger is broken down and then, if you don’t like the bits, pour the blend through a fine mesh. For a green shot, try combining a 2cm length piece of fresh-scrubbed clean ginger with 1/2 small green apple, 1 cup of packed spinach leaves and half an avocado, before adding the juice from 1 large lemon and a small pinch of cayenne pepper.
Blending grains and seeds
Although individual foods can be very healthy, mixing them together is a fantastic way to provide your body with a great variety of essential nutrients. Most health food shops now sell mixed grain and seeds, either ground in bags or in the form of health bars, cereals or drinks. They tend to be expensive and still have to be processed in some way. You can, however, make your own superfood grain mix very easily with the help of a blender.
There are very few whole food supplements which have been evaluated in robust national randomised trials with the exception of the Pomi-T study. This study evaluated the impact of a whole food nutritional supplement containing green tea, pomegranate, turmeric, and broccoli. In 2015, the full study was published in the esteemed ‘Nature’ journal and received extensive global media attention.
Caution must be taken with direct antioxidant supplements such as vitamin E, as well as other supplements which have not been evaluated in randomised trials. Many contain concentrated phytoestrogenic extracts or cloves and peppermint, which may in fact have negative properties. What’s more, unlike Pomi-T, which is now made by a Swiss Pharmaceutical company, the quality assurance of some over the counter products is very inconsistent.
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