What are flavonoids?

Flavonoids are bioactive compounds that are very common in vegetables and fruits. They give color to plants and perform many other important functions in them. They have antioxidant, anti-inflammatory, anti-cancer and detoxifying properties. Flavonoids should be present in the diet in as much as possible to use their health-promoting potential.

Flavonoids are natural plant compounds with antioxidant properties. They are very common in vegetables, fruits and herbs. So far, about 4,000 flavonoid compounds found in leaves, flowers, fruits and seeds of plants have been known and described. Due to the differences in structure, they are divided into flavanones, flavanols, flavones, isoflavones, flavonols and anthocyanins. Flavonoids are dyes that give plants very different colors - from yellow and orange in citrus to navy blue in blackcurrant and blueberries. They also have many important functions: they protect plants against excess UV radiation, against pests, fungi and molds, act as plant hormones, growth regulators and control enzymatic reactions. Flavonoids have a wide range of biological effects confirmed by scientific research.

Properties of flavonoids

Flavonoids have an effect:

• anti-inflammatory
• anti-cancer,
• antiatherosclerotic
• anti-aggregation (reducing the ability of platelets to form clots),
• antiarrhythmic,
• hypotensive (lowering blood pressure),
• diastolic,
• diuretic,
• detoxifying,
• antibacterial,
• antiviral,
• antiallergic.

Food sources of flavonoids

Flavonoids are among the most common compounds in the plant world. Therefore, they are an essential element in the diet. The higher the consumption of vegetables and fruits, the more flavonoids are delivered to the body. The average consumption of flavonoids in Europe is between 100 and 1000 mg per day. The Irish take an average of 1 g of flavonoids with their diet every day. Far Eastern diets, with less processed food and a high content of soy and tea, provide about 2 g, while the extremely processed Western diet even only 50 mg a day. Most of the flavonoids are found in food that is not subjected to heat treatment and long storage. The main sources of flavonoids in the Polish diet are vegetables (onions, tomatoes, peppers, broccoli) and fruits (citrus, apples, blueberries, black currants, grapes). Other foods that contain these compounds include coffee, cocoa, tea, red wine, spices, pulses, and some grains.

The content of particular types of flavonoids in selected food products [mg / 100 g]

Flavanones:

• Artichokes - 12.51
• Grapefruit juice - 18.98
• Orange juice - 18.99
• Oranges - 42.57
• Limes - 46.40
• Lemons - 49.81
• Grapefruits - 54.50
• Dried oregano - 412.13

Flavonols

• Apples - 3.4
• Boiled Brussels sprouts - 5.24
• Fresh figs - 5.47
• Dried and sweetened cranberries - 6.91
• Buckwheat - 7.09
• Chicory - 8.94
• Cherries - 9.41
• American blueberries - 10.59
• Black currant - 11.53
• Cooked asparagus - 15.16
• Fresh cranberry - 21.59
• Goji berries - 31.20
• Red onion - 38.34
• Arugula - 69.27
• Radish - 78.09
• Sorrel - 102.20
• Elderberry juice concentrate - 108.16
• Dried parsley - 331.24
• Fresh capers - 493.03

Flavones

• Kohlrabi - 1.3
• Red grapes - 1.3
• Lemons - 1.9
• Chicory - 2.85
• Celery - 3.90
• Green pepper - 4.71
• Asparagus - 9.69
• Fresh parsley (parsley) - 216.15
• Dried oregano - 1046.46
• Dried parsley (parsley) - 4523.25

Anthocyanins

• Hazelnuts - 6.71
• Cherries - 7.45
• Pears - 12.18
• Dark grapes - 21.63
• Red table wine - 23.18
• Pecan nuts - 25.02
• Strawberries - 27.76
• Red berries - 40.15
• Raspberries - 40.63
• Red cabbage - 63.50
• Red currant - 75.02
• Blackberries - 90.64
• American blueberries - 141.03
• Black currant - 154.77
• Chickpeas - 262.49
• Berries - 285.21
• Aronia - 349.79
• Elderberry juice concentrate - 411.4

Flavanols

• Apple Juice - 5.96
• Apricots - 8.41
• Peaches - 8.6
• Apples - 9.17
• Red table wine - 11.05
• Pecan nuts  - 15.99
• Boiled broad beans- 20.63
• Blackberries - 42.5
• Cocoa powder - 52.73
• Dark chocolate - 108.6
• Black tea infusion - 115.57
• Green tea infusion - 116.15

How do flavonoids work?

The metabolism of flavonoids is not well understood, but it is estimated that at least half of these compounds pass from the gastrointestinal tract into the blood unchanged, and the rest are metabolized mainly in the liver and intestines. Their wide pro-health effect results from the antioxidant activity, which depends on the number of hydroxyl groups and their position in relation to each other. The main mechanisms of the antioxidant activity of flavonoids are:

• capturing free radicals and reactive oxygen species,
• limiting the production of reactive oxygen species in cells by inhibiting the activity of enzymes involved in their formation,
• chelation of copper and iron transition ions, which prevents the formation of free radicals,
• breaking the cascade of free radical reactions,
• protection of low molecular weight antioxidants against oxidation, e.g. vitamins C and E,
• increasing the stability of cell membranes.

Trapping reactive oxygen species and chelating metals is important in disease states associated with oxidative stress such as inflammation, atherosclerosis, diabetes, neurodegenerative diseases and cancer.

Antitumor activity of flavonoids

Since the 1970s, studies have been conducted that provide evidence of the anti-cancer effects of flavonoids. These compounds have been shown to reduce the activity of mutagenic and carcinogenic substances in vitro and to reduce the incidence of tumors in experimental animals. Epidemiological studies also confirm that as the consumption of flavonoids increases, the risk of certain cancers in humans decreases. The antitumor activity of flavonoids results from their antioxidant activity, the ability to block the biotransformation of certain compounds, inhibition of DNA replication and cell proliferation, which prevents the proliferation of cancer cells, as well as the ability to alter cellular metabolism and induce apoptosis (the natural process of programmed cell death).

A diet rich in isoflavones has been found to reduce the risk of hormone-dependent cancers - breast cancer in women and prostate cancer in men. It is also possible to prevent isoflavones in cancer of the thyroid gland, head and neck. Drinking green tea is associated with a lower risk of lung cancer. Consumption of onions and apples, the two main dietary sources of quercetin flavonol, is associated with a reduction in the incidence of prostate, lung, stomach and breast cancer. In addition, moderate wine drinkers have a lower risk of cancer of the lungs, endometrium, esophagus, stomach, and colon.

Although the anti-cancer effects of flavonoids are not fully understood, they are seen as a way to treat and prevent cancer. Some of the more thoroughly studied flavonoids, such as quercetin and catechin gallate, are in clinical trials. With the current increase in the consumption of vegetables and fruit, dietary flavonoids are considered to be an important factor in reducing the risk of cancer.

Influence of flavonoids on the cardiovascular system

The antioxidant properties of flavonoids indicate their role in the prevention of heart and circulatory system diseases. Numerous epidemiological studies have shown that a diet rich in flavonoids (e.g. from 4 cups of green tea a day) has a beneficial effect on the condition of the circulatory system. One of them showed a positive effect of dietary flavonoid consumption in reducing the risk of death from a heart attack by 18% in American adults compared to the group with very little consumption of flavonoids.

The phenomenon of the so-called the French paradox also confirms the role of flavonoids. The French diet is high in fats, especially butter, but the French rarely suffer from arteriosclerosis. Probably responsible for this is a diet rich in flavonoids from vegetables, fruits and red wine, which prevent oxidation of LDL cholesterol, its build-up in the walls of arteries, and increase the concentration of "good" HDL cholesterol. Flavonoids (rutin) together with vitamin C, by stimulating collagen synthesis, strengthen and elasticize blood vessels, thus preventing the formation of blockages and varicose veins. Due to the influence of flavonoids on enzymes, blood pressure is lowered and blood vessel spasms are alleviated. Flavonoids reduce the inflammatory reaction in atherosclerotic processes by inactivating free radicals and inhibiting the influx of leukocytes to the site of inflammation.

Another factor that intensifies atherosclerotic processes is the presence of large amounts of oxygenated LDL in the blood. Flavonoids protect LDL and prevent its oxidation, thus preventing the formation of atherosclerotic plaque in endothelial cells. The antiatherosclerotic effect of flavonoids is also manifested by inhibiting individual enzymes and thus lowering cholesterol levels, increasing endothelial integration, inhibiting the proliferation of vascular smooth muscle cells, and reducing platelet aggregation and the risk of embolism. A randomized, double-blind study showed that oral administration of 500 mg of hesperidin for 3 weeks improves endothelial function, reduces inflammation and has a beneficial effect on the lipid profile of patients with metabolic syndrome.

A very interesting plant rich in flavonoids with great potential in the prevention of atherosclerosis is the Baikal skullcap used in traditional Chinese medicine.

Influence of flavonoids on the nervous system

Scientific research from recent years indicates the influence of flavonoids on the nervous system and their possible use in the prevention and treatment of neurodegenerative diseases related to aging - dementia, Parkinson's disease and Alzheimer's disease. Flavonoid-rich diets contribute to the improvement of cognitive functions, probably due to the protective effect on neurons and accelerating their regeneration. This conclusion was drawn from a 10-year follow-up of 1,640 participants over 65 years of age. Every 2.5 years, their cognitive abilities were tested in a test designed for this purpose. It has been found that the lower the consumption of flavonoids, the faster the loss of cognitive abilities occurs. Neurodegenerative diseases are caused, among others, by by reactive oxygen and nitrogen species, and the flavonoids capture and neutralize them, reducing oxidative damage to neurons. The use of flavonoid-rich ginko biloba extracts has been shown to be beneficial in relieving the symptoms of dementia and Alzheimer's disease. Tangeretin present in citrus fruits may protect against Parkinson's disease. This effect in humans was confirmed in studies in mice where tangeretin was found to inhibit the neurotoxic substance 6-hydroxydopamine that causes Parkinson's.

Flavonoids and diabetes

Diabetes mellitus occurs as a result of impaired insulin secretion by pancreatic beta cells or as a consequence of a decrease in insulin sensitivity. Certain flavonoids have been shown to be anti-diabetic in in vitro and in vivo studies.

• Epicatechin stimulates the synthesis and secretion of insulin.
• Epigallocatechin-3-gallate inhibits glucose synthesis in hepatocytes, i.e. it has a hypoglycemic effect.
• Daidzein, luteolin, and quercetin prevent blood glucose levels from rising rapidly after a meal.
• Flavonoids protect against the appearance of cataracts in diabetics.

Flavonoids and the liver

Some flavonoids, especially silymarin (of which milk thistle is an excellent source) show hepatoprotective (liver protective) effects, reduce lipid peroxidation and stimulate liver regeneration.

Flavonoids and AIDS

In treating AIDS, reducing the multiplication of the virus is essential. Flavonoids may become an important element of therapy. Epicatechin, baicalin, baicalein, quercetin, and myricetin act as an inhibitor of an enzyme essential in HIV development. Epicatechin, EGCG and baicalin can inhibit the penetration of the virus into the host cells, and quercetin inhibits the activity of the viral protein Vpr responsible for virus multiplication.

Flavonoids in dietary supplements

Due to the growing interest in therapies with compounds of natural origin and the broad biological effect of flavonoids, the number of dietary supplements containing these substances is growing. Flavonoids are included in pharmaceutical preparations used in the prevention and treatment of diseases of the heart and circulatory system, varicose veins (rutin, diosmin, hesperidin), liver (silymarin), and to alleviate the symptoms of menopause (isoflavones). Popular ingredients in supplements are extracts of hawthorn flowers and fruits, elderberry and linden flowers, ginko biloba, grapefruit extract and artichoke extract.

Vegetables and fruits contain a whole complex of secondary metabolites, not just the flavonoids themselves. It is not possible to restore the natural composition of bioactive phytochemicals in dietary supplements, which means that they are not as effective as in their original form in food. For the health benefits of flavonoids, it is more important to eat plenty of vegetables and fruits than to administer the isolated flavonoids.

Important - High-dose flavonoids can:

• interact with folic acid, vitamin C and vitamin E,
• disrupt the functioning of the thyroid gland and act as goitrogens (reduce the availability of iodine),
• disrupt metabolic processes involving cytochrome P and phase II enzymes,
• affect the transport of drugs in the body.

Soy isoflavones have estrogenic activity, hence they have a positive effect in perimenopausal women, alleviating symptoms associated with hormonal changes. However, for the same reason, they should not be used by men and women before menopause. Isoflavones contribute to a hormonal state called estrogen dominance that is disadvantageous for both sexes.