Our diet contains a number of different substances that exhibit antioxidant activity or are capable of removing free radicals. The most important antioxidants in our diet are vitamin C, tocopherols (vitamin E), carotenoids (vitamin A source) and especially flavonoids.1
Flavonoids are substancesoccurring in plants whose main function is the protection against harmful external influences (UV radiation), maintenance of the stability of the internal environment (free radical neutralization) and other functions important for the survival of plants. At present, over 4000 flavonoids are known and many of them are being intensively studied because they are one of the most effective antioxidants.
Flavonoids got their name from the Latin word “flavus” – yellow, because they have a naturally yellow colour in the nature.
Flavonoids are essential for normal plant function and survival in plants. In addition to UV protection, flavonoids also have regulatory functions that affect basic life processes and protect plants against foreign organisms. Various types and forms of flavonoids, sometimes referred to as bioflavonoids, are found in many plants, including those that are part of our diet. The best-known sources of flavonoids are forest fruits, grapes, citrus, cocoa beans, peanuts, parsley or onion. Very effective flavonoids are found in tea plants in particular.
Documents on the use of flavonoids for medical purposes are several hundred years old. Many of them were used due to their anti-inflammatory effects and were recommended for many diseases.
In the last 30 years, advanced biochemical techniques have allowed the study of the effects of flavonoids at the level of tissues, cells and individual cell structures. This has brought extensive results from scientific work and research. For example, over 6000 articles were published in 2015.
The research has recently focused on the role of flavonoids in oxidative stress. It is the result of multiple factors. It is an increased production of oxidizing substances, reduction of antioxidant protection and poor repair of oxidative damage.
Flavonoids are effective antioxidants because of their free radical oxygen and fatty acid removal properties.2
They protect the cells from harmful effects of external and internal environment. Flavonoids are one of the strongest known antioxidants.
Flavonoids “quench” oxygen radicals, which are reactive forms of oxygen. In higher concentration, these substances damage the tissues and cells. The inability of the body to get rid of these forms of oxygen is nowadays regarded as one of the most common causes of a number of civilization illnesses.
Inflammation is the response of the organism to the presence of various pathogens (bacteria, viruses, fungi), injuries, toxic substances, but also the reaction of the immune system (hypersensitivity) leading to tissue damage.3
Chronic inflammation is associated with diseases such as rheumatoid arthritis, bronchial asthma, atherosclerosis, Alzheimer's disease and cancer.4-6
The development and spread of inflammation is linked to increased production of freeradicals, which are absorbed by flavonoids. They also affect certain specific enzymes involved in the inflammation.
Flavonoids have been shown to have a similar mechanism of action as non-steroidal anti-inflammatory drugs. 7
The beneficial effect of flavonoids on inflammation can prevent tissue damage and help normalize the immune system's functions.
Flavonoids have proven antibacterial and antiviral activity.
E.g., catechins, whose main source is green tea, are effective against pathogenic food-borne bacteria and therefore have beneficial effects in gastrointestinal disorders.8
The effects of flavonoids on the heart and vessels are multiple and have been studied as one of the first. Epidemiological studies have analysed the effect of flavonoids contained in food on cardiovascular diseases. They have a positive effect on vascular tissue, they are known to reduce the formation of atherosclerotic plaques and thereby reduce the risk of myocardial infarction or stroke.
In 2014, the largest pooled analysis (so-called meta-analysis) of the largest clinical trials monitoring the relationship between flavonoid use and cardiovascular diseases was published. These were studies of several years which included more than 450 thousand individuals.
This pooled analysis has statistically shown that flavonoid intake significantly reduces the risk of heart and vessel diseases.9
The relationship between flavonoids, especially catechins extracted from Camellia sinensis (Tea Plant), and tumour growth has been intensively studied since the 1980s. Epidemiological work was among the first to track the relationship between tea consumption and the incidence of various types of cancer. These effects are now intensively studied in tumour cell lines, which are present in humans, and clinical trials are also under way.
A pooled analysis of the effect of catechins has confirmed their potential benefits in the prevention of certain oncological diseases.10
Flavonoids can have a beneficial effect on a variety of metabolic processes.
They have been shown to increase fat burning and help reduce body weight. Excessive visceral (organ) fat is associated with metabolic disorders leading to poor health and an increased risk of cardiovascular disease. There is a so-called metabolic syndrome, which is a complex of several diagnoses. In addition to overweight and obesity, it includes lipids metabolism disorders, increased blood pressure and elevated blood glucose. People with metabolic syndrome are at high cardiovascular risk. In developed countries, symptoms of metabolic syndrome (sometimes called Syndrome X) are present in up to one-third of the population.
Polyphenol flavonoids also affect caloric intake where they regulate the activity of enzymes involved in the absorption of fats and sugars. 12
Accumulated data show that reactive oxygen species are involved in neurodegenerative diseases such as Alzheimer's or Parkinson's disease.1
These chronic neurological diseases are becoming an increasing economic and social burden for modern society.
Flavonoids are effective antioxidants, have anti-inflammatory effects and affect cell signalling pathways. Their action can provide neuroprotective effects in Alzheimer's disease.13
Biologically active plant substances, including flavonoids, have a well-proven effect on Parkinson's disease based to several mechanisms of action.14
In addition to their antioxidant activity, flavonoids also exhibit good antibacterial activity due to their effect on bacterial division, their cell membrane and metabolism. Substances like quercetin or kaempferol show antimicrobial activity against a wide range of bacteria. Quercetin and luteolin, as well as some other flavonoids, have shown mutually beneficial effects with several antibiotics.15
Various types of flavonoids have been studied for their potential antiviral effects and several have demonstrated antiviral properties not only in in vitro but also in in vivo studies.16
There are a number of studies that deal with the effects of flavonoids on the human immune system.
Flavonoids are a common part of daily food of plant origin. It is a very large group of substances, some of which have a demonstrably beneficial effect on our organism. The question is what specific substances or groups of substances and what amounts we receive. In the past, research has shown that regular intake of flavonoids is most often below the threshold where we could expect their positive effects. It is almost certain that we are not able to receive enough flavonoids from natural sources regularly. If we want to take advantage of their beneficial effects, there is a possibility of dietary supplements. It is preferable to use complex formulations where individual flavonoids can influence each other positively, as is the case with their natural intake. Since they are natural substances of plant origin, of non-toxic chemical structure, there are no issues from the point of view of safety for the human organism.
By typing the phrase “flavonoids”, the scientific search engine finds tens of thousands of studies evaluating their effects on cardiovascular, metabolic, and neurodegenerative diseases, the immune system, chronic inflammations and oncological diseases. The width and depth of exploring the effects of flavonoids in the human body is stunning.
Dietary supplements enable us to receive specific types of flavonoids with extensive evidence of effects on individual organ systems. The antioxidant properties that flavonoids can provide are more accessible and we can benefit from their positive effects.