Additional information

Quercetin, flavonoid

Occurrence in food
The flavonoid quercetin belongs to the group of polyphenols and occurs naturally as a yellow plant pigment, mostly as a glycoside (e.g. in the form of rutin). Particularly high contents are found in some apple varieties, onions, parsley, sage, ginkgo biloba, but also in real buckwheat (tea), green tea, black tea, grapes, dark cherries and berries. Since quercetin is mainly found in the skin of the fruit, the content is higher in red wine than in white wine.

Quercetin – the “king of bioflavonoids
As a secondary plant substance, quercetin has strong antioxidant properties and is considered the most effective antioxidant in the flavonoid substance class. This would put quercetin even above anthocyanins from grapes (OPC) and catechins from green tea (1). It is these antioxidant capacities that make quercetin interesting for human health, as they act as an effective free radical scavenger in the human organism after absorption through the digestive tract (2). Because free radicals are closely linked to numerous pathological processes (e.g., inflammatory responses) in the body, reducing the body’s oxidative burden can have a positive impact on health (3)(4). In basic experiments, it was found that quercetin probably positively influences the antioxidant system in several ways at once:

– direct neutralization of free radicals and thus reduction of oxidative stress load
– Ability to bind reactive metal ions and thus prevent the formation of further oxygen radicals
– Suppression of oxidation of LDL and thus lipid peroxidation.
– Stimulation of glutathione production and consequent improvement of the body’s antioxidant defenses
– Stimulation of antioxidant enzymes to strengthen activity (1).

Quercetin acts as a vitamin saver
The bioflavonoid quercetin has an antioxidant effect towards vitamin C, which means that it recycles oxidized vitamin C and therefore acts as a “vitamin saver” and “vitamin enhancer” (5). In nature, quercetin occurs mostly as a glycoside. The most common source of quercetin is rutin, where it is glycosylated with the sugar rutinose. Through their antioxidant abilities, bioflavonoids also have anticarcinogenic, immunostimulatory (6), and anti-edematous effects.

Antiviral properties of quercetin
Clinical studies show that taking quercetin can relieve the symptoms of respiratory diseases. The immunomodularotic and anti-inflammatory effects of quercetin are probably responsible, at least in part, for this effect (19)(20). In addition, quercetin has antiviral properties, which are achieved by stabilizing the transcription factor NRF2 (21). This leads to the stimulation of endogenous antioxidant and antiviral metabolic processes that counteract viral replication. By decreasing the activity of NRF2 with older age and in the presence of various metabolic diseases such as diabetes, quercetin support is particularly promising in these cases (22). Another antiviral mechanism is opened up by the ionophoric (“ion-carrying”) properties of quercetin, which are discussed particularly in relation to the disease COVID-19. Basic experiments show that zinc ions can inhibit replication of the novel coronavirus. Quercetin provides transport of zinc ions into the cell as an ionophore (23).

General revenue mode
Quercetin should be taken with a meal

References:
Micronutrient Coach® – The great BIOGENA compendium of nutrients. 4th ed. Publishing House of Physicians, Editors: Dr. Christina Schmidbauer and Georg Hofstätter MSc.