What is quercetin?

Quercetin is a naturally occurring flavonoid and belongs to the group of secondary plant compounds. It is found in numerous plant-based foods, especially onions, apples, berries, capers, and leafy green vegetables.^[1].

Biochemically, quercetin is characterized by its polyphenolic structure, which enables it to interact with various molecular targets in the body. After oral ingestion, quercetin is metabolized in the intestine and enters the bloodstream in different conjugated forms.^[4].

In research, quercetin is primarily studied for its antioxidant, inflammation-modulating and immunological properties. It can act directly as a free radical scavenger and also influence signaling pathways involved in inflammatory processes.^[2][3].

Quercetin and immunological processes

A key research area is the Effect of quercetin on the immune system, especially on cells of the innate immune system.

Mast cells play a key role in this process. They are located in mucous membranes, skin, and the respiratory tract, and when activated, they release various mediators, including histamine, cytokines, and leukotrienes. These substances are significantly involved in inflammatory and allergic reactions.

Experimental studies show that quercetin can modulate the activity of these cells. In one frequently cited study, it was observed that Quercetin can inhibit the release of histamine and pro-inflammatory cytokines from human mast cells.^[1].

Furthermore, quercetin influences key signaling pathways such as NF-κB and MAP kinases, which play an important role in the regulation of inflammatory responses.^[2]. These mechanisms suggest that quercetin can intervene in immunological processes at multiple levels.

Quercetin and oxidative stress

In addition to its immunological effects, quercetin is also known for its antioxidant properties.

Oxidative stress arises from an imbalance between the production of reactive oxygen species and the body's ability to neutralize them. This condition is associated with various pathophysiological processes, including chronic inflammation and metabolic dysregulation.

Quercetin can directly neutralize free radicals and simultaneously influence the activity of the body's own antioxidant enzymes.^[3]. These include, among others, superoxide dismutase and glutathione-dependent systems.

In addition, studies show that quercetin also has an indirect antioxidant effect by regulating signaling pathways involved in the cellular stress response. This dual function—direct neutralization and regulatory influence—makes quercetin a frequently studied molecule in the context of oxidative processes.

Inflammation modulation and signal transduction

Another focus of the research is on the effect of quercetin on inflammatory signaling networks.

Inflammatory responses are controlled by complex cascades of signaling molecules. Transcription factors such as NF-κB play a central role in this process, as they regulate the expression of numerous inflammation-related genes.

Quercetin was able to inhibit the activation of these signaling pathways in various experimental models.^[2]. This reduces the production of pro-inflammatory mediators, which is discussed as a possible mechanism for the observed effects.

The influence on MAP kinase signaling pathways also suggests that quercetin can modulate not only individual molecules, but entire regulatory networks.

Bioavailability and pharmacokinetic properties

Despite its diverse biological effects, quercetin has one well-known limitation: its comparatively low bioavailability.

Quercetin is poorly water-soluble and only partially absorbed in the gastrointestinal tract. Furthermore, it undergoes extensive first-pass metabolism, which can significantly reduce the systemically available concentration.^[4].

Bioavailability therefore depends significantly on the specific dosage form. Research is investigating various approaches to overcome this limitation. These include lipid-based formulations, nanoparticles, and liposomal systems.^[5][6].

These technologies aim to improve solubility, increase stability, and facilitate transport through biological membranes.

Liposomal systems as a research approach

Liposomes are vesicular structures made of phospholipids that are similar in composition to biological cell membranes.

They can encapsulate active ingredients and influence their distribution in the body. Studies show that liposomal systems can increase the stability of sensitive molecules and improve their bioavailability.^[6].

Such systems are being increasingly investigated, particularly for poorly water-soluble plant compounds like quercetin. They offer an approach to optimizing pharmacokinetic properties and improving absorption in the body.

Assessment of the current state of research

In summary, quercetin is one of the most intensively studied flavonoids in biomedical research.

The data so far show that this molecule can influence several key biological processes:

• Modulation of mast cell activity and histamine release
• Influence on inflammatory signaling pathways
• antioxidant effects at the cellular level
• Interaction with immunological regulatory mechanisms

At the same time, the clinical relevance of many of these effects remains the subject of further research, particularly with regard to bioavailability, dosage and long-term use.

Sources

[1] Kempuraj D et al. (2012) – PLoS One
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0033805

[2] Mlcek J. et al. (2016) – Molecules
https://www.mdpi.com/1420-3049/21/5/623

[3] boots A.W. et al. (2008) – European Journal of Pharmacology
https://www.sciencedirect.com/science/article/abs/pii/S0014299908002884

[4] D'Andrea G. (2015) – Fitoterapia
https://doi.org/10.1016/j.fitote.2015.09.018

[5] Andres S. et al. (2018) – Molecular Nutrition & Food Research
https://onlinelibrary.wiley.com/doi/10.1002/mnfr.201700447

[6] Moulahoum H et al. (2023) – Food Chemistry
https://www.sciencedirect.com/science/article/pii/S0010854523002400