Inhaltsstoff-Glossar

Quercetin

What Is Quercetin?

Quercetin is a natural pigment belonging to the group of polyphenols and flavonoids. Its name is derived from the Latin word quercus, meaning oak, because the yellow natural dye obtained from the dyer's oak was traditionally used for yellow coloring. In plants, quercetin plays an important role in protecting against oxidative stress, which can also cause cellular damage in humans and is associated with numerous chronic diseases. For this reason, quercetin is being investigated for similar beneficial effects in humans.

Where Does Quercetin Naturally Occur?

As a flavonoid, quercetin is a plant pigment that is widely distributed throughout the plant kingdom and frequently gives flowers and fruit skins their characteristic color. Accordingly, it is found in particularly high concentrations in unpeeled fruits and vegetables such as onions, blackcurrants, and apples. Due to the high quercetin content of oak wood, the skins of red grapes, and the aging of wine in oak barrels, red wine also contains quercetin.

Within plants, flavonoids perform numerous functions. They provide color to attract pollinators, produce an astringent taste that deters herbivores, protect plant cells from oxidative damage caused by ultraviolet radiation, and help prevent colonization of leaves through their antibacterial, antiviral, and antifungal properties. Humans consume quercetin daily through their diet, with an average intake of approximately half a gram per day.

What Is the Primary Function of Quercetin in the Body?

In the past, it was believed that quercetin and other flavonoids exerted their effects primarily through their antioxidant properties, such as neutralizing reactive oxygen species (ROS). Today, it is known that compounds in this group also promote vasodilation, help prevent blood clot formation, and inhibit the enzymes responsible for generating ROS.

Quercetin also regulates inflammation by influencing cellular signaling pathways, inhibiting inflammatory enzymes (cyclooxygenases, COX enzymes), and stimulating certain immune cells. These actions reduce measurable inflammatory mediators such as prostaglandins. The effects of quercetin may therefore help restore physiological balance that has been disrupted by disease or even prevent such imbalances from developing. Indeed, in vivo studies have demonstrated beneficial effects of quercetin on cardiovascular disease, various forms of cancer, and metabolic disorders.

Benefits

May enhance physical and mental performance
May reduce the risk of injuries
May promote muscle growth during high-intensity exercise

What Are the Benefits of Quercetin Supplementation?

Epidemiological studies have demonstrated beneficial associations between flavonoids such as quercetin and a lower risk of cardiovascular disease as well as reduced cardiovascular mortality. A meta-analysis of seven studies involving a total of 587 participants found that quercetin doses exceeding 500 mg per day significantly reduced blood pressure, an important risk factor for cardiovascular disease. Additional systematic reviews of clinical trials confirmed beneficial effects on hypertension and blood lipid profiles when quercetin was taken for more than eight weeks. Even among individuals with normal blood pressure, quercetin supplementation resulted in measurable reductions.

Analyses of preclinical studies have shown that quercetin exerts potent effects against viral respiratory infections by reducing viral load, pro-inflammatory mediators, mucus production, and mortality. Because quercetin inhibits a protease essential for the SARS-CoV-2 virus, researchers have also investigated its potential to suppress coronavirus replication. Initial clinical studies in humans suggest that quercetin may shorten recovery time, reduce the length of hospital stays, and decrease inflammatory markers in the blood.

Quercetin has also shown promise in the fields of diabetes, Alzheimer's disease, and cancer. In animal models, quercetin improved kidney function, reduced diabetes-associated inflammation in the kidneys, and demonstrated neuroprotective effects in Alzheimer's disease. Preclinical studies provide compelling evidence for the anticancer properties of quercetin. However, in many cases, convincing evidence from human clinical trials is still lacking.

In most clinical studies, quercetin was administered at daily doses ranging from 100 mg to 1,000 mg for periods of up to three months. Within this timeframe, it was generally considered safe and well tolerated. Beyond this duration, however, data on long-term safety in humans remain limited. Animal studies suggest that higher doses and prolonged administration may cause liver or kidney toxicity, promote estrogen-dependent cancers, or interfere with the effectiveness of certain medications.