Quercetin is a plant compound found in various foods that has a wide range of beneficial effects on the body. It also seems to be very promising in the prevention and treatment of colds. This is because quercetin can reduce symptom severity and the number of days of illness, as well as susceptibility to upper respiratory tract infections.
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Quercetin, a plant substance against colds
Quercetin is a natural yellow pigment, belongs to the flavonoid group and is found in many foods (see list at the bottom of the text). The plant substance has a variety of effects on health and, according to various studies, can also be used preventively as well as therapeutically for colds.
The Effects of Quercetin
Quercetin in upper respiratory tract infections
The most common upper respiratory infections are the so-called colds, which usually bring symptoms such as a runny or stuffy nose, as well as pain when swallowing, fatigue or cough ⑶. The studies presented below were able to show a positive effect of quercetin preparations in upper respiratory tract infections.
Reduction in days of illness and symptom severity.
A placebo-controlled, double-blind, randomized trial demonstrated that taking quercetin twice daily (in the morning after awakening and between 2 pm and the last meal of the day) for 12 weeks resulted in a reduction in symptom severity and days of illness. Individuals aged 18-85 years with varying fitness levels were studied, but the beneficial effect of quercetin supplements was seen only in those who considered themselves physically fit and were 40 years or older ⑷.
Fewer infections after intense exercise
Further studies focused on the effect of quercetin after intense exercise, as it is associated with an increased risk of upper respiratory tract infections ⑸. This showed a reduction in new cases when quercetin was taken 3 weeks before and 2 weeks after exercise ⑴ ⑹.
Quercetin was also observed to offset increased susceptibility to infection after intense exercise in mice administered an influenza virus ⑸.
Antiviral activity of quercetin
In vitro studies (are conducted outside a living organism) have demonstrated potent antiviral properties of quercetin in both adenoviruses ⑺ and coronaviruses ⑻, both of which can be the cause of upper respiratory tract infections.
For other agents you can use to protect yourself from coronaviruses, see the article Over-the-counter Agents to Prevent Covid-19, where we present a concept that includes quercetin.
Taking Quercetin for colds
Quercetin can therefore have an extremely positive effect on infections of the upper respiratory tract. Moreover, due to the low risk of side effects when taken in the correct dosage (see below), quercetin preparations can be integrated into the prevention and treatment of colds without hesitation. Quercetin can also serve as a good supplement for the prevention of upper respiratory tract infections that can occur after strenuous exercise.
The positive effects of the above studies were due to a dose of 500 mg of quercetin taken twice daily for up to 12 weeks. This showed an increase in quercetin plasma levels without side effects ⑷. We therefore recommend that this duration and dose not be exceeded, as it is not known whether longer-term use is safe.
Combination of quercetin with other vital substances
Additional intake of other phytochemicals (e.g., resveratrol, catechins) and curcumin may enhance the effects of quercetin, so that the beneficial effects may occur even at lower doses ⑼. Furthermore, the effects of quercetin can be enhanced by the additional intake of vitamin C or by bromelain ⑽⑾.
Quercetin is also a zinc ionophore and makes the cell membrane permeable to zinc ions so that the cell can be better supplied with them. Since zinc plays an important role in strengthening the immune system, these two substances can also be combined well.
To learn how you can additionally support your body against colds, see our article What really helps against colds.
What to look for when buying quercetin
Be sure to select high-quality quercetin preparations that do not contain allergens (e.g., gluten or lactose) or unnecessary additives. There are pure quercetin preparations or combination preparations (e.g. quercetin with vitamin C from effective nature) in which the dosages are coordinated (*quercetin with 500 mg per capsule).
Possible side effects
The intake of quercetin through foods such as fruits and vegetables is very healthy and cannot cause any damage to the body. In supplement form, quercetin is considered safe if a dose of 1000 mg/day is not exceeded for 12 weeks. However, it is not known whether larger amounts and longer-term use are safe, as there are no specific studies on this.
Quercetin in high doses (more than 1000 mg/day) may cause headache or tingling in the arms and legs. Intravenous use may also cause kidney damage in case of overdose. If you experience side effects while taking quercetin preparations, contact your doctor.
Possible drug interactions
If you are taking any medications, talk to your doctor before taking quercetin preparations, as quercetin may alter the way the medications work and increase the risk of side effects ⑿.
In children and during pregnancy
Since there are no studies on quercetin and its effectiveness or possible side effects for pregnant women and children under 12 years of age, quercetin preparations should not be taken here as a precaution. In the form of food, quercetin can of course be consumed by these groups of people without concern.
Quercetin in foods
Quercetin is found in a wide variety of fruits and vegetables as well as in cereals and pseudocereals. These include, for example, capers, apples, berries, onions, tomatoes and buckwheat. Particularly good sources of quercetin are the following, whereby the quercetin value always refers – unless otherwise stated – to 100 g of the raw food ⒀:
More Quercetin in old apple varieties.
However, be sure to use apples of older varieties, as new varieties (e.g., Fuji) seem to contain significantly less quercetin. Old apple varieties are generally richer in phytochemicals – and they’re also less allergenic, as we explain in our article on the five benefits of apples.
It is possible that the anti-allergenic effect of old apple varieties is also due to their higher quercetin content. This is because quercetin protects against allergies and allergic reactions. It stabilizes the cell membranes of the mast cells and thus prevents them from releasing histamines and other pro-inflammatory messenger substances too quickly. Histamines are the substances that cause allergic reactions in the body: swelling of mucous membranes, redness and itching.
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