Quercetin Explained Simply

1. What Is Quercetin?

Quercetin is a pigment from plants. It belongs to the group of flavonols, a subgroup of flavonoids. These substances give plants their color and occur in many plants. Quercetin is found, for example, in onions, apples, capers, buckwheat and tea. In plants it is mostly bound to sugar. In research it is studied as a single substance or in certain preparations. As a dietary supplement, the single substance is offered.

2. Where Is It Found, and What Is It Made From?

Quercetin occurs in many plant foods. These include onions, apples, capers, buckwheat and tea. In the plant it sits above all in the outer layers of leaves and fruits. For dietary supplements, quercetin is obtained from plant raw material. In this process the sugar residue can be split off or exchanged. This produces the pure substance or certain sugar forms.

3. What Does Quercetin Do? What Research Shows

Heart and Circulation

The effect on the heart and circulation is the best studied. A summary of several studies found slightly lower blood pressure, especially at higher doses 4. A large review counts quercetin among the substances with medium to good evidence for lower cardiovascular risk factors 1. In men before a bypass operation, quercetin improved vascular relaxation, but not in women. Atrial fibrillation, a cardiac arrhythmia, occurred less often after the operation than with placebo (sham treatment) 8. Another study found no difference in blood pressure after a single dose with a test meal 15.

Inflammation and Immune System

For inflammation and the immune system, the picture is unclear. Several studies showed no difference from placebo (sham treatment), for example in vaccine responses 11 or in inflammation values after a test meal 15. A review on polyphenols in food in rheumatoid arthritis found overall lower disease activity 2. In a study in COVID-19, inflammation values in the blood fell more with a mixture of quercetin, vitamin C and bromelain than with the usual treatment alone; however, intubations or deaths were not less frequent 6. Mixture preparations do not allow any statement about quercetin alone.

Cellular Ageing

For cell aging, there are several indications. In a study with men, quercetin reduced oxidative damage to the genetic material after intensive interval training 20. In men with coronary heart disease, quercetin reversed aging and inflammation patterns in vascular cells 8. A study on a mixture of isoquercetin and sodium nitrite found no difference in oxidative stress 9. The studies are predominantly small and short.

Absorption and Metabolism

For absorption, small studies tested formulations intended to improve absorption. A study found better absorption for quercetin in a gel formulation with fenugreek components than for unformulated quercetin 16. Another study showed better absorption for a quercetin complex with phospholipids from food lecithin than for unformulated quercetin 17. The results apply in each case only to the formulation studied. Direct comparisons of the different forms are still pending.

Physical Performance and Recovery

For physical performance, the findings are mixed. In older adults in strength training, maximum strength increased more with quercetin sugar forms than with placebo 13. After one week with quercetin sugar forms, the strength threshold at which additional muscle fibers are recruited fell compared with baseline; with placebo it did not 14. In a study without random assignment, muscle strength declined more after light or moderate exertion with quercetin sugar forms than with placebo 19. The studies are small and tested sugar forms; other sports studies concerned mixtures.

Blood Sugar and Metabolism

For blood sugar and metabolism, there are initial indications. A study with quercetin from onion skin extract with a test meal showed no difference in blood lipids, insulin and glucose in overweight adults with high blood pressure 15. A review on dietary supplements in polycystic ovary syndrome found overall lower fasting insulin, most pronounced with vitamin D 5. The strength of evidence is low. The finding applies to a single administration with a test meal.

Allergies and Histamine

For allergies and histamine, the statements have so far relied on review articles. A review on natural substances in allergic rhinitis and asthma attributes to quercetin the inhibition of the release of alarm signals and the stabilization of mast cells 24. A review cites small clinical studies with quercetin alone that reported fewer symptoms and better quality of life 25. Two further reviews report fewer nasal symptoms 27 and fewer inflammatory cells 28. Larger controlled trials (with a comparison group) with a clearly named form are still pending.

Other Investigations

In further investigations, quercetin in bars lowered the frequency and severity of acute mountain sickness in healthy men during ascent to high altitude 7. In adults with autism spectrum disorder, social responsiveness improved within the group, but there was no difference between the groups 10. The findings apply to the form studied in each case. Because of small participant numbers and short duration, larger studies are needed to confirm them.

4. How Was Quercetin Taken in the Studies?

The studies predominantly used the pure substance quercetin, partly as a hydrous form. Sugar forms such as isoquercetin as well as a complex with phospholipids were also used. Some investigations tested mixed preparations, for instance with vitamin C and bromelain, with sodium nitrite or with resveratrol and further substances. In every study, form and dose are important because the forms are absorbed differently. Results from mixed preparations do not apply to quercetin alone.

5. Is Quercetin Safe? Side Effects in the Studies

The studies predominantly report good tolerability of quercetin sugar forms and isoquercetin. Serious adverse events were not attributed to the preparation 1112. In a study with a grape skin extract that contains, besides quercetin, also ellagic acid and resveratrol, occasional mild gastrointestinal complaints occurred, such as bloating and soft stool 22. Many studies are small and short. The results apply only to the respective form and preparation studied. With mixed preparations, it cannot be said which share comes from quercetin.

All details with tables and sources: full expert page on Quercetin