Quercetin
Quercetin is a plant compound of the group known as flavonoids, found in many fruits and vegetables, particularly onions and apples. In food it is usually bound to natural sugars rather than standing alone; supplements normally supply it in its free form.
Quercetin is one of the plant compounds known as flavonoids. In laboratory test systems flavonoids act as antioxidants, which is why the literature groups them that way. The Linus Pauling Institute is explicit about an important qualification: flavonoids are rapidly and extensively changed in intestinal and liver cells, so what appears in the bloodstream are by-products whose biological activities are likely to differ from those of the original compound. This page describes the ingredient itself and is not a statement about any particular product. Dietary supplements are not intended to prevent, treat or cure any disease.
About this ingredient
Quercetin belongs to the flavonoids, a large family of plant compounds. In plants it is almost never present on its own — it is attached to natural sugars, which is why some labels use other names for it, such as rutin.
Dietary sources
Quercetin is not an essential nutrient, so there is no Thai RDI and no EU nutrient reference value. The richest food sources are onions, apples, kale, sweet cherries, grapes, red cabbage, green beans, tea and capers; in apples it is concentrated in the peel. Ordinary dietary intake is far below the amounts in supplements. Estimates for the US population put mean intake from natural sources at about 5.9 mg a day and the 90th percentile at about 14.7 mg a day, with a maximum estimate of 258 mg a day, whereas supplements commonly deliver several hundred milligrams. Most commercial raw material is extracted from the flower buds of the Japanese pagoda tree or from onion skins.
Forms & absorption
Quercetin raw materials differ in form, and the form affects how well the body takes it up. In food, quercetin is usually attached to natural sugars, and how quickly it is taken up depends on which food it comes from; supplements normally use quercetin on its own. That form does not dissolve easily in water, so some products are made with added fat or with technology that improves solubility. Because the body takes up only a small part of it, the number on the label does not tell you how much reaches the bloodstream.
Research status
There is a reasonable body of human research on quercetin as a single compound, at doses from about 3 mg to 1,000 mg a day, mostly running for no more than three months. A safety review of that literature reports that adverse effects were rarely reported and mild where they occurred, while stating in the same breath that adequate scientific data are not currently available to assess the safety of long-term use beyond 12 weeks at high supplemental doses of 1,000 mg or more. A central methodological limitation is that quercetin is detected in plasma only as metabolites, so further study is needed on what those metabolites do. No claim has been approved by the Thai FDA for the products on this site. This page therefore presents quercetin here as an ingredient, not as a benefit.
Safety & considerations
Thai FDA guidance on the use of key ingredients in dietary supplements, dated 8 July 2024, lists quercetin as item 346 at not more than 1 g per day, and the same figure recurs as a condition on several raw materials that contain quercetin, among them onion at item 10 and strawberry at item 142. On safety, the Linus Pauling Institute states that no adverse effects have been associated with high dietary intakes of flavonoids from plant foods, and explains this by their relatively low bioavailability and rapid metabolism and elimination — but supplements are a different case. One specific point is that taking quercetin in supplement amounts may affect how the body handles some medicines, so it could interact with them. Flavonoids can also reduce the absorption of iron from plant foods. If you take prescription medicines or are pregnant or breastfeeding, speak to a doctor or pharmacist before starting, and always follow the amount printed on the label. The published safety literature notes that adequate scientific data for assessing the long-term use (more than 12 weeks) of high supplemental doses of 1,000 mg or more are not currently available, which is the level at which the Thai ceiling sits. Dietary supplements are not intended to prevent, treat or cure any disease.
Quality markers
Three things are worth checking on a quercetin label. First, whether the milligram figure refers to quercetin itself or to an extract containing a stated percentage of it — an extract standardised at 95 % needs about 526 mg to deliver 500 mg of quercetin. Second, the botanical source, usually Japanese pagoda tree flower buds or onion skin, which should be declared. Third, what else is in the same formula: vitamin C, zinc or vitamin D each carry their own ceilings and count towards whatever else you are taking. Every dietary supplement sold in Thailand also carries an FDA (อย.) registration number on the label.
- Linus Pauling Institute (Oregon State University) – Flavonoids ↗
- Andres et al. (Mol Nutr Food Res 2018) – Safety aspects of the use of quercetin as a dietary supplement ↗
- Thai FDA – Advice on the use of key ingredients in dietary supplements (8 July 2024), item 346 Quercetin ↗
This page is general educational information about a nutrient — it describes the nutrient itself and is not a statement about any particular product. Dietary supplements are food, not medicine, and have no effect in preventing or treating disease. The references listed above are public-institution sources, not product claims.
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