Quick answer: quercetin is a flavonoid found in onions, apples, capers, berries and tea. On its own it is poorly absorbed, so enhanced forms such as phospholipid (phytosome) and liposomal quercetin have been developed, and some show much higher blood levels. However, evidence that quercetin supplements improve health outcomes in people is limited. Treat bold immune or allergy claims with caution.
What quercetin is
Quercetin is one of the most common flavonoids, plant compounds that give colour to fruits and vegetables. In laboratory studies it acts as an antioxidant and affects inflammatory pathways, which is why it attracts attention. Many effects seen in cells or animals, however, use concentrations that are hard to reach in humans.
Food sources
- Capers (one of the richest sources)
- Red onions and shallots
- Apples (especially the skin)
- Kale, broccoli and leafy greens
- Berries, cherries and grapes
- Black and green tea
A diet rich in fruits and vegetables provides quercetin along with fibre, vitamins and many other beneficial compounds.
Why absorption is a problem
Quercetin dissolves poorly in water, and much of it is changed in the gut and liver before reaching the bloodstream. Plain quercetin powder therefore produces low blood levels. Enhanced forms aim to fix this:
| Form | Approach | Evidence |
|---|---|---|
| Quercetin phytosome | Quercetin bound to sunflower phospholipids | A 2019 human study reported substantially higher blood levels than plain quercetin |
| Liposomal quercetin | Quercetin in phospholipid spheres | Mainly laboratory data and small studies |
| Isoquercetin / EMIQ | Modified quercetin with sugar groups | Better absorbed in some studies |
| With bromelain or vitamin C | Combination formulas | Limited evidence that combinations improve absorption |
For a general comparison of carriers, see Liposomal vs Micellar vs Nano.
What human research shows
- Blood pressure: some trials and meta-analyses report small reductions at doses of 500 mg a day or more, but results vary.
- Exercise and endurance: small, mixed effects.
- Allergies and immune health: mostly laboratory and animal research; good human evidence is lacking.
- Infections: quercetin has been promoted for viral infections, but strong clinical trial evidence is lacking. It should not replace vaccines or medical treatment.
Safety
- Doses up to about 1,000 mg a day for up to 12 weeks were generally well tolerated in studies; headache and tingling have been reported.
- Quercetin may interact with medications metabolised by liver enzymes, including some antibiotics, blood thinners and cyclosporine. Ask a pharmacist.
- People with kidney disease should avoid high doses.
- Not well studied in pregnancy and breastfeeding.
- Enhanced forms raise blood levels more, so interactions may be more likely.
Choosing a quercetin product in Canada
- Check the NPN and approved claims (how to check).
- Look at the amount of quercetin per dose and the delivery form.
- Prefer forms with published human absorption data.
- Review medications with a pharmacist.
Frequently asked questions
Is liposomal quercetin worth it?
If you take quercetin at all, an enhanced form makes more sense than plain powder. Phytosome quercetin currently has more published human absorption data than most liposomal products.
Can I get enough quercetin from food?
Food provides much smaller amounts than supplements, but eating onions, apples, berries and greens regularly is a proven healthy habit.
Sources
- Riva A, et al. Improved oral absorption of quercetin from quercetin phytosome, a new delivery system based on food grade lecithin. European Journal of Drug Metabolism and Pharmacokinetics, 2019.
- Review literature on quercetin bioavailability and clinical studies.
- Health Canada. Licensed Natural Health Products Database.
This article is for general information only and is not medical advice. See our medical disclaimer.
