Quercetin: Supplement Forms and Which to Choose

Quercetin is a plant flavonoid found in onions, apples, capers and tea, and it is sold in an increasing number of forms. The main problem with quercetin is poor absorption, and different formulations try to solve that in different ways. Our editorial team compares the options and explains how to choose sensibly.

Quercetin and the absorption problem

Quercetin belongs to the flavonol subgroup of flavonoids. In food it mostly occurs bound to sugars as glycosides, such as quercetin glucosides in onions and rutin in buckwheat. The typical British diet provides perhaps a few tens of milligrams per day, depending heavily on fruit, vegetable and tea intake.

Interest in quercetin stems from laboratory studies showing antioxidant, anti-inflammatory and antiviral activity. In sport, it has been studied for endurance performance and recovery, with meta-analyses suggesting at most a small effect on endurance capacity.

The challenge is bioavailability. Pure quercetin aglycone is poorly soluble in water and extensively metabolised in the gut wall and liver, so only a small fraction appears in blood as free quercetin. Most circulates as conjugated metabolites, whose activity may differ from laboratory findings.

This is why formulation matters. The same nominal dose can lead to very different blood levels depending on the chemical form and delivery system used in the product.

The main forms on the market

Quercetin dihydrate is the most common and cheapest form, often supplied as 250–500 mg capsules. It is essentially aglycone with bound water. Many studies used this form, but absorption is limited, and taking it with a meal containing fat may help somewhat.

Isoquercitrin and enzymatically modified isoquercitrin (EMIQ) are glycoside forms. Glucose attachments allow better uptake via intestinal transporters, and EMIQ in particular has been shown in human studies to produce substantially higher plasma levels than aglycone at the same dose.

Quercetin phytosome combines quercetin with phospholipids from sunflower lecithin. A pharmacokinetic study reported markedly higher blood levels compared with unformulated quercetin, which allows lower doses. Rutin, the form in buckwheat, is absorbed more slowly because it must first be broken down by gut bacteria.

Combinations: vitamin C, bromelain and others

Many products combine quercetin with vitamin C, on the theory that vitamin C can regenerate oxidised quercetin. This makes chemical sense, but there is little human evidence that the combination produces better health outcomes than either ingredient alone.

Bromelain, an enzyme from pineapple, is often added to support absorption. Data supporting this are mostly anecdotal or from limited research. Bromelain itself may affect blood clotting and can cause allergic reactions in people sensitive to pineapple.

Quercetin also appears in senolytic research, combined with the cancer drug dasatinib. These are experimental studies under medical supervision and do not support self-medication; dasatinib is a prescription medicine with serious side effects.

When comparing products, focus on the amount and form of quercetin itself rather than on lists of additional ingredients, which often serve marketing more than clinical purposes.

Doses and how to choose

Studies on exercise and immune outcomes most often used around 500–1000 mg per day of quercetin dihydrate, usually taken for several weeks. Enhanced-absorption forms are often used at lower doses, as stated by manufacturers based on their pharmacokinetic data.

When deciding, think about your goal. For a general dietary approach, eating onions, apples, berries, kale and tea regularly provides quercetin along with many other beneficial compounds, at no extra cost and with no safety concerns.

If you choose a supplement, an enhanced form may deliver more quercetin per milligram, but it typically costs more. Compare price per effective dose rather than price per capsule, and look for transparent labelling of the form and quantity.

• Check the exact form: dihydrate, isoquercitrin, EMIQ or phytosome.
• Look for the amount of quercetin per capsule, not only the total blend weight.
• Prefer products with batch testing or certificates of analysis.
• Take aglycone forms with a meal to improve absorption.

Safety and interactions

Quercetin from food is safe. Supplemental doses up to about 1000 mg per day for up to 12 weeks have generally been well tolerated in studies, with occasional headaches and tingling reported. Long-term safety at high doses is less well established.

High doses given intravenously have been associated with kidney toxicity in clinical research; this is not relevant to normal oral use but underlines that more is not automatically better. People with kidney disease should seek advice before taking quercetin supplements.

Quercetin can inhibit some drug-metabolising enzymes and transporters in laboratory studies, potentially affecting medicines such as ciclosporin or certain antibiotics. The clinical significance is uncertain, so anyone on regular medication should speak to a pharmacist.

Pregnant and breastfeeding women should stick to food sources, as safety data for supplements in these groups are lacking. In the UK, quercetin supplements are sold as food and cannot claim to prevent or treat disease.

Editorial conclusions

Quercetin is poorly absorbed in its basic form, which is why several enhanced formulations have been developed.

EMIQ, isoquercitrin and phytosome products produce higher blood levels, while dihydrate remains the cheapest and most studied option.

Diet remains an excellent source, and supplements should be chosen by form and dose, with medical advice if you take other medicines.

Our editorial team suggests reading next about polyphenols and exercise recovery, antioxidants and training adaptation, and how to read supplement labels.

References

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2. Riva A, Ronchi M, Petrangolini G, et al. Improved oral absorption of quercetin from quercetin phytosome, a new delivery system based on food grade lecithin. Eur J Drug Metab Pharmacokinet. 2019;44(2):169–177.
3. Manach C, Williamson G, Morand C, et al. Bioavailability and bioefficacy of polyphenols in humans. I. Review of 97 bioavailability studies. Am J Clin Nutr. 2005;81(1 Suppl):230S–242S.
4. Andres S, Pevny S, Ziegenhagen R, et al. Safety aspects of the use of quercetin as a dietary supplement. Mol Nutr Food Res. 2018;62(1):1700447.
5. Nieman DC, Henson DA, Gross SJ, et al. Quercetin reduces illness but not immune perturbations after intensive exercise. Med Sci Sports Exerc. 2007;39(9):1561–1569.
6. Hollman PC, de Vries JH, van Leeuwen SD, et al. Absorption of dietary quercetin glycosides and quercetin in healthy ileostomy volunteers. Am J Clin Nutr. 1995;62(6):1276–1282.