Quercetin: available forms and which one to choose

On store shelves quercetin is sold in capsules, tablets, powders and 'immune' complexes, and the prices for what seems to be the same substance differ several-fold. The reason lies in the forms: aglycone, glycosides, phytosomes and modified molecules are absorbed differently. The editors explain how they differ and how to choose a product deliberately.

Why the form matters

Quercetin aglycone is a molecule with very low water solubility. In the intestine it dissolves poorly, and without that absorption is impossible. Therefore, from a large dose of pure powder only a small fraction reaches the blood, and the rest is broken down by the microbiota of the large intestine.

Nature long ago 'solved' this problem in its own way: in plants quercetin is bound to sugars. Hollman and co-authors showed that quercetin glucosides from onions are absorbed better than the aglycone, while rutin from tea or buckwheat - worse and more slowly. Accordingly, the bioavailability of quercetin from different foods differed substantially.

Supplement manufacturers use this logic: some simply sell cheap aglycone, others - glycosides with better absorption, and still others - technological forms in which the molecule is 'packaged' into phospholipids. For the buyer this means that 500 mg on the label of different products do not equal the same amount of quercetin in the blood.

At the same time it is worth remembering: higher bioavailability has been proven mainly by concentration in plasma. Whether it translates into a better clinical or athletic result has been studied much less.

The main forms of quercetin in supplements

The most common and cheapest form is quercetin aglycone, often as the dihydrate. The dihydrate is a crystalline form with two water molecules; it is more stable during storage, and water has little effect on the active-substance content. It was precisely the aglycone that was used in most sports studies, at a dose of 1000 mg per day.

Rutin is also sold separately, often in 'vascular' complexes. As a source of quercetin it is the least effective among those listed: the peak in the blood comes later and is lower, because only the bacteria of the large intestine can break down rutinose.

Isoquercetin (quercetin-3-O-glucoside) is broken down already in the small intestine, and its absorption is higher than that of the aglycone. Some 'improved' products are based on it, although the cost of such supplements is noticeably higher.

Enzymatically modified isoquercetin (EMIQ) is isoquercetin to which additional glucose residues have been attached. Such a molecule dissolves well in water; in studies in rats, Makino and co-authors (2009) showed that it is absorbed more easily than other glycosides and the aglycone.

Technologies for increasing bioavailability

The best-known technology is the phytosome: a complex of quercetin with phosphatidylcholine (lecithin). Phospholipids facilitate the passage of the molecule across the membranes of the intestinal epithelium. In a study by Riva and co-authors (2019) the phytosomal form provided several-fold higher plasma concentrations of quercetin than the ordinary aglycone at the same dose.

Other approaches - micronization (reducing particle size), nanocapsules, cyclodextrin complexes and solutions in oil. Some of them have convincing pharmacokinetic data, some - only marketing claims. If the manufacturer writes about 'increased bioavailability', it is worth looking for a reference to a specific study of that very raw material.

A simple and free way to slightly improve the absorption of ordinary aglycone is to take it with food that contains fats. Guo and co-authors (2013) showed that fatty food increased the bioavailability of quercetin.

An important nuance: a form with higher bioavailability requires a smaller dose to achieve the same concentration in the blood. Therefore, when choosing a phytosome or EMIQ, you should not automatically take 'the same 1000 mg' that were used in studies with the aglycone.

Complexes and combined products

Quercetin is often sold as part of complexes. The most common combinations have a certain logic, but the level of evidence for the advantage of the combination specifically, rather than the individual components, is usually low.

• Quercetin + bromelain:a classic pair in 'seasonal' products; bromelain is credited with improving absorption, but there is little convincing data for humans.
• Quercetin + vitamin C:vitamin C may help regenerate oxidized quercetin; the combination is popular in immune formulas.
• Quercetin + zinc:the marketing relies on laboratory data about zinc transport into cells; the clinical significance has not been proven.
• Quercetin + EGCG, omega-3:studied in sports research by Nieman's group; the results are mixed.

The downside of complexes is less transparency. Each component adds its own risks and possible interactions, and the dose of quercetin in them is often lower than in studies. For a person who wants to control exactly what they take, a single-ingredient product is more convenient.

Powders and drinks with quercetin are encountered less often. The aglycone has a slightly bitter taste and dissolves poorly, so for drinks water-soluble forms such as EMIQ are used more often.

Chewable tablets and 'vitamin gummies' with quercetin usually contain small doses and a significant amount of sugar. As a way to get the dose from studies they are of little use.

How to read the label and choose a product

The first thing to pay attention to is the exact name of the form. The label 'quercetin 500 mg' without any clarification usually means the aglycone or dihydrate. If a phytosome or EMIQ is used, the manufacturer writes about it, often indicating the trade name of the raw material.

1. Find out the form: aglycone, dihydrate, isoquercetin, EMIQ, phytosome or rutin.
2. Check whether the dose of quercetin itself is stated, rather than of the entire complex with lecithin.
3. Assess the auxiliary components: bromelain, soy lecithin, dyes, sugar.
4. Look for independent quality-testing certificates, especially if you are an athlete.
5. Compare the price per effective dose, not per capsule.

For phytosomal products the total mass of the complex is often stated, where quercetin makes up only a part. For example, 500 mg of phytosome does not equal 500 mg of quercetin - check the active-substance content.

For most people who want to 'try quercetin' in the context of training, ordinary aglycone taken with food is a reasonable choice: it is precisely the one with the most studies. Improved forms are justified if a smaller number of capsules matters or if a high dose of aglycone is poorly tolerated.

Editors' conclusions

The forms of quercetin differ substantially in absorption: rutin and the aglycone have the lowest bioavailability, isoquercetin - higher, and EMIQ and phytosomes - the highest among the common variants.

Higher bioavailability is confirmed by the plasma level, but does not necessarily mean a better athletic result. The most efficacy studies have been conducted specifically with the aglycone.

When choosing, read the form, the real quercetin content and the auxiliary components, and compare the price per effective dose.

Next we recommend reading 'How to take quercetin: dosage, timing, duration', 'What to combine quercetin with' and 'Myths about quercetin'.

References

1. Hollman PC, de Vries JH, van Leeuwen SD, Mengelers MJ, Katan MB. Absorption of dietary quercetin glycosides and quercetin aglycone in healthy ileostomy volunteers. Am J Clin Nutr. 1995;62(6):1276–1282.
2. Hollman PC, van Trijp JM, Buysman MN, et al. Relative bioavailability of the antioxidant flavonoid quercetin from various foods in man. FEBS Lett. 1997;418(1–2):152–156.
3. Makino T, Shimizu R, Kanemaru M, et al. Enzymatically modified isoquercitrin, alpha-oligoglucosyl quercetin 3-O-glucoside, is absorbed more easily than other quercetin glycosides or aglycone after oral administration in rats. Biol Pharm Bull. 2009;32(12):2034–2040.
4. Riva A, Ronchi M, Petrangolini G, Bosisio S, Allegrini P. 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.
5. Guo Y, Mah E, Davis CG, et al. Dietary fat increases quercetin bioavailability in overweight adults. Mol Nutr Food Res. 2013;57(5):896–905.
6. Kawabata K, Mukai R, Ishisaka A. Quercetin and related polyphenols: new insights and implications for their bioactivity and bioavailability. Food Funct. 2015;6(5):1399–1417.
7. Nieman DC, Henson DA, Maxwell KR, et al. Effects of quercetin and EGCG on mitochondrial biogenesis and immunity. Med Sci Sports Exerc. 2009;41(7):1467–1475.