Safety Aspects of the Use of Quercetin as a Dietary Supplement

Abstract

The flavonoid quercetin is frequently found in low amounts as a secondary plant metabolite in fruits and vegetables. Isolated quercetin is also marketed as a dietary supplement, mostly as the free quercetin aglycone, and frequently in daily doses of up to 1000 mg d -1 exceeding usual dietary intake levels. The present review is dedicated to safety aspects of isolated quercetin used as single compound in dietary supplements. Among the numerous published human intervention studies, adverse effects following supplemental quercetin intake have been rarely reported and any such effects were mild in nature. Published adequate scientific data for safety assessment in regard to the long-term use (>12 weeks) of high supplemental quercetin doses (ࣙ1000 mg) are currently not available. Based on animal studies involving oral quercetin application some possible critical safety aspects could be identified such as the potential of quercetin to enhance nephrotoxic effects in the predamaged kidney or to promote tumor development especially in estrogen-dependent cancer. Furthermore, animal and human studies with single time or short-term supplemental quercetin application revealed interactions between quercetin and certain drugs leading to altered drug bioavailability. Based on these results, some potential risk groups are discussed in the present review.

Dietary quercetin supplements: Assessment of online product informations and quantitation of quercetin in the products by high‐performance liquid chromatography

“…In addition to quercetin, its metabolites (e.g., isorhamnetin, quercetin‐3'‐ O ‐sulfate, and quercetin glucuronides) can also affect the function of enzymes and transporters, including CYP2C9, CYP3A4, OATs, and GLUT (glucose transporter) (Csepregi et al, ; Kimura et al, ; Miron et al, ; Poór et al, ). As reported in animal and human studies, quercetin significantly affects the pharmacokinetics of several drugs by interacting with CYP3A4, P‐gp, and other biotransformation enzymes and transporters (Andres et al, ). Although quercetin–drug interactions are not well documented, special attention should be given to patients who concomitantly take quercetin products and chemotherapeutic or cardiovascular medications with narrow therapeutic index (Andres et al, ; Mason & Lavallee, ; Tsai et al, ).…”

“…The price ranged between $6.5 and $56.3 (USD; average: $24.2) per package including shipping, with pure bulk quercetin powder being the least expensive alternative ($6.5 for a 25‐g package, including shipping). The most commonly recommended daily dose of quercetin aglycone as a dietary supplement was 500 mg (Andres et al, ), and the average product cost of formulated products (excluding bulk powder) is $0.37/500 mg/day (±0.2 SD). Delivery time ranged between 2 and 17 days (average: 7.7 days).…”

“…To minimize the risk of adverse effects, critical safety aspects and possible high‐risk patient groups must be established. These include patients with impaired kidney function and with estrogen‐dependent cancer, because quercetin may aggravate kidney failure and may promote tumor formation in these patients, respectively (Andres et al, ). Quercetin and some of its metabolites may produce pharmacokinetic interactions with several drugs, primarily due to modulation of biotransformation enzymes and drug transporters (Miron et al, ; Srinivas, ).…”

“…Although quercetin–drug interactions are not well documented, special attention should be given to patients who concomitantly take quercetin products and chemotherapeutic or cardiovascular medications with narrow therapeutic index (Andres et al, ; Mason & Lavallee, ; Tsai et al, ). Furthermore, patients on medications with narrow therapeutic window (e.g., warfarin or digoxin) should be made aware of the potential interactions of quercetin‐containing products with drugs, and they should consult with their health care provider prior to taking any herbal or dietary supplement (Andres et al, ; Woodward, Deyo, Donahue, Deal, & Hawes, ).…”

Effects of quercetin supplementation on glycemic control among patients with metabolic syndrome and related disorders: A systematic review and meta‐analysis of randomized controlled trials

“…Recently, antioxidant supplements have been widely evaluated to decrease the progression of metabolic disorders (Akbari et al, ). Quercetin is a well‐recognized flavonoid with antioxidant effects that is found naturally in food such as onions, apples, red wine, and tea (Andres et al, ). Extensive evidence has demonstrated its favorable impacts on metabolic disorders including hypertension, obesity, insulin resistance, and cardiovascular disease (Chen, Jiang, Wu, & Fang, ; Overman, Chuang, & McIntosh, ).…”