A critical review of the data related to the safety of quercetin and lack of evidence of in vivo toxicity, including lack of genotoxic/carcinogenic properties
Introduction
Quercetin [3,3′,4′,5,7-pentahydroxyflavone, CAS no. 117-39-5] is one of several naturally-occurring dietary flavonol compounds belonging to a broad group of polyphenolic flavonoid substances (see Fig. 1). Flavonoids are characterized by a phenyl benzo(γ)pyrone-derived structure consisting of two benzene rings (A and B in Fig. 1) linked by a heterocyclic pyran or pyrone ring (C in Fig. 1) (Kühnau, 1976, Morand et al., 1998). In plants, the flavonol aglycone is most commonly present conjugated at the 3-position of the unsaturated C-ring with a sugar moiety, forming O-β-glycosides such as quercitrin or rutin (Merck, 2001). Quercetin can be obtained from plants via extraction of the quercetin glycosides followed by hydrolysis to release the aglycone and subsequent purification. Flavonols exhibit numerous biological and pharmacological effects, including anti-oxidant, chelation, anti-carcinogenic, cardioprotective, bacteriostatic, and secretory properties (Gross et al., 1996, Middleton et al., 2000, PDRNS, 2001). In plants, these compounds are involved in energy production (Theoharides et al., 2001) and exhibit strong anti-oxidant properties, possibly protecting plants against harmful ultraviolet rays (Wiczkowski et al., 2003).
The Joint FAO/WHO Expert Committee on Food Additives evaluated quercetin for use in food in 1977 (JECFA, 1977), but the limited amount of toxicity data available at the time of the evaluation precluded establishing an acceptable daily intake (ADI). Subsequently, in 1998 the International Agency for Research on Cancer evaluated quercetin for its potential carcinogenic risk to humans, assigning an overall Group 3 classification (i.e., not classifiable as to its carcinogenicity to humans) (IARC, 1999). Until recently, quercetin has been marketed in the United States primarily as a dietary supplement (Theoharides and Bielory, 2004), with recommended daily doses of supplemental quercetin of 200–1200 mg (PDRNS, 2001). In 2004, food-grade quercetin of high purity (i.e., 98.5% and higher, and marketed as QU 985, QU 995, QU 998, and QU 1000) was self-affirmed as generally recognized as safe for use in a variety of foods at use-levels in the range of 0.008–0.5% or 10–125 mg/serving (Quercegen Pharma, LLC (personal communication), 2004). Based on the specified use-levels of quercetin in foods such as breakfast cereals, chewing gum, fats and oils, frozen dairy desserts and mixes, grain products and pastas, hard and soft candies, milk and plant protein products, beverages and beverage bases, and processed fruits and fruit juices, it was calculated that under a worst-case scenario of estimating intake, a heavy-end consumer of quercetin (90th percentile) would not be exposed to more than 4.70 mg quercetin/kg body weight/day (226 mg quercetin/person/day) from the intended addition of quercetin to foods. In Japan, quercetin is permitted as a food additive under the List of Existing Food Additives (MHLW, 1996).
Because of the prevalence of quercetin in the diet and its potential clinical and food applications, the safety of quercetin has been evaluated extensively in a variety of genotoxicity assays and a full range of acute, subchronic, chronic, and reproductive toxicity studies. In an attempt to reconcile the differences observed between in vitro results demonstrating quercetin-related mutagenic activity and the absence of carcinogenicity in vivo, several reviews of some of the data available for quercetin were conducted. Most recently, Okamoto (2005) provided an extensive overview of the safety data available for quercetin. Presently, the bioavailability and anti-oxidant properties of quercetin appear to be two areas of intense research. Specifically, validation of quercetin as a potent anti-oxidant in vivo, but also realizing its potential for pro-oxidant activity following oxidation, are of prime interest in an effort to determine its clinical applicability and acceptability for use in food. Moreover, the pro-oxidant state of quercetin, as a consequence of its potent anti-oxidant activity, may provide insight into its apparent in vitro mutagenicity. Regardless, the available data suggest that in vivo protective mechanisms adequately limit any potential for adverse effects related to quercetin pro-oxidant activity.
The present review is a critical evaluation of the safety of quercetin, which expands on some of the points presented by Okamoto (2005) and considers additional data to further support the absence of dietary quercetin-related carcinogenicity in vivo. Searches of several scientific literature databases (e.g., PubMed, MEDLINE®, EMBASE®, and BIOSIS Previews®) were conducted through January 2007 and only papers related to mechanism of action, metabolic fate, genotoxicity and mutagenicity, potential short- and long-term animal toxicity and carcinogenicity, and human safety were selected for inclusion in this review. The implications of the in vitro results and their usefulness in determining the potential for quercetin toxicity in vivo, in light of the largely negative results obtained in animal studies, are assessed, with special emphasis placed on the pro-oxidant properties of quercetin as a potential mechanism for its in vitro mutagenicity.
Section snippets
Natural dietary occurrence of quercetin
Flavonols occur ubiquitously in the human diet as glycosides, with wide distribution in the edible portions of food plants, including berries, citrus, and various other fruits, leafy vegetables, roots, tubers and bulbs, herbs and spices, legumes, and cereal grains, as well as in tea and cocoa (Brown, 1980). Fruits and vegetables, particularly apples, cranberries, blueberries, and onions, are the primary sources of naturally-occurring dietary quercetin of the typical Western diet and contain the
Biological properties of quercetin
As cited by Middleton et al., 2000, Erlund, 2004, anti-oxidant, anti-carcinogenic, anti-inflammatory, and cardioprotective properties are several key biological functions ascribed to quercetin. Middleton et al. (2000) stressed the anti-carcinogenic properties of quercetin and other flavonoids. Galati and O’Brien (2004) also reviewed the ability of certain flavonoids to prevent tumor development and also raised the possibility of flavonoid–drug interactions. It remains to be determined whether
Absorption and metabolism
Since the potential toxicity of quercetin, as well as any of its putative beneficial pharmacological effects are largely dependent on its bioavailability following oral administration, the absorption, distribution, metabolism, and excretion of quercetin have been extensively studied in laboratory animals and humans. As depicted in Fig. 2, quercetin may be O-methylated, primarily resulting in the formation of 3′-O-methylquercetin (isorhamnetin) and to a smaller extent, 4′-O-methylquercetin
In vitro genotoxicity studies
In vitro, quercetin consistently tested positive for mutagenic activity in most standard strains of Salmonella typhimurium (Bjeldanes and Chang, 1977, Hardigree and Epler, 1978, Seino et al., 1978, Brown and Dietrich, 1979, Ochiai et al., 1984, Stoewsand et al., 1984, Ueno et al., 1984, Hatcher and Bryan, 1985, Busch et al., 1986, Rueff et al., 1986, Rueff et al., 1992, Crebelli et al., 1987, Schimmer et al., 1988, Nguyen et al., 1989, Vrijsen et al., 1990, NTP, 1992, Czeczot, 1994, Cross et
Clinical and epidemiological studies related to the safety of quercetin
Although limited, clinical data relating to the safety of quercetin were available from a number of randomized, double blind clinical tolerance and efficacy studies (Ferry et al., 1996, Shoskes et al., 1999, Kiesewtter et al., 2000, Lozoya et al., 2002). In studies in which quercetin or plant extracts containing quercetin glycosides were provided to study subjects for oral ingestion for periods of up to 12 weeks at dose levels ranging between 3 and 1000 mg quercetin/day, no compound-related
Discussion
Interest in the use of quercetin in various food applications has been increasing due to its potential anti-oxidant and other beneficial health properties; however, the toxicity profile for quercetin is complicated by consistent findings of positive mutagenic effects in vitro and reports of positive findings in two carcinogenicity studies (Pamukcu et al., 1980, NTP, 1992). Numerous additional animal studies have consistently failed to demonstrate any relationship between quercetin
Conclusion
From a critical evaluation of the available literature on the biological effects of quercetin, including data related to safety, it may be concluded that quercetin, at estimated dietary intake levels, would not produce adverse health effects.
Acknowledgements
The authors greatly appreciate the helpful comments provided by Prof. I.M. Rietjens, Dr. T.C. Theoharides, and Dr. M. Ono during the preparation of this review and also wish to thank the Defense Advanced Research Projects Agency (DARPA) and Dr. B. Giroir, Dr. K. DeMarco, and Dr. S. Kayar for their efforts and support.
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