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Longevity Conferences 2023
January 19, 2023
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Quercetin: a protective flavonoid
Onions, in particular, are regarded as the most important quercetin source in the human diet. Its possible uses may be preventing cardiovascular diseases, diabetes, viral infecti
Compound description
Quercetin is a naturally occurring flavonoid compound, first derived from the oak tree (Quercetus in Latin). Flavonoids are a class of polyphenols (compounds that contain multiple phenol groups) that form in plants Due to their chemical structure that potentially enables them to quench free radicals, polyphenols sparked interest in their antioxidant and anti-inflammatory properties.
Quercetin is present (mainly in the form of glycosides) in multiple fruits (apples, grapes) and vegetables (peppers, onions), as well in other food items, such as wine and tea (1). Onions, in particular, are regarded as the most important quercetin source in the human diet. Estimated dietary quercetin intake ranges from 3 to 40 mg/daily in Western diets but can reach up to 250 mg/daily in the active fruits and vegetable consumers (2).
Properties
Multiple in vitro studies demonstrated numerous biological effects of quercetin: antioxidant, anti-inflammatory, immunoprotective, and anticarcinogenic. This discovery led to the discussions of quercetin's possible uses in preventing cardiovascular diseases, diabetes, viral infections, or cancers (3). However, in the human and animal trials, quercetin (similarly to other polyphenols) showed a much more moderate effect for many of its biological activities than in vitro. The main reason behind this is its low oral bioavailability. Another possible explanation is that in vitro experiments use the aglycone form of quercetin (which is not present in the diet).
Use as a supplement
However, some effects received confirmation, though they might be hard to systematize. In dentistry, quercetin or quercetin-doped treatments, when applied locally, showed consistent anti-inflammatory and antibacterial effects (4). Supplementation with quercetin reduced mortality rate and average viral load during lower respiratory tract infections across 11 animal studies (5). As for promises of cancer prevention, some meta-analyses (6,7) confirmed a strong negative correlation between quercetin consumption and cancer incidence. The results are not straightforward as, for example, quercetin intake significantly reduces cancer risks in smokers while not affecting non-smokers (6). Also, it is essential to note that these effects are observed either in the case of high-dose (preferably intravenous) treatment or dietary intake (where an additional protective effect might be obtained from other compounds) (8). Promising results have been obtained for quercetin as a supporting therapy in ovarian cancer (9), but more research is needed to address the bioavailability problem and unravel the details of the mechanism behind. Studies of the anti-inflammatory properties of quercetin show inconclusive and contradictory results while supporting some putative benefits for chronically ill patients (10). Positive effects of quercetin consumption were also observed in lowering blood pressure (at doses > 500 mg/day) (11), glucose levels in diabetics (12), and low-density lipids in obese individuals (13).
As in most cases, levels of quercetin obtained through diet are insufficient to observe a significant effect; it has been popularly used as a supplement. However, due to the problem of bioavailability and quick metabolism of large doses, the focus shifted from using quercetin as a standalone supplementation to combining it with other flavonoids. When combined, the flavonoids exhibit a synergistic effect, which allows them to obtain desirable outcomes and maintain a lower dosage (8). When used as an anti-aging supplement, quercetin is frequently combined with resveratrol to achieve a better effect.
Side effects
In 2010, the American Food and Drug Administration confirmed that high-purity quercetin is generally recognized as safe. A mean intake of 200 mg/daily is considered safe for all age groups (14). In Canada, the maximum daily dose of quercetin is limited to 1200 mg/daily.
For a use exceeding 12 weeks, it is highly recommended to consult your healthcare professional as the data on long-term safety is limited. Additional caution applies to pregnant and breastfeeding women. In general, oral intake in humans seems to be well-tolerated, and only a very low incidence of adverse effects has been observed up to date.
In our Marketplace, you can find pure quercetin from the DoNotAge vendor with a recommended dosage of 800 mg/serving (two capsules).
References
1. D’Andrea G. Quercetin: A flavonol with multifaceted therapeutic applications? Fitoterapia. 2015 Oct;106:256–71.
2. Lin J, Zhang SM, Wu K, Willett WC, Fuchs CS, Giovannucci E. Flavonoid Intake and Colorectal Cancer Risk in Men and Women. Am J Epidemiol. 2006 Oct 1;164(7):644–51.
3. ElAttar TM, Virji AS. Modulating effect of resveratrol and quercetin on oral cancer cell growth and proliferation: Anticancer Drugs. 1999 Feb;10(2):187–94.
4. Yang H, Li K, Yan H, Liu S, Wang Y, Huang C. High-performance therapeutic quercetin-doped adhesive for adhesive–dentin interfaces. Sci Rep. 2017 Dec;7(1):8189.
5. Brito JCM, Lima WG, Cordeiro LPB, Cruz Nizer WS. Effectiveness of supplementation with quercetin‐type flavonols for treatment of viral lower respiratory tract infections: Systematic review and meta‐analysis of preclinical studies. Phytother Res. 2021 Sep;35(9):4930–42.
6. Woo HD, Kim J. Dietary Flavonoid Intake and Smoking-Related Cancer Risk: A Meta-Analysis. Davis KR, editor. PLoS ONE. 2013 Sep 19;8(9):e75604.
7. Chang H, Lei L, Zhou Y, Ye F, Zhao G. Dietary Flavonoids and the Risk of Colorectal Cancer: An Updated Meta-Analysis of Epidemiological Studies. Nutrients. 2018 Jul 23;10(7):950.
8. Russo M, Spagnuolo C, Tedesco I, Bilotto S, Russo GL. The flavonoid quercetin in disease prevention and therapy: Facts and fancies. Biochem Pharmacol. 2012 Jan;83(1):6–15.
9. Vafadar A, Shabaninejad Z, Movahedpour A, Fallahi F, Taghavipour M, Ghasemi Y, et al. Quercetin and cancer: new insights into its therapeutic effects on ovarian cancer cells. Cell Biosci. 2020 Dec;10(1):32.
10. Ou Q, Zheng Z, Zhao Y, Lin W. Impact of quercetin on systemic levels of inflammation: a meta-analysis of randomised controlled human trials. Int J Food Sci Nutr. 2020 Feb 17;71(2):152–63.
11. Serban M, Sahebkar A, Zanchetti A, Mikhailidis DP, Howard G, Antal D, et al. Effects of Quercetin on Blood Pressure: A Systematic Review and Meta‐Analysis of Randomized Controlled Trials. J Am Heart Assoc. 2016 Jul 6;5(7):e002713.
12. Bule M, Abdurahman A, Nikfar S, Abdollahi M, Amini M. Antidiabetic effect of quercetin: A systematic review and meta-analysis of animal studies. Food Chem Toxicol. 2019 Mar;125:494–502.
13. Guo W, Gong X, Li M. Quercetin Actions on Lipid Profiles in Overweight and Obese Individuals: A Systematic Review and Meta-Analysis. Curr Pharm Des. 2019 Oct 21;25(28):3087–95.
14. Andres S, Pevny S, Ziegenhagen R, Bakhiya N, Schäfer B, Hirsch-Ernst KI, et al. Safety Aspects of the Use of Quercetin as a Dietary Supplement. Mol Nutr Food Res. 2018 Jan;62(1):1700447.
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Compound description
Quercetin is a naturally occurring flavonoid compound, first derived from the oak tree (Quercetus in Latin). Flavonoids are a class of polyphenols (compounds that contain multiple phenol groups) that form in plants Due to their chemical structure that potentially enables them to quench free radicals, polyphenols sparked interest in their antioxidant and anti-inflammatory properties.
Quercetin is present (mainly in the form of glycosides) in multiple fruits (apples, grapes) and vegetables (peppers, onions), as well in other food items, such as wine and tea (1). Onions, in particular, are regarded as the most important quercetin source in the human diet. Estimated dietary quercetin intake ranges from 3 to 40 mg/daily in Western diets but can reach up to 250 mg/daily in the active fruits and vegetable consumers (2).
Properties
Multiple in vitro studies demonstrated numerous biological effects of quercetin: antioxidant, anti-inflammatory, immunoprotective, and anticarcinogenic. This discovery led to the discussions of quercetin's possible uses in preventing cardiovascular diseases, diabetes, viral infections, or cancers (3). However, in the human and animal trials, quercetin (similarly to other polyphenols) showed a much more moderate effect for many of its biological activities than in vitro. The main reason behind this is its low oral bioavailability. Another possible explanation is that in vitro experiments use the aglycone form of quercetin (which is not present in the diet).
Use as a supplement
However, some effects received confirmation, though they might be hard to systematize. In dentistry, quercetin or quercetin-doped treatments, when applied locally, showed consistent anti-inflammatory and antibacterial effects (4). Supplementation with quercetin reduced mortality rate and average viral load during lower respiratory tract infections across 11 animal studies (5). As for promises of cancer prevention, some meta-analyses (6,7) confirmed a strong negative correlation between quercetin consumption and cancer incidence. The results are not straightforward as, for example, quercetin intake significantly reduces cancer risks in smokers while not affecting non-smokers (6). Also, it is essential to note that these effects are observed either in the case of high-dose (preferably intravenous) treatment or dietary intake (where an additional protective effect might be obtained from other compounds) (8). Promising results have been obtained for quercetin as a supporting therapy in ovarian cancer (9), but more research is needed to address the bioavailability problem and unravel the details of the mechanism behind. Studies of the anti-inflammatory properties of quercetin show inconclusive and contradictory results while supporting some putative benefits for chronically ill patients (10). Positive effects of quercetin consumption were also observed in lowering blood pressure (at doses > 500 mg/day) (11), glucose levels in diabetics (12), and low-density lipids in obese individuals (13).
As in most cases, levels of quercetin obtained through diet are insufficient to observe a significant effect; it has been popularly used as a supplement. However, due to the problem of bioavailability and quick metabolism of large doses, the focus shifted from using quercetin as a standalone supplementation to combining it with other flavonoids. When combined, the flavonoids exhibit a synergistic effect, which allows them to obtain desirable outcomes and maintain a lower dosage (8). When used as an anti-aging supplement, quercetin is frequently combined with resveratrol to achieve a better effect.
Side effects
In 2010, the American Food and Drug Administration confirmed that high-purity quercetin is generally recognized as safe. A mean intake of 200 mg/daily is considered safe for all age groups (14). In Canada, the maximum daily dose of quercetin is limited to 1200 mg/daily.
For a use exceeding 12 weeks, it is highly recommended to consult your healthcare professional as the data on long-term safety is limited. Additional caution applies to pregnant and breastfeeding women. In general, oral intake in humans seems to be well-tolerated, and only a very low incidence of adverse effects has been observed up to date.
In our Marketplace, you can find pure quercetin from the DoNotAge vendor with a recommended dosage of 800 mg/serving (two capsules).
References
1. D’Andrea G. Quercetin: A flavonol with multifaceted therapeutic applications? Fitoterapia. 2015 Oct;106:256–71.
2. Lin J, Zhang SM, Wu K, Willett WC, Fuchs CS, Giovannucci E. Flavonoid Intake and Colorectal Cancer Risk in Men and Women. Am J Epidemiol. 2006 Oct 1;164(7):644–51.
3. ElAttar TM, Virji AS. Modulating effect of resveratrol and quercetin on oral cancer cell growth and proliferation: Anticancer Drugs. 1999 Feb;10(2):187–94.
4. Yang H, Li K, Yan H, Liu S, Wang Y, Huang C. High-performance therapeutic quercetin-doped adhesive for adhesive–dentin interfaces. Sci Rep. 2017 Dec;7(1):8189.
5. Brito JCM, Lima WG, Cordeiro LPB, Cruz Nizer WS. Effectiveness of supplementation with quercetin‐type flavonols for treatment of viral lower respiratory tract infections: Systematic review and meta‐analysis of preclinical studies. Phytother Res. 2021 Sep;35(9):4930–42.
6. Woo HD, Kim J. Dietary Flavonoid Intake and Smoking-Related Cancer Risk: A Meta-Analysis. Davis KR, editor. PLoS ONE. 2013 Sep 19;8(9):e75604.
7. Chang H, Lei L, Zhou Y, Ye F, Zhao G. Dietary Flavonoids and the Risk of Colorectal Cancer: An Updated Meta-Analysis of Epidemiological Studies. Nutrients. 2018 Jul 23;10(7):950.
8. Russo M, Spagnuolo C, Tedesco I, Bilotto S, Russo GL. The flavonoid quercetin in disease prevention and therapy: Facts and fancies. Biochem Pharmacol. 2012 Jan;83(1):6–15.
9. Vafadar A, Shabaninejad Z, Movahedpour A, Fallahi F, Taghavipour M, Ghasemi Y, et al. Quercetin and cancer: new insights into its therapeutic effects on ovarian cancer cells. Cell Biosci. 2020 Dec;10(1):32.
10. Ou Q, Zheng Z, Zhao Y, Lin W. Impact of quercetin on systemic levels of inflammation: a meta-analysis of randomised controlled human trials. Int J Food Sci Nutr. 2020 Feb 17;71(2):152–63.
11. Serban M, Sahebkar A, Zanchetti A, Mikhailidis DP, Howard G, Antal D, et al. Effects of Quercetin on Blood Pressure: A Systematic Review and Meta‐Analysis of Randomized Controlled Trials. J Am Heart Assoc. 2016 Jul 6;5(7):e002713.
12. Bule M, Abdurahman A, Nikfar S, Abdollahi M, Amini M. Antidiabetic effect of quercetin: A systematic review and meta-analysis of animal studies. Food Chem Toxicol. 2019 Mar;125:494–502.
13. Guo W, Gong X, Li M. Quercetin Actions on Lipid Profiles in Overweight and Obese Individuals: A Systematic Review and Meta-Analysis. Curr Pharm Des. 2019 Oct 21;25(28):3087–95.
14. Andres S, Pevny S, Ziegenhagen R, Bakhiya N, Schäfer B, Hirsch-Ernst KI, et al. Safety Aspects of the Use of Quercetin as a Dietary Supplement. Mol Nutr Food Res. 2018 Jan;62(1):1700447.
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Dr. Ana Baroni MD. Ph.D.
Scientific & Medical Advisor
Quality Garant
Ana has over 20 years of consultancy experience in longevity, regenerative and precision medicine. She has a multifaceted understanding of genomics, molecular biology, clinical biochemistry, nutrition, aging markers, hormones and physical training. This background allows her to bridge the gap between longevity basic sciences and evidence-based real interventions, putting them into the clinic, to enhance the healthy aging of people. She is co-founder of Origen.life, and Longevityzone. Board member at Breath of Health, BioOx and American Board of Clinical Nutrition. She is Director of International Medical Education of the American College of Integrative Medicine, Professor in IL3 Master of Longevity at Barcelona University and Professor of Nutrigenomics in Nutrition Grade in UNIR University.
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