Science behind Quercetin not as promising as marketing claims

Quercetin is a type of plant-based chemical, or phytochemical, known as a flavonoids that give many fruits, flowers, and vegetables their color. It is found in many plants and foods, such as red wine, citrus fruits, onions, green tea, apples, berries, and many other. Quercetin is also available as a dietary supplement and is often packaged with bromelain (an enzyme found in pineapple) because both are anti-inflammatories [1]. It is one of the most prominent antioxidants.

Natural Sources

Quercetin can be found in wide variety of natural foods, such as nuts, apples, grapes, onions, berries, broccoli and black tea [9].

Bioavailability

It was initialy thought that quercetin has low bioavailability, however later studies reported that it can be detected in plasma 15-30 mins after ingestion, reaches peaks concentrations after 120-180 mins and returns to baseline after 24 hours [9].

Quercetin Supplementation in Athletes

It is added to sport supplements with claims that is a powerful antioxidant and has positive effects on exercise endurance with faster recovery.

Several laboratory studies show quercetin may have anti-inflammatory properties [1,3]. Quercetin has been demonstrated to be a potent antioxidant in vitro but this effect was absent in vivo [4].

A study by researchers from Appalachian State University showed that chronic quercetin ingestion does not exert protection from exercise-induced oxidative stress and inflammation. Furthermore, findings in study conducted on ultramarathon runners suggested that oral quercetin supplementation does not alter blood plasma lipid, antioxidant capacity or oxidative damage [5]. A double-blind, randomized, placebo controlled trial on sixty-three runners who were ingesting 1000 mg of Quercetin a day for three weeks failed to attenuate muscle damage, inflammation, increase plasma cytokine or hormone levels [6]. A single, very high dose (2000 mg) also failed to improve exercise performance in moderately fit soldiers in heat [13]. In 60 male athletes, 8 week-long supplementation with quercetin and vitamin C did not improve exercise performance but it did however reduce muscle damage and body fat percentage [14]. In contrast to these studies, MacRae and Mefferd [10] reported that administration of 1200 mg of quercetin daily for 6 weeks significantly improved high-intensity cycling time trial performance through enhancement of power output in elite cyclists. Furthermore, Davis et al. [11] reported that 7 days of quercetin (1000 mg) supplementation increased VO_2max  and time to fatigue on bicycle ergometer in untrained men and women. Meta-analysis published in 2013 [12] reported that quercetin supplementation improves endurance performance by 0,74 ± 1,04% compared with placebo.

It can be concluded that quercetin in unlikely to improve athletic performance, at least not at studied doses.

Other Possible Uses

Quercetin has also been promoted as being effective against a wide variety of diseases, including cancer. According to cancer.org [7]: “While some early lab results appear promising, as of yet there is no reliable clinical evidence that quercetin can prevent or treat cancer in humans.”

Quercetin Side Effects and Risks

Quercetin is generally considered safe. It may cause headache, stomach upset, [1] and tingling of the arms and legs [8]. Very high doses might cause kidney damage (reported when given intravenously) [8]. There have also been some reports of nausea when supplements were taken in high doses [7].

(Other common names: Bioflavonoid, Bioflavonoid Complex, Bioflavonoid Concentrate, Bioflavonoid Extract, 3,3′,4’5,7-Penthydroxyflavone, Quercetina, Citrus Bioflavonoid Extract, Citrus Flavones, Citrus Flavonoids, Sophoretin, Xanthaurine, Meletin, Quercetine)

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References

1. University of Maryland Medical Center (UMMC) – Quercetin. Retrieved from www.umm.edu at 6. May 2013
2. Stewart, Laura K., et al. “Qurcetin transiently increases energy expenditure but persistently decreases circulating markers of inflammation in C57BL/6J mice fed a high-fat diet.” Metabolism: clinical and experimental 57.7 Suppl 1 (2008): S39.
3. Davis, J. Mark, et al. “Quercetine increases brain and muscle mitochondrial biogenesis and exercise tolerance.” American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 296.4 (2009): R1071-R1077.
4. McAnulty, Steven R., et al. “Chronic quercetiin ingestion and exercise-induced oxidative damage and inflammation.” Applied Physiology, Nutrition, and Metabolism 33.2 (2008): 254-262.
5. Quindry, John C., et al. “Oral quercti supplementation and blood oxidative capacity in response to ultramarathon competition.” International journal of sport nutrition and exercise metabolism 18.6 (2008): 601-616.
6. Nieman, David C., et al. “Quecetine ingestion does not alter cytokine changes in athletes competing in the Western States Endurance Run.” Journal of Interferon & Cytokine Research 27.12 (2007): 1003-1012.
7. American Cancer Society, Inc. Retrieved from www.cancer.org at 6. May 2013
8. Find a Vitamin or Supplement – Quercetine. Retrieved from WebMD.com at 6. May 2013
9. Boots, Agnes W., Guido RMM Haenen, and Aalt Bast. “Health effects of quercetinn: from antioxidant to nutraceutical.” European journal of pharmacology 585.2 (2008): 325-337.
10. MacRae, Holden SH, and Kari M. Mefferd. “Dietary antioxidant supplementation combined with quercetni improves cycling time trial performance.” International journal of sport nutrition and exercise metabolism 16.4 (2006): 405-419.
11. Davis, J. Mark, et al. “The dietary flavonoid quercetin increases VO2max and endurance capacity.” International journal of sport nutrition and exercise metabolism 20.1 (2010): 56-62.
12. Pelletier, Denis M., Guillaume Lacerte, and Eric DB Goulet. “Effects of quercetin supplementation on endurance performance and maximal oxygen consumption: a meta-analysis.” International journal of sport nutrition and exercise metabolism 23.1 (2013): 73-82.
13. Cheuvront, Samuel N., et al. “No effect of nutritional adenosine receptor antagonists on exercise performance in the heat.” American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 296.2 (2009): R394-R401.
14. Askari, Gholamreza, et al. “Does quercetin and vitamin C improve exercise performance, muscle damage, and body composition in male athletes?.” Journal of Research in Medical Sciences 17.4 (2012).