There is increased interest among athletes to use quercetin to enhance performance and improve health, but not all studies support this benefit. In vitro en animal studies indicate that quercetin has unique biological properties that are likely to improve performance and reduce infection risk during intense exercise. Quercetin stimulates mitochondrial biogenesis and has anti-inflammatory, antioxidant and psycho-stimulant activity.
A study by the Appalachian State University, Boone, USA, investigated the effect of quercetin with or without epigallocatechin 3-gallate on 39 trained cyclists. They received the phytochemicals 2 weeks before, during and 3 days after the 3 day period of exercise. The researchers found that a combination of quercetin and epigallocatechin 3-gallate increased the granulocyte oxidative burst activity and decreased after the exercise the levels of C-reactive protein (CRP), and plasma interleukin 6 (IL-6) and interleukin 10 (IL-10). They concluded that a 2 week quercetin supplementation and epigallocatechin 3-gallate effectively increased granulocyte oxidative burst activity and reduced inflammation, 3 days after a heavy exertion in trained cyclists.
Davis and co-workers at the University of South Carolina demonstrated the performance benefits of quercetin on fitness without exercise training . Previous in vitro studies demonstrated an improved mitochondrial biogenesis after treatment with quercetin. Davis and co-workers were the first to test the in-vivo effects of quercetin on mitochondrial biogenesis exercise tolerance. They examined the effects of quercetin feedings in mice on markers of mitochondrial biogenesis and on endurance exercise tolerance. Quercetin increased markers of mitochondrial biogenesis and improved endurance capacity and voluntary wheel-running activity. They concluded that quercetin may have important implications for enhancement of athletic and military performance.
Two studies were no able to demonstrate a protective effect of quercetin on exercise-induced oxidative stress. In a first study, Quindry and co-workers of Appalachian State University, USA, were not able to show any performance benefits of quercetin supplementation during an ultramarathon challenge . Ultramarathon exercise can result in blood oxidative stress and the supplementation with an antioxidant, such as quercetin, may be a potential countermeasure this oxidative stress. They tested the effect of quercetin in a double-blind experiment, involving 63 participants and found that quercetin supplementation did not affect race performance or trolox-equivalent antioxidant capacity. They concluded that oral quercetin supplementation does not alter blood plasma lipid or aqueous-phase antioxidant capacity or oxidative damage during an ultramarathon challenge. Another study by McAnulty and co-workers, conducted at the same university, concluded that quercetin ingestion does not exert protection from exercise-induced oxidative stress and inflammation . They treated 40 cyclists with 1000 mg quercetin or placebo each day for 6 weeks before and during 3 days of cycling. No effects of quercetin on oxidative or inflammation markers were observed.
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 Oral quercetin supplementation and blood oxidative capacity in response to ultramarathon competition.
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