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ABSTRACT: Quercetin exerts strong anti-oxidative, anti-inflammatory, anti-pathogenic, and immune regulatory effects in vitro and in animal-based studies. Epidemiologic data indicate reduced rates of cardiovascular disease and various types of cancer in groups self-selecting diets high in quercetin. Several recent quercetin supplementation studies in human athletes have focused on potential influences as a countermeasure to post-exercise inflammation, oxidative stress, and immune dysfunction, in improving endurance performance, and in reducing illness rates following periods of physiologic stress. When quercetin supplementation is combined with other polyphenols and food components such as green tea extract, isoquercetin, and fish oil, a substantial reduction in exercise-induced inflammation and oxidative stress occurs in athletes, with chronic augmentation of innate immune function. Quercetin supplementation (1,000 mg/day for two to three weeks) also reduces illness rates in exercise-stressed athletes. Animal studies support a role for quercetin as an exercise mimetic for mitochondrial biogenesis, and recent data in untrained human subjects indicate modest enhancement in skeletal muscle mitochondrial density and endurance performance. Quercetin has multiple bioactive effects that support athletic endeavor, and research continues to better define optimal dosing regimens and adjuvants that amplify these influences.
KEY WORDS: Exercise, flavonoids, immune function, inflammation, oxidative stress
INTRODUCTION
Polyphenols are a large class of colorful, plant-based, phenolic organic compounds. Flavonoids, a polyphenolic subgroup, comprise several thousand compounds classified into five subgroups (USDA Nutrient Data Laboratory, 2007). One of these subgroups, the flavonols, includes quercetin, a widely distributed and investigated flavonoid.
Food sources for quercetin include tea, onions, apples, peppers, blueberries, and dark green vegetables (USDA Nutrient Data Laboratory, 2007; Chun et al., 2007) (Figure 1). Quercetin accumulates in the outer and aerial tissues (skin and leaves) because its biosynthesis is stimulated by light. Flavonol intake is only about 13 mg/day for U.S. adults, with quercetin representing three-fourths of this amount (Chun et al., 2007). Human subjects can absorb significant amounts of quercetin from food or supplements, and elimination is quite slow, with a reported halflife ranging from 11 to 28 h (Conquer et al., 1998; Edwards et al., 2007; Egert et al., 2008, 2009; Manach et al., 2005).
Quercetin supplementation in both animal and human studies does not cause adverse symptoms or harmful physiologic effects (Harwood et al., 2007; Knab et al., 2010; Utesch et...