Exercise-Induced Oxidative Stress, Muscle Damage and Inflammation in Prolonged High-Intensity Intermittent Exercise: Effect of Quercetin
Holland, Camilla Rebecca
Soccer is characterised by prolonged periods of high-intensity intermittent exercise, the metabolic and mechanical demands of which can induce oxidative stress, skeletal muscle damage, and associated inflammation as part of the adaptive response to exercise. However, the negative effects of these responses include fatigue and muscle soreness, which can compromise recovery and impair performance. There is paucity in the literature regarding these phenomena following intensified periods of soccer, such as microcycles of a competitive in-season, where athletes undertake daily training sessions and competitive matches with minimal recovery periods. Therefore, study one explored associations between oxidative stress (assessed via urinary MDA, a biomarker of lipid peroxidation) and high-intensity training load (GPS & HR derived variables) in a cohort of professional soccer players throughout microcycles of a competitive in-season. Results showed that urinary MDA decreased significantly over the season, but was not associated with high-intensity training load. Furthermore, lipid peroxidation was lower in professional soccer players when compared to recreational players. The observed progressive reduction in lipid peroxidation in professional soccer players may theoretically be explained as an adaptive response to regular participation in soccer training. To further increase understanding of the physiological demands imposed on athletes within a training microcycle, study two profiled the short-term response of biomarkers indicative of (i) oxidative stress, (ii) muscle damage, and (iii) inflammation, following a 3-day intensified period of simulated soccer in trained athletes; with results compared to a control group. The 3-day exercise protocol did not alter oxidative stress but induced transient cellular damage, which was accompanied by an inflammatory response, evidenced by significant post-exercise increases in (i) CK, (ii) LDH, (iii) CRP, (iv) IL-6, (v) MCP-1, (vi) total leukocyte and neutrophil counts. These findings may have negative implications for performance, as recovery may be incomplete prior to subsequent sessions; strategies that facilitate recovery by counteracting muscle damage and inflammation may be beneficial during this time. Quercetin supplementation has been proposed as a nutritional recovery strategy that could benefit athletes during intensified periods of exercise, as it is postulated that quercetin has antioxidant, anti-inflammatory, and analgesic properties. Studies investigating the use of quercetin in intermittent exercise have been limited; therefore, study three expanded on study two, by examining the efficacy of prolonged quercetin supplementation to attenuate exercise-induced cellular damage and associated inflammation. For practical application, measures of perceptual recovery were also determined. Results revealed quercetin supplementation to be ineffective in alleviating (i) cellular damage, (ii) inflammation, (iii) perceived fatigue, and (iv) muscle soreness, in comparison to a placebo. Thus, the use of quercetin to improve recovery by counteracting cellular damage and inflammation during intensified periods of exercise characteristic of soccer, appears not to be supported. The NF-κB classical pathway was studied to elucidate underlying mechanisms of exercise-induced inflammation but findings revealed no effect of the 3-day exercise protocol, or quercetin supplementation, on NF-κB activity. In conclusion, results showed a progressive reduction in a biomarker of lipid peroxidation throughout a competitive in-season in professional soccer players, but no association between lipid peroxidation and high-intensity training load. Interestingly, a 3-day intensified period of simulated soccer induced acute transient cellular damage and inflammation but no alterations in oxidative stress. Therefore, this thesis provides novel data regarding intensified periods of soccer and simulated soccer. Moreover, the results add to the current literature debating the use of antioxidant supplementation in athletes, providing no significant evidence to support the use of quercetin supplementation as a recovery strategy. An overarching theme of this thesis was biomarker monitoring, which could be used to help sports scientists/coaches prescribe training loads and recovery strategies that negate the undesirable effects of exercise-induced muscle damage and associated inflammation in susceptible athletes when recovery time is limited.
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