In vitro and some animal models have shown that quercetin, a polyphenol derived from plants, has a wide range of biological actions including anti-carcinogenic, anti-inflammatory and antiviral activities; as well as attenuating lipid peroxidation, platelet aggregation and capillary permeability. This review focuses on the physicochemical properties, dietary sources, absorption, bioavailability and metabolism of quercetin, especially main effects of quercetin on inflammation and immune function. According to the results obtained both in vitro and in vivo, good perspectives have been opened for quercetin. Nevertheless, further studies are needed to better characterize the mechanisms of action underlying the beneficial effects of quercetin on inflammation and immunity.

Objective: The aim of this study was to evaluate associations between flavonoid intake at dinner versus breakfast and the odds of non-alcoholic fatty liver disease (NAFLD) using data from the National Health and Nutrition Examination Survey (NHANES). Methods: According to total flavonoids, flavonoids intake at breakfast and dinner, or flavonoids intake differences at dinner and breakfast (Δ = dinner-breakfast), we divided 3240 participants from the NHANES (2007–2010) into five groups. Logistic regression analyses were performed to examine associations between total flavonoids, flavonoid intake at breakfast and dinner or Δ flavonoids and NAFLD after the adjustment of several confounding factors. Dietary substitution models were used to estimate the association between replacing 5% of flavonoid intake at dinner with that at breakfast and the odds of NAFLD. Results: For total flavonoids intake and flavonoids intake at breakfast, when compared to the lowest quintile (Q1), participants in the highest quintile of total flavonoids intake had lower odds of NAFLD, with an adjusted odds ratio (OR) for NAFLD of 0.78 (95% confidence interval (CI): 0.61–0.98) and a p-trend of 0.076. Participants in the highest quintile of flavonoid intake at breakfast had an adjusted OR for NAFLD of 0.87 (95% CI: 0.69–1.09) and a p-trend of 0.035, also showing a trend toward lower odds of NAFLD. Conversely, for Δ flavonoids, participants in the highest quintile had higher odds of NAFLD, with an adjusted OR for NAFLD of 1.27 (95% CI: 1.01–1.60) and a p-trend of 0.093. When total flavonoid intake in participants remained constant, replacing 5% of dietary flavonoid intake at dinner with that at breakfast was associated with 7% lower odds of NAFLD. Conclusions: Higher flavonoid intake at breakfast than at dinner may be associated with lower odds of NAFLD. Future research should focus on the potential role of breakfast-timed flavonoid intake in preventing the onset of NAFLD.