['Quercetin bioavailability exhibits high interindividual variation for reasons that remain unclear. We conducted a 24-h pharmacokinetic study to investigate whether individual differences in circulating antioxidants, oxidative stress and inflammation, and intestinal permeability affect quercetin bioavailability.', 'Healthy adults (n\xa0=\xa09 M/7 F; 34.3 ± 4.5 y; 27.0 ± 1.7 kg/m(2)) ingested 1095 mg quercetin aglycone with a standardized meal. Plasma antioxidants, biomarkers of oxidative stress and inflammation, and endotoxin were measured at baseline (0 h), and quercetin and its methylated metabolites isorhamnetin and tamarixetin were measured at timed intervals for 24 h.', 'Plasma pharmacokinetics of quercetin, isorhamnetin and tamarixetin were highly variable between participants (CVinter = 37-96%). Plasma vitamin C concentrations (34.6 ± 2.5 μmol/L), but no other antioxidants, were inversely correlated to the Cmax and AUC0 to 24 h of total quercetin (Qtotal; sum of quercetin, isorhamnetin and tamarixetin; r = -0.52 to -0.53; P < 0.05). Plasma endotoxin (0.13 ± 0.01 EU/mL), a surrogate marker of intestinal permeability, was correlated to Qtotal Cmax (r = 0.45; P < 0.05) and tended to be correlated to Qtotal AUC0 to 24 h (r = 0.38; P = 0.07). Additionally, vitamin C was inversely related to C-reactive protein, myeloperoxidase, and endotoxin (r = -0.46 to -0.55; P < 0.05), whereas endotoxin was positively correlated to C-reactive protein (r = 0.73; P < 0.05).'] Is quercetin bioavailability associated with inadequate plasma vitamin C status and greater plasma endotoxin in adults?