2.1. Construction of the Diabetic Rat Model

Two-month-old Sprague–Dawley male rats were purchased from the Bengbu Medical College Experimental Animal Center (Bengbu, China). The diabetic rat model was established by high-fat feeding for 4 weeks and intraperitoneal injection of streptozotocin (STZ, 50 mg/kg). The successful establishment of the model was confirmed when the level of fasting blood glucose was over 11 mmol/L on 2 serial measurements or over 14 mmol/L in a single evaluation. The rats were divided into the normal (CON-Group), diabetic (DM-Group), interval dosing+diabetic (Qod-Group), and daily dosing+diabetic (Qd-Group) groups (n = 6). The quercetin stock solution was prepared according to the ratio of 50 mg quercetin dissolved in 1 mL DMSO solution. In the treatment group, the rats were fed quercetin at a dosage of 160 mg.kg⁻¹.d⁻¹. According to the body weight of each rat, an appropriate amount of quercetin stock solution was mixed with 5 mL of drinking water and fed to the rats by gavage. In the case of untreated groups, a mixture of DMSO and drinking water without quercetin was used as a control. The rats in each group were housed for 6 months under the same conditions. All animals were raised in the Bengbu Medical College Animal Experiment Center, separated in different cages according to experimental groups. The experimental environment was managed by the Animal Center: a room temperature of 26°C and 12 h light-dark cycles were maintained, and the breeding environment was cleaned regularly by professionals. All experimental procedures were approved by the Medical Ethics Committee of the First Affiliated Hospital of Bengbu Medical College (approval no. BYYFY-2021KY03).