Abstract: BACKGROUND: Overweight or obesity is the most significant risk factors for knee osteoarthritis. Time-restricted diet shows an effective effect in preventing overweight or obesity. Whether infrapatellar fat pad, an important component of the knee joint, is affected by time-restricted diet and thus influences knee osteoarthritis remains unclear.
OBJECTIVE: To employ a high-fat diet-induced rat model to investigate the effect of weight loss by time-restricted diet on infrapatellar fat pad, thereby providing evidence for early prevention and treatment of obesity-related knee osteoarthritis. 
METHODS: Fifteen male Sprague-Dawley rats were randomly divided into three groups (n=5 per group). Rats in the control group were allowed to eat at will for 24 hours and were fed the defined control diet (12% fat); rats in the high-fat diet group were allowed to eat at will for 24 hours and were fed the high-fat diet (45% fat); and rats in the high-fat diet with time-restricted diet group were fed the high-fat diet only from 9:00 (2 hours after the light) to 17:00 (2 hours before the dark). After 8 weeks of feeding, mDixon-Quant sequence was used to assess proton density fat fraction in the infrapatellar fat pad and subcutaneous adipose tissue in the right inguinal region. ELISA was used to quantify differences adipokine. Sirius red staining was used to evaluate changes in fibrosis of the infrapatellar fat pad. The expressions of uncoupling protein-1 in the infrapatellar fat pad and subcutaneous adipose tissue in the right inguinal region as well as leptin, adiponectin and tumor necrosis factor-α in the infrapatellar fat pad were detected by immunohistochemistry staining. 
RESULTS AND CONCLUSION: After 8 weeks of feeding, compared with the high-fat diet group, the body mass of rats in the control group (P=0.036) and the high-fat diet with time-restricted diet group (P=0.003) was significantly reduced. The proton density fat fraction in the infrapatellar fat pad in the high-fat diet group was significantly higher than that in the control group (P < 0.001) and the high-fat diet with time-restricted diet group (P=0.004), while there was no significant difference in the proton density fat fraction of the subcutaneous adipose tissue among the three groups. The serum leptin levels of rats in the high-fat diet group were significantly higher than those in the control group (P=0.030) and the high-fat diet with time-restricted diet group (P=0.018). Compared with the high-fat diet group, the infrapatellar fat pad fibrosis characterized by Sirius red staining in the control group (P < 0.001) and the time-restricted diet group (P=0.003) was significantly decreased. The expression of leptin in IFP of the high-fat diet group was significantly higher than that of the control group (P < 0.001) and the high-fat diet with time-restricted diet group (P < 0.001). The expression of adiponectin in the infrapatellar fat pad of the high-fat diet group was significantly lower than that of the control group (P=0.004) and the high-fat diet with time-restricted feeding group (P=0.048). However, there was no positive expression of uncoupling protein-1 in the infrapatellar fat pad and subcutaneous adipose tissue of all the three groups, and no positive expression of tumor necrosis factor-α in the infrapatellar fat pad. To conclude, time-restricted diet could retard the fibrosis of the infrapatellar fat pad, reduce the proton density fat fraction of the infrapatellar fat pad, and affect the level of adipokine in serum and infrapatellar fat pad. Time-restricted diet may become a simple and effective option for the treatment and prevention of obesity-related knee osteoarthritis.

Key words: time-restricted diet, infrapatellar fat pad, fibrosis, obesity, knee osteoarthritis, weight loss, adipokine, animal experiment