**5. Conclusions**

Diabetes mellitus and its complications have become an important public health problem. Epidemiological studies found that drinking tea could reduce the risk of diabetes mellitus and diabetic complications, and among these studies, green tea, black tea, and oolong tea were in the majority, while epidemiological studies on white tea, dark tea, and yellow tea were less common. In addition, experimental studies have shown that tea could protect against diabetes mellitus and diabetic complications by improving insulin resistance, activating the insulin signaling pathway, playing an insulin-like role, improving oxidative stress, and alleviating inflammatory response. Further, tea has synergistic effects with certain antidiabetic drugs. Moreover, clinical trials have shown that tea played a positive role in the prevention and treatment of diabetes mellitus and its complications. Additionally, different types of tea have different main bioactive ingredients, which may be applicable to different diabetic complications. Therefore, tea could be used as a beverage, or be developed into functional foods or nutraceuticals, for the prevention and managemen<sup>t</sup> of diabetes mellitus and its complications, such as diabetic nephropathy, diabetic cardiovascular disease, and diabetic retinopathy. In the future, more bioactive components in tea for the prevention and managemen<sup>t</sup> of diabetes mellitus and its complications should be separated and identified, especially for the dark tea. The molecular mechanisms of tea and its bioactive components should be further studied. In addition, because of the differences of doses and effects of tea between experimental and clinical studies, it is still difficult to conclude whether the effective doses from animal studies might have beneficial effects on human. Therefore, more clinical trials should be carried out to verify the protective effects of tea on diabetes mellitus and its complications. In addition, special attention should be paid to the safety of tea and tea products.

**Author Contributions:** Conceptualization, J.-M.M., R.-Y.G. and H.-B.L.; writing—original draft preparation, J.-M.M., S.-Y.C. and X.-Y.X.; writing—review and editing, X.-L.W., Y.-F.W., S.-X.C., P.-Z.Z., R.-Y.G., and H.-B.L.; supervision, R.-Y.G. and H.-B.L.; funding acquisition, R.-Y.G. and H.-B.L.

**Funding:** This study was supported by the National Key R&D Program of China (No. 2018YFC1604400), Shanghai Basic and Key Program (No. 18JC1410800), the Agri-X Interdisciplinary Fund of Shanghai Jiao Tong University (No. Agri-X2017004), and the Key Project of Guangdong Provincial Science and Technology Program (No. 2014B020205002).

**Conflicts of Interest:** The authors declare no conflict of interest.
