Epidemiological studies found that drinking tea could reduce the risk of diabetes mellitus and diabetic complications. 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. Tea has been observed to act as a potent neuroprotector in the retina. This evidence concerns the gene INS and diabetes mellitus.