Diabetes mellitus has become a serious and growing public health concern. It has high morbidity and mortality because of its complications, such as diabetic nephropathy, diabetic cardiovascular complication, diabetic neuropathy, diabetic retinopathy, and diabetic hepatopathy. Epidemiological studies revealed that the consumption of tea was inversely associated with the risk of diabetes mellitus and its complications. Experimental studies demonstrated that tea had protective effects against diabetes mellitus and its complications via several possible mechanisms, including enhancing insulin action, ameliorating insulin resistance, activating insulin signaling pathway, protecting islet β-cells, scavenging free radicals, and decreasing inflammation. Moreover, clinical trials also confirmed that tea intervention is effective in patients with diabetes mellitus and its complications. Therefore, in order to highlight the importance of tea in the prevention and management of diabetes mellitus and its complications, this article summarizes and discusses the effects of tea against diabetes mellitus and its complications based on the findings from epidemiological, experimental, and clinical studies, with the special attention paid to the mechanisms of action.

Tea is a popular beverage and shows very strong in vitro antioxidant activity. However, the relationship among in vitro and in vivo antioxidant activities in teas is seldom reported. In this study, in vivo antioxidant and hepatoprotective activities of 32 selected Chinese teas were evaluated on a mouse model with acute alcohol-induced liver injury. The results showed that most teas significantly reduced the levels of alanine transaminase, aspartate transaminase, alkaline phosphatase, triacylglycerol, and total bilirubin in the sera of mice at a dose of 400 mg/kg. In addition, most teas greatly decreased the malondialdehyde level and increased the levels of superoxide dismutase, glutathione peroxidase, and glutathione in the liver of mice, indicating the antioxidant and hepatoprotective activities of teas. Furthermore, the in vivo antioxidant activity of dark tea was stronger than that of green tea, opposite to the results of the in vitro study. Among these 32 teas, Black Fu Brick Tea, Pu-erh Tea, and Qing Brick Tea showed the strongest antioxidant and hepatoprotective activities. Moreover, total phenolic content as well as the contents of epicatechin, gallocatechin gallate, and chlorogenic acid were found to contribute, at least partially, to the antioxidant and hepatoprotective actions of these teas. Overall, teas are good dietary components with antioxidant and hepatoprotective actions.

The structure and hypoglycemic activity of tea polysaccharides has been extensively studied, while there are few reports on the characterization and hypoglycemic activity of dark tea polysaccharides. The crude dark tea polysaccharide (CDTPS) was optimally extracted from Fuzhuan dark tea. Six polysaccharide fractions (namely DTPS-1, DTPS-2, DTPS-3, DTPS-4, DTPS-5, and DTPS-6) were isolated from CDTPS, and their physicochemical, structural, and biological properties were compared and analyzed. The results revealed that the compositions, structural characteristics, and biological properties of the six DTPSs were different. Therein, DTPS-4 and DTPS-6 had looser morphology, faster solubility, and a more stable structure. Additionally, DTPS-4 had the optimum in vitro antioxidant capabilities, and DTPS-6 had the strongest in vitro hypoglycemic capabilities. In addition, a correlation analysis revealed that the molecular weight and uronic acid content were significantly related to their antioxidant and hypoglycemic activities. Our results indicated that DTPS-4 and DTPS-6 could be further developed into functional foods or additives, respectively.