**1. Introduction**

Diabetes mellitus, one of the most common metabolic disorders in the world, is featured by hyperglycemia caused by either decreased insulin secretion or insulin resistance [1]. The incidence of diabetes mellitus in adults has been increasing in the last decades [2,3]. Diabetes mellitus has been the fifth leading cause of death in the world, and the International Diabetes Mellitus Federation predicted that 592 million people worldwide will su ffer from diabetes mellitus by the year 2035 [4]. Numerous studies have demonstrated that diabetes mellitus, especially type 2 diabetes mellitus (T2DM), could induce diverse complications, such as diabetic nephropathy, diabetic cardiovascular complications, neuropathy, eye, and liver complications, which have been the major causes of its

morbidity and mortality [4,5]. Therefore, it is necessary and urgen<sup>t</sup> to find e ffective strategies for the prevention and managemen<sup>t</sup> of diabetes mellitus and its complications [6].

As a popular drink worldwide, tea has many bioactivities and healthy benefits, such as antioxidant, anticancer, hepatoprotective, cardioprotective, anti-obesity, improving intestinal flora, and antidiabetic effects [7–14]. Due to the di fferent characteristics and fermentation degrees caused by various manufacturing processes, tea can be classified into six main categories, including unfermented green tea, slightly fermented white tea, partly fermented yellow tea, semi-fermented oolong tea, fully fermented black tea, and post-fermented dark tea [15,16]. Moreover, tea contains many bioactive components, especially polyphenols, such as catechins (Figure 1), flavonols, theaflavins, and thearubigins, which have the potential to decrease the risk of diabetes mellitus and its complications [17].

**Figure 1.** Chemical structures of main catechins in tea.

The e ffects of tea against diabetes mellitus and its complications have been widely studied. Epidemiological research found that the consumption of tea was negatively associated with the risk of diabetes mellitus and its complications [18,19]. Further, recent in vitro, in vivo, and clinical trials further supported the e ffects of tea on the prevention and treatment of diabetes mellitus and its complications [20,21]. In addition, tea is a potential hypoglycemic substance with low cost, good patient compliance, and fewer side e ffects compared with many synthetic hypoglycemic drugs [22]. In order to provide an updated understanding of tea targeting diabetes mellitus and its complications, we searched related literature of epidemiological, experimental, and clinical studies based on PubMed and Web of Science databases and reviewed and discussed the protective e ffects of tea against diabetes mellitus and its complications, highlighting the related molecular mechanisms.
