**4. Clinical Trials**

Several clinical trials have assessed the role of tea in treating diabetes mellitus and diabetic complications.

A double-blind, randomized, controlled clinical trial (RCT) found that green tea could improve bone mineral levels in patients from Brazil with diabetes mellitus [153]. Another RCT observed that green tea extract significantly reduced bone resorption markers and altered bone conversion in T2DM patients [154]. Furthermore, a phase I clinical trial involving 63 patients with T2DM found that drinking 4 cups of green tea per day for two months significantly reduced body weight and systolic blood pressure [155]. Another RCT conducted in Taiwan observed that green tea extract significantly improved insulin resistance and increased glucagon-like peptide 1 [156]. It was proven that green tea could reduce risk factors of diabetes mellitus such as average arterial pressure, waist-to-hip ratio, and glutamic-pyruvic transaminase, but had little effect on fasting blood sugar and hemoglobin A1c (HbA1c), which may be related to the short time span of the study, for subjects in Mauritius [21]. Another RCT conducted in British showed that green tea extract could reduce proteinuria in diabetic patients [41]. Further, it was reported that postprandial blood sugar could be decreased in people drinking black tea [39]. A clinical trial conducted in Kuwait also revealed that drinking black tea for one year could significantly reduce HbA1c level and pro-inflammatory CD3+ CD4+ IL-17<sup>+</sup> cells, and eliminate serum total cholesterol, thereby preventing diabetes mellitus and its complications [22]. Moreover, another trial demonstrated that black tea could protect against diabetes mellitus and diabetic cardiovascular disease through its anti-inflammatory and antioxidant properties [58]. Oolong tea was proven to be an effective adjunctive oral hypoglycemic substance for T2DM in a clinical trial in diabetic patients from Taiwan taking normal hypoglycemic drugs [157].

In brief, clinical trials involving different countries and different people showed that tea could prevent and manage diabetes mellitus and its complications, mainly by improving insulin resistance and decreasing postprandial blood sugar (Table 3).


**Table 3.** The effects of tea on diabetes mellitus and its complications based on clinical trials.

N/A, not available; RCT, randomized, controlled clinical trial; FBG, Fasting blood glucose; T2DM, type 2 diabetes mellitus; PTH, parathyroid hormone; HbA1C, hemoglobin A1c.
