Mozaffari-Khosravi H, Ahadi Z, Barzegar K. The effect of green tea and sour tea on blood pressure of patients with type 2 diabetes: a randomized clinical trial. J Diet Suppl. 2013;10(2):105-115.

Diabetes mellitus, a common chronic metabolic disorder, is increasing in prevalence worldwide, with about 6.4% of adults aged 20 to 79 years reported in 2010 to have the disease.¹ Herbal therapies are often used to prevent and control the complications of diabetes. Specifically, the chemical constituents of tea, including flavonoids, are thought to help control the metabolic and vascular complications of the disease. Green tea (GT; Camellia sinensis) and hibiscus (sour tea; Hibiscus sabdariffa) are reported to have beneficial health effects. These authors conducted a randomized, clinical trial to compare the effects of drinking these 2 teas in patients with mild hypertension and type 2 diabetes mellitus.

Supported by the Yazd Diabetes Research Center in Yazd, Iran, the authors recruited 100 patients who fit the following criteria: aged between 30 and 60 years with a 5-year history of diabetes; blood pressure of 120-139/80-89 mmHg; no nephropathic or other expressive symptoms; no insulin use; no consumption of antioxidant complements, vitamins, minerals, or fish oil for the previous 6 months; and no history of renal, thyroid, hepatic, or cardiovascular disorders.

Patients were randomly assigned to either the hibiscus tea (HT) group (n=46) or the GT group (n=48); there were 6 patient dropouts, leaving a total of 94 patients who completed the study. Patients were given similar GT and HT bags and instructed to drink the tea 3 times daily (2 hours after each meal) for 4 weeks. Both types of tea were purchased from "reliable herbal shops and approved by the naturist practitioners." Those in the GT group were instructed to boil the 3 g GT bags in 150 mL of water and wait 5 minutes before drinking the tea. In the HT group, the patients boiled the 3 g HT bags in 150 mL of water, waited 10 to 15 minutes, and then drank the tea in one sitting.

Overnight fasting blood samples were drawn before and after the intervention to measure fasting blood sugar (FBS) levels. Body measurement parameters, including body weight, height, and body mass index (BMI), were recorded at baseline and at the end of 4 weeks. The patients' systolic blood pressure (SBP) and diastolic blood pressure (DBP) were measured on day 1, day 15, and at the end of the study. A 24-hour dietary recall questionnaire was used to determine the patients' daily dietary intake. The authors report an 89% compliance rate in the HT group and 91% in the GT group. No adverse effects were reported. No within- or between-group changes were seen in mean FBS concentration, weight, or BMI during the 4-week study.

The SBP and DBP of both groups decreased significantly at the end of the study compared with baseline (P<0.001); however, the mean differences were not statistically significant between the 2 groups at the study's end. Compared with baseline values, blood pressure decreased by 39.6% in the GT group and by 43.5% in the HT group. Pulse pressures, body weight, and BMI decreased in both groups, though not significantly, and the mean changes were also not statistically different between the 2 groups at the study's end.

According to the authors, GT is thought to affect blood pressure through its polyphenol epigallocatechin-3-gallate (EGCG), which increases nitric oxide (NO) production.² The mechanism accounting for the decreased blood pressure associated with HT may be the release of NO from the endothelium of vessels and the prevention of calcium penetration to vascular smooth muscle cells.³

This study is limited by the lack of a control group, no double-blinding, and no measurement of sodium, potassium, or calcium concentrations or specific amounts of the various active constituents of tea, such as caffeine.

Acknowledging those limitations, the authors conclude that their study "revealed that mildly hypertensive type 2 diabetic individuals who drink three glasses of green or sour [hibiscus] tea daily for 4 weeks show significant decreased systolic and diastolic blood pressures," and they suggest that patients with diabetes who are hypertensive drink those teas daily.

―Shari Henson

References

¹Azimi-Nezhad M, Ghayour-Mobarhan M, Parizadeh MR, et al. Prevalence of type 2 diabetes mellitus in Iran and its relationship with gender, urbanisation, education, marital status and occupation. Singapore Med J. 2008;49(7):571-576.

²Wolfram S. Effects of green tea and EGCG on cardiovascular and metabolic health. J Am Coll Nutr. 2007;26(4):373S-388S.