Hypertension is a common disorder in humans. Te a drinking can lower blood pressure. There are many Chinese traditional prescriptions, with tea as a major constituent, used in the treatment of hypertension and coronary disease in Chinese traditional medicine. A survey on the relationship between hypertension and tea drinking in 964 adults was carried out by Zhejiang Medical University of China during the 1970s. Results showed that the average rate of hypertension was 6.2% in the group who drank tea as habit, and 10.5% in the group who did not. Clinical experiments showed that hot water extract of green tea possessed a degree of blood pressure lowering effect. An experiment in vivo carried out on rats fed with diet supplemented with 0.5% crude catechins showed that the blood pressure in treated rats was 10–20 mm Hg lower than that in the control group (). A clinical experiment using green tea on high blood pressure patients was conducted at the Anhui Medical Research Institute of China. Results showed that a 10 g tea intake daily treatment over half a year, decreased the blood pressure by 20–30% ().
A study was conducted to determine whether the effect in vitro is reflected in the lowering of blood pressure of animals after oral feeding of tea. Crude catechins (extracted from green tea and composed mainly of EGCG) were fed in the diet to spontaneously hypertensive rats (SHR) or stroke-prone SHR (SHRSP) and their blood pressure was measured by the tail cuff method. The mean systolic blood pressure of SHR fed a diet containing 0.5% crude catechins from the age of 5 weeks increased with time, but remained significantly lower than that in the control group (P<0.05). At the age of 16 weeks, the diets of the two groups were exchanged. As a result, the blood pressure curves of these two groups crossed in 2 weeks and this situation remained. In SHRSP given 1% saline as drinking water, addition of 0.5% crude catechins to the diet not only tended to suppress the blood pressure to a non-significant extent relative to the control group, but also clearly prolonged the survival period before death due to stroke. Ishigaki and Hara () carried out the blood pressure lowering experiment by using tea polyphenols (dosage: 400 mg/kg,×2 in succession of three months) on 21 volunteer-adults (10 male and 11 female). The blood was sampled. Results showed that the blood pressure of those patients whose pressure was around 160 mmHg was decreased significantly. Yokozawa et al. () showed that rats orally administered with 2 mg green tea tannin tended to have a lower systolic blood pressure. A further increase to 4 mg produced a significant decrease in the systolic blood pressure, 7% lower than that in the control rats. Similar changes produced by green tea tannin were observed in the diastolic pressure value. Kobayashi et al. () reported that high doses of theanine (1500-2000 mg/kg) significantly decreased the blood pressure of spontaneously hypertensive rats (SHR). The dose-dependent changes in the blood pressure values (systolic, diastolic and mean) resulted from the administration of theanine to experimental animals. Theanine entered the brain via the blood-brain barrier after the intragastric administration. Results showed that brain serotonin concentration was significantly decreased by theanine.
The effects of caffeine on blood pressure depend on dose and route of administration. Intravenous administration frequently produces an initial fall in blood pressure (seen only after larger doses) followed by a secondary rise. In contrast, following oral caffeine intake in animals and human, the initial blood pressure fall is only rarely seen and the maximal plasma concentration are much smaller than those seen following intravenous administration (). Caffeine may influence blood pressure to a greater extent during stress; however, studies on the blood pressure effects of caffeine in humans indicate that it is not deleterious in essential hypertension. It was suggested that the plasma levels of caffeine needed to exacerbate renovascular hypertension in their study could be reached by moderate to heavy caffeine users. Omori () first developed a new type of tea (Gabaron tea) by means of anaerobic treatment of fresh leaves and found it contained large amounts of Y-aminobutyric acid (170-270 mg %), 8-10 times higher than that in common green tea (). Clinical experiments conducted in Japan and China Taiwan showed that Gabaron tea possesses a significant blood pressure lowering effect (), the blood pressure in the treated animals was 14-17% (25-30 mmHg) lower 20 days after treatment. According to a clinical experiment on 13 hypertension patients, the blood pressure in 7 patients was reduced significantly, and 6 were insignificantly reduced (). However, the blood pressure rose rapidly to that of the control rats when the administration of Gabaron tea ceased. Thus, it was recommended that the administration of Gabaron tea should be conducted continuously for the purpose of lowering the blood pressure of hypertensive patients.
Taniguchi et al. () reported that hot water extracts of green tea showed a prolonged hypotensive effect in anesthetized rabbits. The major active principle was (-)- gallocatechin gallate [(-)-GCG]. (-)-GCG at a dose of 0.1 mg/kg (i.v.) effectively reduced the blood pressure of anesthetized rabbits and at 0.5 mg/kg lowered it by 20-40 mmHg for an extended period of time.
Regarding the mechanism of hypertension-depressing effect, hypertension is regulated by angiotensin, the inactive angiotensin I is converted into angiotensin II with potent blood pressure raising action by angiotensin I transferase (ACE). So, those compounds inhibiting the activity of ACE also showed the blood pressure lowering effect. Investigation proved that tea components, especially theaflavin digallate and EGCG, showed obvious inhibiting effects (). In an experiment comparing the inhibiting ability of various kinds of tea on the angiotensin I transferase, it was shown that green tea extracts possessed the most potent inhibitory ability. The ACE activity in the green tea treatment (1 g in 200 ml hot water) was 1% in comparison with 100% in the control (). Of course, there are many mechanisms of action of antihypertensive drugs, such as ß-blockers, central α-stimulants, angiotensin converting enzyme inhibitors, Ca antagonists (). Based on the experimental data reported by Tollins and Raiz (), angiotensin enzyme inhibitors may be superior to Ca antagonists in halting the progression of renal dysfunction. Besides, tea polyphenols and caffeine may ameliorate the development of hypertension by improving the renal circulatory state. So, it was regarded that the blood pressure depressing effect of tea is resulted from its direct action in the kidney, inducing the activation of the kinin-kallikrein-postaglandin system in the kidney ().
Selections from the book: “Tea: Bioactivity and Therapeutic Potential”. Edited by Yong-su Zhen, Zong-mao Chen, Shu-jun Cheng, and Miao-lan Chen. Series “Medicinal and aromatic plants – industrial profiles”. 2002.