Cardiovascular diseases, together with cancers, are the main killing diseases of humans in the world. Of the cardiovascular diseases, atherosclerosis is one of the most prevalent. Atherosclerosis is primarily caused by hypercholesterolemia in which excess cholesterol accumulates in the blood vessels and oxidation of low-density cholesterol (LDL) leads to foci of endothelial abnormalities associated with the process of atherosclerosis (). It deteriorates further with the oxidation of lipids in the blood. Therefore, in order to maintain the cardiovascular system in good condition, it is very important to prevent not only an excessive increase of cholesterols in the blood, but also the oxidation of lipids in the blood. Hypertension is another major factor that can affect the health of the cardiovascular system. In this article, the antioxidative, hypolipidemic, hypotensive and the obesity-depressing activity of tea will be discussed.
Blood Lipid and Cholesterol Lowering Effect
Excessive lipids in blood is a common disorder of middle aged or old aged men and women. High serum-lipid includes high cholesterol and triglyceride content in blood. The cholesterol includes low-density cholesterol (LDL), ultralow-density cholesterol (VLDL) and high-density cholesterol (HDL). Among those, low-density cholesterol and ultralow-density cholesterol have harmful roles in promoting the formation of atherosclerosis. On the other hand, high-density cholesterol is a kind of beneficial cholesterol and plays a role in preventing the occurrence of atherosclerosis. Tea drinking showed the effect of decreasing the serum lipid level, the contents of low-density cholesterol and VDL cholesterol. Green & Harari () in their study of 650 men in six factories in Israel in 1986, found a statistically significant inverse relationship between tea drinking and the level of both total and low-density cholesterol cholesterol. A study on 4700 men and 4500 women in Finland revealed that increasing tea consumption was associated with slight reductions in plasma cholesterol level (). An epidemiological survey including 7710 men and 8222 women was carried out in Norway and showed a negative relationship between tea drinking and the serum lipid level ().
Over the past ten years, several investigations have been conducted on rats, mice and rabbits to test the protective effect of tea on plasma lipids, plasma cholesterol level and/or atherosclerosis. As reported (), in animals fed with high fat/high cholesterol atherogenic diets, the administration of tea (green tea or black tea) which was added in the drink or the diet resulted in reducing the plasma level of cholesterol.
A clinical experiment on hyperlipidemia patients was carried out by a research group in Quanming Medical Institute of China, after daily drinking of 15 g Yunnan Tucha (a kind of compressed tea) for one month, the lipid-lowering effect was comparable to that of Atromid-S (a popular lipid-lowering medicine) (). Iwata et al. () reported that the plasma level of triglyceride and phospholipid on healthy human female volunteers was decreased significantly after drinking Oolong tea. Recently, Nakachi et al. () reported the results of a cohort study conducted in Japan. The survey covered 8553 people aged over 40 years old living in a town in Saitama, Japan, and asked about 90 lifestyle factors, such as present and past eating habits, history of consumption of tea, tobacco and alcohol, history of disease, present state of health. Out of the 8553 persons surveyed by the questionnaire, 3625 people gave blood samples that were subjected to biochemical and immunological assays during 1986–1990. Results showed that the consumption of green tea was significantly associated with lower serum lipid concentration and low density lipoprotein. An increase in green tea consumption substantially decreased serum total cholesterol and triglyceride concentration. This strong association remained almost unaltered even after age, cigarette smoking, and alcohol consumption were adjusted for using a statistical method. Increased consumption of green tea was associated with an increased serum concentration of HDL, which is often referred to as good cholesterol. As a result, a significant decrease in the atherogenic index, which is an important index for atherosclerosis, was observed in the group that drank the most green tea. A similar cross sectional study of effects of drinking green tea on cardiovascular disease was conducted on 1371 men aged over 40 years by Imai et al. () and similar results were reported. The administration of 0.5–1.0% EGCG, crude catechin or 1.5–2.0% Oolong tea supplemented diet can lead to the decrease of total cholesterol, free cholesterol, low-density cholesterol cholesterol and triglyceride in plasma and body fluid significantly; moreover, it can increase high-density cholesterol cholesterol simultaneously (). ECG and EGCG were more effective in increasing the serum cholesterol excretion via feces ().
The atherogenic index can be decreased significantly after the feeding of 1–2% catechins () as well as ECG and EGCG () in experimental animals with a high cholesterol diet. A similar relationship was established between jasmine tea drinking and the inhibition of LDL-oxidation (). Further research showed that tea catechins not only decrease the serum lipid and cholesterol as well as increase the excretion of body lipid, but also effectively reduce the cholesterol absorption from the intestine by reducing the solubility of cholesterol in mixed micelles (). Among the various catechins, the gallate ester type catechins (EGCG, ECG) was most effective in precipitating micellar cholesterol than those free catechins (EC, EGC) (). Thus, the hypocholesterolemic activity observed with catechin feeding can be also attributed to the inhibition of intestinal cholesterol absorption (). According to the investigation of Ikeda et al. (), it is likely that EGCG forms insoluble coprecipitates with cholesterol in mixed micellar solution (). Thus, the insoluble coprecipitation of cholesterol and catechins cannot be absorbed from the intestine. However, the mode of binding between cholesterol and EGCG needs to be clarified by further investigation. The effects of green tea polyphenols on body weight and the contents of lipids and peroxidized lipid in rat plasma, kidney and liver were investigated. The diet of young rats from 3 weeks of age until they were 19 months old was supplemented with green tea polyphenols (0.5% and 1.0%). In the older animals, peroxidized lipids were significantly lower in the 1-% group (at 13–19 months old). At 19 months old, triglycerides, total cholesterol and phospholipids were significantly decreased in the serum of the 1-% feed group. This suggests a hypocholesterolemic effect from long-term feeding of green tea constituents (). In a similar investigation carried out in China, the results showed that the serum total cholesterol and triglyceride levels were significantly reduced and serum high-density cholesterol level was significantly increased compared to the controls (). Based on these results, some antihyperlipid agents have been developed in Japan, in which tea catechins were used as the active principles ().
There are also several negative relationships between the tea drinking and lipoprotein levels. In 1985, Klatsky et al. conducted a large epidemiological study of serum lipids and the tea intake in 22,000 males and 25,000 females and reported no statistically significant relationship. In the same experiment, coffee was found to be positively and significantly associated with serum cholesterol levels. Similar results were reported by Kark et al. in 1985 from their study of 1000 males and 500 females in Jerusalem. There was no relationship between tea drinking and cholesterol levels before adjustment for covariables, although there was a slight rise (6 mg/dl) in plasma cholesterol at the highest levels of tea drinking in males after covariable adjustment. On the other hand, female tea drinkers had a slightly lower plasma cholesterol level after similar analysis. In a study of 900 males and 1200 females, Haffner and his colleagues () confirmed the lack of relationship between tea drinking and low-density cholesterol level. Coffee once again was found to be positively correlated. Data from the heart study of 5858 Japanese men in Honolulu were essentially identical to those reported by Haffner et al. that tea drinking was not related to the cholesterol levels.
The Effects of Tea on the Cardiovascular System: Prospects
As mentioned above, tea is not only a nutritional and flavored beverage, but also a physiologically function-modulating one. The obesity-depressing, anti-atherosclerosis, cardiotonic, blood-lipid depressing, blood-pressure depresssing activities from tea drinking can be traced to medical books in ancient China (). A large amount of investigation has shown that tea and tea components protect against cardiovascular diseases as a result of their cholesterol-lowering and antioxidative actions (). Also, some epidemiological studies have demonstrated a protective effect of flavonoid intake of tea consumption on mortality from cardiovascular diseases () and a reduction of lipid risk factors of cardiovascular disease (). However, some studies cannot establish these associations (). There may be various reasons for these conflicting results. Atherosclerosis, cardiovascular disease and associated disorders are polygenic and multifactorial in their origins. In addition to abnormal lipid metabolism as a risk factor, there are other risk factors present. Another possibility may be the total flavonoids and other dietary components, and not tea per se, that is responsible for the beneficial effects, thus diminishing the effects of tea. Further investigations and epidemiological studies under carefully controlled conditions are needed. Until now, although the clear-cut conclusion on the positive effect of tea and tea components on cardiovascular disease cannot be drawn, a lot of encouraging results have been obtained. The huge worldwide consumption of three billion cups per day, apparent lack of toxicity and cheap price may attract to the development of tea drinking as an adjunctive therapy for prevention of some chronic diseases.
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.