15 minute read
Tea and Cardiovascular Disease
from Tea & Your Health
by Dilmah
Heart disease and stroke are the leading causes of death in the world at present. Narrowing of arteries due to plaque formation restricts the blood supply to the heart and brain. Complete blockage of the already narrowed arteries due to blood clots cause heart attacks and stroke. Several risk factors, which include high cholesterol and fat levels in blood, high blood pressure, diabetes, oxidative stress, inflammation and factors that favour blood clotting, are involved in plaque formation in the arteries.
Detailed research on both tea consumption and the above risk factors have revealed, that flavonoids in tea have the ability to reduce these risk factors. In addition, epidemiological studies have been carried out in several countries, where large numbers of individuals from the populations have been followed for a long period of time. These studies have revealed that regular tea consumption could reduce the risk of heart disease and stroke. As plaque formation is a slow process, taking several years, individuals could adopt healthy life styles and diets that reduce the risk. Regular tea consumption could be a part of such healthy diet.
Incidence of Heart Disease
Incidence of non-communicable diseases (NCD’s) such as heart disease, stroke, diabetes, high blood pressure and cancer have increased over the years, the rate of increase in the last five years being much higher than previously. According to the World Health Organization (WHO), leading causes of death at present are the NCD’s; heart disease is number one with 8.5 million deaths in 2012 and number two is stroke. Improper lifestyle is the main cause for the increased incidence of heart disease and diet is the major contributory factor in our lifestyle that affects health. (See chapter 3 for details on importance of a healthy diet for a healthy life)
Development of Heart Disease
Although there are other forms of heart diseases such as defects in heart valves, increased or decreased heart rate etc., cardiovascular disease (CVD) is the main cause of death. CVD refers to diseases of the heart (cardio) and blood vessels (vascular). A continuous blood supply to the heart muscle is essential for the continuous pumping of blood by the action of the heart muscle. Coronary arteries supply blood to different parts of the heart including heart muscles. When there is a blockage in any of these arteries, the blood supply to the heart could be partially or fully interrupted. The place where the blockage occurs (whether in a main artery or a branch artery) and the extent of the blockage will determine the severity of ‘heart attack’. It could be either fatal in the case of a complete blockage in a major coronary artery, or a minor heart attack in the case of a partial blockage in a branched artery. This type of heart disease is also called coronary heart disease (CHD) and ischemic heart disease. (IHD). Ischemia means inadequate flow of blood to a particular area of body.
Development of heart disease is a gradual process which takes place over many years, through the gradual narrowing of arteries. Healthy arteries are flexible and elastic. With age, arteries could become thick and stiff, known as hardening of arteries or arteriosclerosis, leading to restriction of blood flow. Atherosclerosis refers to the build-up of fats, cholesterol and other substances in and on the artery walls, resulting in plaque formation, which can restrict the blood flow. Atherosclerosis is a specific type of arteriosclerosis, but the terms are often used interchangeably.
Atherogenesis is known as the deposition of fat and cholesterol on the artery walls. However, it is a complex interaction and exchange of material, between the cells of the artery wall and the blood. Messages exchanged between molecules in the blood and cells in the artery wall also play an important role in atherogenesis. The process starts with the expression or formation of ‘cell adhesion molecules’ on the inner surface of the artery wall, which promote the adhesion of white blood cells to the artery wall, resulting in the promotion of the deposition of fat and the migration of muscle cells, thereby enlarging the area between the endothelium and muscles. Gradually, further depositions, calcification and cell death occur, resulting in plaque formation.
Rupture of the plaque’s protective fibrous cap most commonly causes lethal coronary thrombosis (formation of a blood clot).
Cerebrovascular Diseases (Stroke)
A similar process could take place in the brain, also due to the same risk factors. This could result in partial or complete blockage of blood supply to the brain resulting in stroke or death. The factors which reduce the risk for heart disease would also reduce the risk of stroke through similar mechanisms.
Risk Factors for Heart Disease
The main factors that influence the expression of adhesion molecules and plaque formation are as follows;
• Elevation of blood cholesterol, triglycerides and fat • High blood pressure (hypertension), • Products associated with high blood sugar in diabetes (glycated proteins or lipids), • Oxidative stress, • Pro-inflammatory cytokines due to inflammation or derived from excess adipose tissue due to obesity (see chapter 5 for details on inflammation)
Therefore, elevated blood fat, high blood pressure, obesity, oxidative stress, inflammation and diabetes are main risk factors for heart diseases. Atherogenesis is a slow gradual process that takes place over several years. Generally, by the time of diagnosis it would have progressed to a critical stage. Therefore, it is important that healthy individuals should always try to reduce the risk factors, by changing their lifestyles, such as by the regular consumption of food items or beverages that reduce the risk factors. Scientific research carried out in the last few decades indicate that regular consumption of tea could reduce the risk of heart disease. According to these research studies tea could reduce the risk through several mechanisms.
Mechanisms of Risk Reduction by Tea
Lowering of Cholesterol in Blood
Lipids (fat) and cholesterol (a particular type of lipid found in animals) are bound with proteins and form spherical particles for transportation through the blood stream, to different parts of the body. These particles are known as lipoproteins. Lipoproteins are grouped according to their density as High Density Lipoproteins (HDL) and Low Density Lipoproteins (LDL). HDL transports cholesterol and lipids from blood to organs whilst LDL transports cholesterol from organs to blood.
High levels of cholesterol in low density lipoproteins (LDL cholesterol) have been identified as a risk factor for heart disease. Therefore, LDL cholesterol is known as ‘bad cholesterol’ whilst cholesterol in high density lipoproteins (HDL cholesterol) are known as ‘good cholesterol,’ as that could remove cholesterol from arteries. Often medical practitioners recommend lipid profile
(or lipid panel) test to find the levels of these risk factors in the blood.
Research had shown that tea consumption could reduce blood cholesterol levels, especially the ‘bad cholesterol’. Recently, a systematic review of all research done on tea consumption and blood cholesterol levels have indicated, that black tea1 and green tea2 consumption could reduce the LDL or bad cholesterol levels.
Both black tea3 and green tea4 have the ability to reduce cholesterol absorption from the diet into the blood stream, which contributes to the reduction of cholesterol.
Reduction of Risk by Antioxidants in Tea
Formation of oxidised molecules, particularly oxidation of fat and proteins in the blood stream, increases the risk of their deposition and incorporation into the atherosclerotic plaque. ‘Oxidative stress,’ or high amounts of ‘reactive oxygen species’ (ROS), and ‘free radicals’ (FR) in the body could, therefore, contribute to an increase in the plaque formation in arteries. Antioxidants in tea could neutralize the ROS and FR and reduce the ‘oxidative stress’ in the body, contributing to the reduction of plaque formation in arteries. Oxidation of lipids known as ‘lipid peroxidation’ is an important risk factor for plaque formation in the arteries. Dietary components could have a protective effect on the lipid peroxidation and tea has been identified as one of the dietary components with a protective property.5
Effect of Tea on High Blood Pressure
According to the World Health Organization there are at least 970 million people, worldwide, who have elevated blood pressure (hypertension). Hypertension is a risk factor for coronary heart disease and the single most important risk factor for stroke. It causes about 50% of ischaemic strokes. Hypertension stresses the blood vessels, causing them to clog or weaken and can lead to atherosclerosis and narrowing of the blood vessels, increasing the likelihood of blocks from blood clots or bits of fatty material, breaking off from the lining of the blood vessel wall. Damage to the arteries can also create weak places that rupture easily. High blood pressure also could contribute to the rupture of the fibrous cap on atherosclerotic plaques, resulting in the formation of clots blocking the blood flow.
Dietary salt is a significant factor, in raising blood pressure in people with hypertension and in some people with normal blood pressure. Therefore, salt reduction programs are carried out in many countries as a measure to reduce heart disease risk. The scientific name for salt is sodium chloride and sodium is the ‘culprit’ for the elevation of blood pressure. As tea is very low in sodium it would be a suitable beverage for the reduction of risk of high blood pressure.
The review of a number of research studies have demonstrated that long term consumption of black and green tea could significantly reduce blood pressure.6, 7
Endothelial dysfunction is a pathological state of the endothelium (the inner lining of blood vessels) that could lead to high blood pressure and heart disease. With advancing age the proper functioning of the endothelium tends to deteriorate. Endothelial function is a broad function but it mostly affects the constriction and dilation of blood vessels, which helps in keeping the blood pressure constant. Therefore, endothelial dysfunction could lead to high blood pressure.
Some components in the diet could improve the endothelial function. Flavonoids in tea have been identified as a dietary component that could improve the endothelial function, thereby, reducing the risk of blood pressure and heart disease.8, 9
Effect of Tea on Inflammation
Inflammation is an essential part of the body’s healing system. Inflammation’s aim is to defend the body against bacteria, viruses, and other foreign invaders, to remove debris, and to help repair damaged tissue (See Chapter 5 for details on inflammation). Without inflammation, simple injuries would aggravate and simple infections could be deadly. However, inflammation should remove the threat and repair the damage and the body should quickly come back to the normal state.
Prolonged or chronic inflammation could lead to many health issues. Chronic inflammation is involved in all stages of atherosclerosis, the process that leads to plaque and clogged arteries. Further, inflammation results in oxidative stress, which is another risk factor for heart disease. Therefore, chronic inflammation increases the risk of heart disease and strokes.
As tea contains high amounts of antioxidants, it could reduce the effects of chronic inflammation.
Effect of Tea on Blood Clotting
Blood clots could completely block already narrowed coronary arteries resulting in a ‘heart attack’. Therefore, often, individuals at risk are prescribed to continually use medicine which reduces the possibility of blood clot formation. Research indicates that tea components have the ability to reduce clot formation in blood to a certain extent. Platelets, a type of cells in blood, are involved in the blood clotting process. Both green tea and black tea have the ability to interfere with the process and reduce the blood clotting.10, 11 This would also contribute to reduce the risk of heart disease.
Effect of Tea on Diabetes
It has been established that diabetes increases the risk of heart diseases. Tea is known to reduce the risk factors for diabetes. Therefore, tea would contribute in the reduction of the risk for heart disease through its ability to reduce the risk of diabetes. Further, high blood sugar results in the attachment of the sugars to other molecules such as lipids and proteins. Such molecules are known as ‘Advanced Glycation End products’ (AGE). Production of AGE has been implicated in the development and progression of many NCD’s, including heart disease. Formation of
AGE increases the possibility of deposition on the plaques in arteries, leading to an increase in the stiffness in arteries and oxidative stress, thus contributing to increased risk of heart disease. Tea could interfere with the production of AGE and their oxidation thereby contributing to the reduction of risk for heart disease due to formation of AGE.12
Population Studies
Results from research on tea and cardiovascular disease, further reinforced by studies on a large number of individuals, have indicated that tea has the ability to reduce the risk for cardiovascular disease through several mechanisms. The ‘Seven countries study’ was the first such large scale study carried out, over a long period, to establish the relationship between life style factors and coronary heart disease.
Large groups with contrasting life styles, eating habits and different risk levels for CHD, were chosen from seven countries, namely, USA from the American continent, Netherlands, Finland, Italy, Greece and former Yugoslavia from the European continent and Japan from the Asian continent. Initially, exploratory pilot studies had been carried out in Italy, Spain, South Africa and Japan from 1952 to 1956 and further pilot studies were done, from 1956 to 1957 in Finland, Italy, and Greece. The first phase of the seven countries study was from 1958 to 1983 and the second from 1984 to 1999. An important finding of the seven countries study was that flavonoid intake was inversely
related to the CHD.13 Tea had been identified as a major source of flavonoid intake.14
Many other similar studies have been carried out later. A review of twenty two such studies have revealed that increased tea consumption is associated with reduced risk of CHD.15
Conclusion
A large number of research studies carried out on the effect of tea on heart disease indicate that flavonoids in tea act through several mechanisms, to reduce the risk of heart disease and stroke. These mechanisms include lowering of blood lipids (fat) by reduced absorption of lipids from the digestive tract and reduction of the oxidation • Tea could reduce cholesterol levels in blood • Tea could reduce blood pressure • Tea could reduce oxidative stress which is a risk factor for heart disease. • Tea could reduce the risk of diabetes which increases the risk of heart disease. • Tea could reduce inflammation which is a risk factor for heart disease. • Research studies carried out on large groups of people in several countries have shown that regular tea consumption could reduce the risk of heart disease.
of lipids, by the antioxidant flavonoids in tea, which would reduce their deposition on arteries. Improvement of the endothelial function of the arteries by tea improves the expansion and constriction of the arteries when necessary, thus assisting to maintain constant blood pressure. Low sodium (or salt) content in tea also makes it a beverage suitable for regular consumption, as high salt intakes could lead to high blood pressure. Reduction of inflammation and blood clotting by tea flavonoids also contribute to reduce the risk of heart disease. Reduction of the risk of diabetes by tea indirectly contributes to the reduction of the risk of heart disease.
Population studies carried out using large populations, to determine the risk factors for heart disease also confirm that tea consumption reduces the risk of heart disease.
Song Jiaoren, 1882- 1913
REFERENCES
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