Both green tea and black tea come from the leaves of the same plant, Camellia sinensis. However, the final processing of the leaves is different. The leaves for black tea are fully oxidized, while those for green teas are lightly steamed before being dried.
Black teas and green teas both contain similar amounts of flavonoids. However, they differ in their chemical structure. Green teas contain more of the simple flavonoids called catechins, while the oxidization that the leaves undergo to make black tea converts these simple flavonoids to the more complex varieties called theaflavins and thearubigins.
The health benefits of flavonoids
Although the oxidization process modifies the type of flavonoids present, the total level and their overall antioxidant activity is similar in both teas. Research is now suggesting that antioxidants, such as those found in both green tea and black tea, may have a protective role to play in certain health conditions such as heart disease, stroke and some cancers.
The health benefits of green tea
Green tea, specifically, has been associated with protection against certain types of cancers, including lung, stomach and its pre-cancerous condition, gastritis. Moreover, an observational study in Japan found that the regular consumption of green tea (more than 3 cups a day) might be protective against the recurrence of breast cancer in the early stages.
The possible protective mechanism of green tea is unclear, although a number of in-vitro and animal studies are attempting to explain this, including a study that found that the green tea polyphenol-epigallocatechin (EGCG) inhibited the DNA replication in leukemia cells, resulting in the death of these cells.
In addition to its potential anti-carcinogenic and antioxidant effects, other studies have shown green tea to have anti-inflammatory, antithrombotic, cholesterol lowering, antiviral and antibacterial properties.
Green tea and weight loss
Preliminary research published in the American Journal of Clinical Nutrition suggests that an extract from green tea may help with weight loss by speeding up fat oxidation. In this study, researchers conducted a 6 week study of 10 healthy men in their 20's, and found that those men who were given a green tea extract used more calories in a day than those who did not.
Green tea's antioxidant EGCG stimulates the body to burn calories, notably fat. In a Swiss study, a daily dose of 270mg EGCG (the amount in 2 to 3 cups of green tea) caused men to burn 4% more energy - about 80 extra calories a day. Green tea did not increase heart rate, and the calorie burning was not due to caffeine.
Research published in an issue of Archives of Internal Medicine is the first human study to find that a tea product lowers cholesterol. A clinical trial testing a theaflavin-enriched green tea extract is the first human study to find that a tea product lowers cholesterol.
Japanese scientists at the National Defense Medical College in Saitama have reported that green tea seems to confer a protective effect against the risk of heart attack in humans. Hirano R, Momiyama Y, Takahashi R, Taniguchi H, Kondo K, Nakamura H, Ohsuzu F. Comparison of green tea intake in Japanese patients with and without angiographic coronary artery disease. Am J Cardiol 2002;Nov 15;90:1150-3
The Harvard School of Public Health, looked at patient survival rates after a heart attack. They found that the more tea that patients drank in the year before their heart attack, the better their chances of surviving in the years after.
Researchers from the Boston University School of Medicine report that both short- and long-term tea drinking appear to improve blood-vessel functioning in patients with coronary heart disease.
Researchers continue to search for more evidence that define the healing properties of tea. For updated information, please check regularly with the American Cancer Society, American Heart Disease, National Cancer Institute, or other sources related to the health benefits of tea.
Blood Sugar Reduction
There are two main avenues of attack in trying to prevent or control diabetes: you can enhance your insulin function, which helps control blood sugar levels, or you can suppress your blood sugar levels in some other way (careful control of your diet being the most obvious). An intriguing aspect of tea is that it may have both effects, not just the insulin-enhancing effect.
The lack of control of blood sugar, leading to glucose intolerance and ultimately diabetes is one of the leading causes of poor health. The incidence of diabetes is expected to double in the next two to three decades. A study determined the potential improvements in the control of blood sugar by the consumption of tea and determined the active ingredients in tea. Tea, as normally consumed, was shown to increase insulin activity more than 20-fold in a laboratory assay.
Green tea and skin protection
A number of animal studies have shown that topical treatment or oral consumption of green tea polyphenols inhibits chemical carcinogen or ultraviolet radiation-induced skin tumors in different animal models. The treatment of green tea polyphenols on skin has been shown to have a beneficial effect on the biochemical pathways involved in skin inflammation, cell proliferation and chemical tumor promoters. These results have been confirmed in a human model, where topical application of green tea polyphenols protected against UV light-induced DNA damage. Based on results mainly from animal studies, many companies are now supplementing their skin care products with green tea extracts.
Tea and Antioxidant properties
Increasing evidence is highlighting the role antioxidants may have in protecting against certain conditions, such as heart disease, stroke and cancers. It has been proposed that the mechanisms leading to these diseases may be promoted by free radicals, and that antioxidants may oppose the action of these molecules. In addition to the well known antioxidants such as Vitamins C and E, there is growing research demonstrating the potentially beneficial effects of plant-derived antioxidants, such as polyphenols, found in fruits, vegetables, nuts, cereals and drinks such as tea and red wine.
Free radicals explained
Free radicals are unstable molecules that include the hydrogen atom, nitric oxide (NO) and molecular oxygen (O2). These naturally occur in the body as a result of chemical reactions during normal cellular processes. They can also be formed in response to excess pollution, too much UV sunlight and exposure to cigarette smoke. In an attempt to stabilize, they attack other molecules in the body, potentially leading to cell damage and triggering the formation of another free radical, resulting in a chain reaction. Some scientists believe that this type of free radical action has been implicated in certain chronic diseases, such as cancer, heart disease, stroke, rheumatoid arthritis, cataracts and Alzheimer's disease.
Protective mechanisms of antioxidants
Antioxidants are compounds that help to inhibit the many oxidation reactions caused by free radicals, thereby preventing or delaying damage to the cells and tissues. Their mechanisms of action include:
In this way antioxidants limit the free radical damage from:
- Scavenging reactive oxygen and nitrogen free radical species
- Decreasing the localized oxygen concentration thereby reducing molecular oxygen's oxidation potential
- Metabolizing lipid peroxides to non-radical products
- Chelating metal ions to prevent the generation of free radicals
Some antioxidants are synthesized within the cells themselves (endogenous) and others need to be provided in the diet.
- Oxidizing Low Density Lipoprotein (LDL) cholesterol, which may increase the risk of atheriosclerosis
- Promoting platelet adhesion, which can lead to thrombosis thereby increasing the risk of heart disease or stroke
- Damaging the cell's DNA, which may lead to cancer
- Blocking the normal endothelial cell function and vasodilatation in response to nitric oxide, a potential mechanism for heart disease and cancer
- Triggering inflammation
- Impairing immune function
Sources of dietary antioxidants
Traditionally, dietary antioxidants were thought of as Vitamins E and C and the carotenoid - carotene. In recent years there has been particular interest in the antioxidant activity and health benefits of other phytochemicals.
Tea flavonoids demonstrate antioxidant activity, and while not a replacement for fruit and vegetables, the antioxidant activity of tea has been compared to that of fruit and vegetables in a number of studies. Another study found that one or two cups of tea has the same 'radical scavenging capacity' as five portions of fruits and vegetables, or 400mg Vitamin C equivalents.
Health benefits of tea flavonoids
For many years it has been known that the plant polyphenols are antioxidant in-vitro. In fact,many common flavonoids are several times more potent than Vitamins C or E. This growing interest in the antioxidant activity of phenolic compounds has led to increased research into their potential health benefits.
Heart Disease and Stroke
Several reports indicate that tea flavonoids inhibit the oxidation of LDL cholesterol in-vitro.
A reduction in blood lipids has been demonstrated in animal studies.
Certain tea flavonoids exhibit anti-inflammatory actions in animals. Atheriosclerosis is a disease with a strong inflammatory component.
Improvements in blood vessel function, specifically the vascular endothelium, has been seen in patients with established CHD.
Several in-vitro studies and one human trial have found that platelet aggregation can be inhibited by various flavonoids.
The antioxidant activity of tea flavonoids may account for the results of a number of epidemiological studies suggesting that they may have a protective role in conditions such as cardiovascular disease.
In-vitro studies have demonstrated that the initiation stage of cancer can be prevented by the action of tea flavonoids.
Tea polyphenols have been shown to inhibit DNA synthesis of leukemia cells and lung carcinoma cells.
Animal studies have shown that tea and its flavonoids protects against many types of cancer; e.g.: skin tumors in mice, lung cancer in mice and digestive cancer in mice and rats.
Tea extracts exhibit inhibitory effects against Salmonella typhi, Campilobacter jejuni, Campilobacter coli, Helicobacter pylori, Shigella, Clostridium, Pseudomonas, Candida and others.
Green tea and various catechins have exhibited inhibitory effects on the growth of cariogenic bacteria by preventing the adherence and growth of bacteria at the tooth surface.
It is well known that fruit and vegetables are good sources of antioxidants. However, what is less well known is the amount of antioxidants present in tea. The major group of antioxidants in tea are flavonoids that appear to be digested, absorbed and metabolized by the body. There is a wealth of evidence demonstrating that tea and flavonoids exhibit beneficial effects in animal and in-vitro studies, and provide a promising area of research for future human studies.
So, as well as eating more fruit and vegetables, antioxidant intake can be enhanced by drinking more tea, helping to promote overall health and well-being.