The 21st century is characterised by a search for health-promoting foods and beverages. Green tea is one such beverage that is increasingly being used in western countries to obtain health benefits. But how true are the claims that green tea can cure heart disease, cancer, alleviate skin conditions, induce weight loss etc.?
A recent paper published by the SA Vitamin Information Centre (VIC) on 'Health benefits of green tea, green tea extract, and EGCS' provides us with an update on research findings in this interesting field.
What is green tea?
Green tea, like black tea, is produced from the Camellia sinensis plant. To manufacture green tea, the leaves of this plant are picked and heated rapidly to inactivate enzymes and to prevent fermentation.
So-called Oolong tea is partially fermented and black tea is fully fermented tea made from the leaves of the same plant. The difference between these three types of tea, therefore, lies in the extent of the fermentation process the tea leaves are exposed to.
What does green tea contain?
Green tea contains a number of different chemicals called polyphenols, such as flavonols, flavonoids and phenolic acid. Flavonols, which are also known as catechins, are the most important polyphenols that occur in green tea. The most important flavonol is called epigallocatechin gallate or EGCG.
These chemicals have been hailed as protective agents that can ward off a variety of degenerative diseases. However, a great deal of the 'evidence' that is used to motivate the use of green tea as a cure for a variety of diseases, is anecdotal and we need to have a look at the results of sound scientific studies to decide if green tea has health benefits or not.
Green tea and heart disease
- A large Japanese study with 8552 subjects found that individuals who drank 10 or more cups of green tea a day experienced less coronary heart disease than those who drank three or fewer cups a day. Strangely this positive effect was only evident in male subjects, female subjects did not benefit.
- Another smaller Japanese study obtained similar results with green tea, once again only having a protective effect in men and not in women.
- In contrast, a third Japanese study found no link between green tea intake and incidence of coronary heart disease.
- A fourth very large study identified that green tea lowered total cholesterol concentrations, but had no effect on LDL-cholesterol (so-called 'bad cholesterol) or triglycerides (another blood fat that is often raised in heart disease).
The authors of the VIC paper conclude that these studies have identified a possible link between heart disease and green tea, but that additional studies are required to clarify why the protective effect is only found in men and not in women, and why total cholesterol levels are lowered by green tea and LDL-cholesterol levels remain unchanged.
Green tea and obesity
It has been suggested that green tea increases energy expenditure thereby causing weight loss. Once again the presently available study results are highly divergent.
- One study identified no effect of green tea on energy expenditure.
- In contrast, a second study using green tea extract, found a significant increase in 24-hour energy expenditure and fat oxidation. The 24-hour increase in energy expenditure amounted to approximately 4% or 328 kJ (over time, such an increase in the amount of energy required by the human body, could lead to weight loss).
The conclusion that can at present be reached, is that green tea or rather green tea extract, has the potential to promote weight loss, but that we are still in the very early phases of research and that two studies do not constitute sufficient proof that green tea is a slimming aid.
Green tea and cancer
- A Japanese study found a significant decrease in the incidence of cancer in women who consumed more than 10 cups of green tea a day, but no improvement in men.
- Other studies did not find a protective effect.
The authors conclude that although there is some evidence that green tea consumption may have a protective effect against various types of cancer, other studies found no link between green tea and reduction of cancer risk.
Green tea and antioxidant properties
The human body is constantly exposed to the buildup of harmful chemicals called free radicals and any compound or antioxidant that can counteract the effects of free radicals is beneficial. Evidence is accumulating that green tea and the flavonols it contains will protect cells against damage from free radicals.
Green tea and dental health
- One study determined that green tea extract strongly inhibits various microorganisms such as Escherichia coli, which occur in the mouth and contribute to tooth decay.
- Another Japanese study identified that green tea extract in the form of tablets has strong anti-plaque effects.
It is evident that green tea has some beneficial effects, but that a great deal of additional research is required before we can make any recommendations about its use as a universal solution to all kinds of diseases.
If you enjoy drinking green tea, it is a good idea to continue doing so while we wait for scientists to produce more conclusive evidence that green tea promotes good health. – (Dr Ingrid van Heerden, DietDoc)
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