Thursday January 6 2011
The study was not conducted in humans
“A cup of green tea a day appears to protect against Alzheimer’s disease and other forms of dementia,” reported The Daily Telegraph. It said that researchers have found the drink may also protect against cancer.
This study looked at the effects of a concentrated green tea extract that had been treated in the laboratory to mimic the effects of normal digestion. The researchers detected over 30 major compounds, called polyphenols, that remained active after being “digested”. The extract was then tested to see whether it protected rat nerve cells from the toxic effects of certain chemicals and a protein linked to Alzheimer’s disease.
Overall, this study adds to a large volume of research into green tea. However, the findings do not provide conclusive evidence that green tea combats either Alzheimer’s or cancer. A systematic review of 51 studies found that green tea is not protective against cancer. A good diet, plenty of exercise and a healthy lifestyle are important in reducing the risk of some cancers.
Where did the story come from?
The study was carried out by researchers from the Medicinal Plant Research Group at the School of Agriculture in Newcastle University in collaboration with the Scottish Crop Research Institute and the GGS Indraprastha University in India. The study was supported by a grant from the Mental Health Foundation. It was published in the peer-reviewed journal Phytomedicine.
The Daily Mail, Daily Telegraph and Metro reported this research and mentioned previous human studies, without clarifying that this study was conducted on animal cells grown in the laboratory.
What kind of research was this?
This laboratory study investigated whether polyphenol compounds from green tea protect animal cells from the toxic effects of certain chemicals.
The researchers explain that both green and black tea are thought to be protective as they are high in polyphenols, chemicals that protect cells by neutralising toxic free radicals. They say that human studies have suggested that free radicals can cause nerve cell death and that other studies show an association between a protein called beta-amyloid, nerve cell death and dementia. The laying down and accumulation of beta-amyloid peptides in the brain may play a role in the development of Alzheimer’s disease. It has also been suggested that polyphenols may have anti-cancer properties.
The researchers were particularly interested in the chemical composition of digested green tea, and whether it was as effective as undigested green tea in neutralising free radicals.
What did the research involve?
Though the researchers acknowledge that results of previous research have been conflicting, they point to studies supporting the theory that green tea polyphenols might improve thinking (cognition) in animals and humans. In particular, they discuss a type of polyphenol called a flavonol, found in tea and thought to have anti-cancer, cholesterol-lowering and nerve-protection properties.
The researchers first brewed a pot of green tea. After 45 minutes, the mixture was concentrated and then went through a simulated digestion using enzymes and chemicals found in the stomach and small intestine, including acid, pepsin, bile salts and pancreatin. The resulting mixture, called “colon-available” green tea extract (CAGTE), was analysed by liquid chromatography-mass spectrometry, which identifies the individual molecular contents of the mixture. This content was compared to an analysis of the undigested green tea.
The CAGTE was tested for its ability to protect cells that had been grown from rat nerve tumour cells from death. To induce cell death, the cells were incubated with either hydrogen peroxide, a source of free radicals, or beta-amyloid, the protein linked to Alzheimer’s disease. The concentrations of chemicals used were sufficient to kill about half of the cells treated. Different concentrations of CAGTE were tested to see if they could prevent this cell death. This was done by pre-incubating the rat nerve cells with CAGTE 24 hours before exposing them to hydrogen peroxide or beta-amyloid.
What were the basic results?
When they looked for changes to tea following digestion, the researchers found that the flavanol derivatives were reduced in digested green tea extract compared to green tea itself. The overall concentration of polyphenols remained similar in both green tea and digested green tea extract.
The digested green tea extract (CAGTE) significantly protected the rat nerve cells from the toxic effect of hydrogen peroxide at all concentrations of CAGTE tested. The lowest concentrations seemed to provide the best protection.
Lower concentrations of CAGTE were also able to protect the cells against death caused by beta-amyloid. However, higher concentrations of CAGTE reduced the number of cells compared to the untreated cells. This could be due to CAGTE killing some of the cells or stopping them from dividing.
How did the researchers interpret the results?
The researchers say their results show that green tea metabolites (the contents of the green tea extract) can reduce cell death caused by hydrogen peroxide and beta-amyloid protein in the laboratory. The say this provides the basis for further study of flavanols as a potential way of alleviating some of the effects of Alzheimer’s disease.
This laboratory research evaluates in more detail how chemicals found in green tea affect nerve cells. By carefully assessing the content of green tea and how it is digested, the researchers have advanced what is known about this research area. However, some caution must be taken in applying this research to humans. A large epidemiological study would be the most appropriate way of initially testing the effects of drinking tea in humans.
- As the tea extract was concentrated before being applied to the cells, it is not known what the effect of simply drinking green tea would be.
- It is not known for certain if the cell protection seen in these isolated rat nerve cells in the laboratory is directly applicable to the human diseases Alzheimer’s and cancer.
- The reduction in cell numbers seen with higher CAGTE concentrations is reported by the authors to be “in line with” the reputed anti-cancer properties of green tea polyphenols. However, the researchers were not specifically looking at effects of green tea on the growth of cancer cells. It is too soon to draw any conclusion about any anti-cancer effects.
Overall, this study adds to a large volume of research into green tea. However, the findings do not conclusively show that green tea combats either Alzheimer’s disease or cancer. A 1999 systematic review of 51 studies found that, while green tea is safe if consumed regularly in moderation, its anti-cancer properties remain unproven.