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Antioxidants in Green Tea - Bad News and Good News

Wherever one reads about green tea, be it on tea specific websites/blogs or health and wellness-sites and books, one aspect is on most sites omnipresent and dominant: the health value of the antioxidants in green tea. Antioxidants have the ability to block oxidation reactions, resulting in a reduction or even prevention of cell damage which is mainly caused by so-called free radicals. This well promoted and marketed antioxidant-effect is the main factor for the green tea-boom in recent years. But as it happens so often: scientific studies, carried out under experimental conditions in vitro or on animals, are misinterpreted but, however, quickly recognized as a new opportunity to make a business. So happened in the case of green tea.

Green tea, there is no doubt, is rich in catechins. Catechins belong to flavan-3-ols, a subclass of flavonoids. Flavonoids comprise about 5,000 plant compounds with a common chemical structure (polyphenol). But do the catechins in green tea, namely (-)-epicatechin (EC), (-)-epicatechin gallate (ECG), (-)-epigallocatechin (EGC) and (-)-epigallocatechin gallate (EGCG), act as antioxidants as propagated?

The Bad News

No. According to the well-known Linus Pauling Institute, who`s mission is “to promote optimal health through cutting-edge nutrition research and trusted public outreach“, they scarcely do; their antioxidant-effect is negligible! Contrary to what is spread, all Flavonoids, i.e. Catechins too, have a very low bioavailability. The absorption into the blood is very limited and most of the Flavonoids leave the body without having caused any effect. For catechins e.g. in green tea, the bioavailability is only about 2% of the total intake! “Unfortunately, many test tube experiments published in the scientific literature showing strong antioxidant activity of flavonoids have used very high, unphysiological concentrations and parent chemical structures (rather than metabolites) of flavonoids. When these results are conveyed to the public, a misunderstanding may occur about the actual biological significance of flavonoids, leading consumers to believe incorrectly that flavonoids have important antioxidant value.
However, in the human body, flavonoids are not effective antioxidants because, even with high levels of dietary intake, cellular flavonoid concentrations are 100-1,000 times lower than concentrations of other important cellular antioxidants, including vitamin C, vitamin E, uric acid, and glutathione. For these reasons, the relative contribution of dietary flavonoids to antioxidant function in the body must be very small and physiologically negligible.” (“Controversy: What are the True Biological Functions of Superfruit Antioxidants?” 2009-04-01 - Balz Frei, PhD, Director, Linus Pauling Institute, Oregon State University, Corvallis, OR). And exactly that happened!

The Good News

Other than being an effective antioxidant, it`s getting more and more clear to scientists, that Flavonoids modulate so-called “cell-signal pathways”. These pathways are important for the regulation of a number of cell-activities like cell growth etc. The increase or decrease of the “cell-signaling pathways”-activity is regulated by the cell`s response on stress or other signals. To activate this mechanism, only a low flavonoid-concentration within the cells is needed. This corresponds with the low flavonoid-concentration that`s not eliminated after the intake and stays in the body.

“The results of many laboratory experiments suggest that flavonoids may selectively inhibit a group of cell-signaling enzymes called kinases, which are important in maintaining normal cell function. Increased activity of these kinases appears to be needed at the start of various chronic diseases; flavonoids may selectively inhibit kinases, thereby lowering chronic disease risk… The results indicate that changes in cell signaling by flavonoids could help prevent cancer…[and]… could help prevent cardiovascular diseases (mainly heart attacks and strokes).” (“Controversy: What are the True Biological Functions of Superfruit Antioxidants?” 2009-04-01 - Balz Frei, PhD, Director, Linus Pauling Institute, Oregon State University, Corvallis, OR; see also: “Flavonoids: antioxidants or signalling molecules?” Williams RJ, Spencer JP, Rice-Evans C. Free Radic Biol Med. 2004; 36:838-849).

Conclusion

Flavonoids like the Catechins in green tea are rarely (if ever) a potent antioxidant. But there`s a good chance that they have an affect on so called “cell-signaling pathways, “possibly influencing the onset and progression of cancer or cardiovascular diseases.” („Controversy: What are the True Biological Functions of Superfruit Antioxidants.“ 2009-04-01 - Balz Frei, PhD, Director, Linus Pauling Institute, Oregon State University, Corvallis, OR).

Going through scientific publications, as I did, can be confusing. Here two randomly picked statements from the same (co-)author (Henning SM):

1. “Polyphenols are antioxidants, and EGCG, due to its high levels, is widely accepted as the major antioxidant in green tea.” (“Catechin and caffeine content of green tea dietary supplements and correlation with antioxidant capacity.” Seeram NP, Henning SM, Niu Y, Lee R, Scheuller HS, Heber D., J Agric Food Chem. 2006 Mar 8;54(5):1599-603

2. „Tea polyphenols have strong in vitro antioxidant activity. Due to their limited bioavailability, however, their contribution to in vivo antioxidant activity may depend on the form of administration.“ (“Bioavailability and antioxidant effect of epigallocatechin gallate administered in purified form versus as green tea extract in healthy individuals.” Henning SM, Niu Y, Liu Y, Lee NH, Hara Y, Thames GD, Minutti RR, Carpenter CL, Wang H, Heber D, J Nutr Biochem. 2005 Oct;16(10):610-6.

This example clearly demonstrates that we always have to be very careful and question each statement and author. Reading just one single publication or even only an extract in a magazine or on a website is not enough and can be misleading.

We will have to wait for future research and studies to see if the above made conclusion about the mode of action of Catechins in green tea can be verified – or not: “Tea catechins... are effective scavengers of reactive oxygen species in vitro and may also function indirectly as antioxidants through their effects on transcription factors and enzyme activities. The fact that catechins are rapidly and extensively metabolized emphasizes the importance of demonstrating their antioxidant activity in vivo.“ (“Tea Catechins and Polyphenols: Health Effects, Metabolism, and Antioxidant Functions.” Critical Reviews in Food Science and Nutrition, Volume 43, Issue 1, 2003)

If the health benefits will be verified, then green (and black!) teas from Darjeeling should be your No.1 choice as your daily beverage. Why? Please have a look at What makes Darjeeling Tea so special?


Dr. Hans-Juergen Langenbahn
Dr. Hans-Juergen Langenbahn

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