Antioxidants in Tea

Wikipedia: Antioxidant
Last Updated: Jan. 13, 2014
Antioxidants are chemicals that prevent oxidative stress inside living cells, by chemically blocking oxidation reactions. Oxidative stress involves free radicals, highly reactive molecules that can catalyze reactions inside living cells: excessive amounts of free radicals can result in cell damage, mutations, and can contribute to chronic diseases like cancer. Antioxidants work by acting as radical scavengers, neutralizing free radicals by reacting with them so that they cannot cause further damage.

Which antioxidants are in tea?

Catechin molecule
Catechin molecule
Tea contains numerous different chemicals which act as antioxidants. Green and white tea are rich in chemicals called catechins, which are a form of flavanol monomers, a type of flavonoid. Flavanols are also called flavan-3-ols, and are also found in other plant-based foods and beverages. The catechins include epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), and epigallocatechin gallate. The catechins generally impart an astringent quality and a bitter taste to tea.

Green tea also contains a small amount of Vitamin C, an antioxidant which is also an essential nutrient. Other types of tea do not contain Vitamin C, or only contain traces.

Antioxidants in oxidized (black or oolong) teas:

When tea is oxidized, the catechins are converted into other chemicals, called theaflavins and thearubigins, different chemicals which still act as antioxidants. The theaflavins and thearubigins are sometimes called tannins, although tea does not contain tannic acid. These chemicals are responsible for the darker color of black and more heavily-oxidized oolong teas.

The theaflavins of black tea and catechins of green tea have been directly compared in a laboratory setting for their antioxidant activity, and were found to be equally effective as antioxidants.[1]

Is green tea or white tea higher in antioxidants than black tea or other oxidized teas?

Green Tea Leaves and a cup of golden tea
Chun Mee Green Tea
from Harney & Sons
No. While it is true that the oxidation processes used to create black tea and oolong tea does result in a lower catechin concentration, the catechins are converted to theaflavins and thearubigins, which are mostly absent from unoxidized teas. The oxidation process changes the relative amounts of catechins vs. theaflavins and thearubigins, but does not necessarily result in a lower total antioxidant content.

Furthermore, a study that examined the antioxidant content of a number of different teas found that the amount of antioxidants (both catechins and theaflavins) varies widely from one tea to the next. It is impossible to generalize about one class of tea, such as black tea, green tea, white, or oolong, having a higher or lower antioxidant content.[2]

Do the antioxidants in tea have health benefits?

There is a widespread public perception that antioxidants are generally beneficial to health. This is not necessarily true, and this impression is primarily a result of tea companies and nutritional supplement companies exaggerating health claims in order to promote their products. One of the most common claims is that the antioxidants in tea prevent cancer. The Linus Pauling Institute has reviewed the scientific literature surrounding tea and cancer and has concluded that while there is some evidence that tea flavonoids may prevent cancer, they probably do so through some other biological pathway, and not due to their antioxidant activity.[3] While there is significant scientific evidence backing some of the health benefits of tea, there is also a great deal of exaggeration circulated by various less reputable sources, concerning the health benefits of the antioxidants in tea.

Although antioxidants can have positive effects, they can also have negative effects on health as well. For example, antioxidants such as the theaflavins in black tea can bind to nonheme iron (the principal form of iron found in plant and dairy sources and in most nutritional supplements). Consumption of beverages rich in polyphenolic compounds, including black tea, peppermint tea, cocoa, and chamomile tea, has been shown to reduce the absorption of iron; in one study, 79-94% of the absorption was inhibited by black tea, and chamomile inhibited 47% of iron absorption.[4] Based on these results, it is best to drink tea well before or after a meal, and not during the meal, especially if one is concerned with iron absorption. For more information, you can read our article on tea and iron absorption.

Does milk interfere with the antioxidants in tea?

There has been some confusion and controversy surrounding the question of whether or not adding milk to tea interferes with or blocks the health benefits of antioxidants in the tea. Some early studies found fairly conclusive evidence that milk binds to certain antioxidants in the tea, thus preventing their absorption into the bloodstream. These results have been confirmed by measuring the level of catechins in the bloodstream. However, later work on black tea has found that the total antioxidant absorption, and the ability of the tea to prevent oxidative stress, is unaffected by adding milk.[5]

Are antioxidants unique to tea or the tea plant?

No. In fact, even the catechins, the main antioxidants in unoxidized teas, are found in other plants, including cocoa beans. Antioxidants are common and abundant in plant sources, including fruits, vegetables, and herbal teas. The antioxidants in some herbal teas, including rooibos, have been studied and compared to those in tea.

References:

1. Lai Kwok Leung et. al., Theaflavins in Black Tea and Catechins in Green Tea Are Equally Effective Antioxidants, Journal of Nutrition, 2001, Vol. 131, pp.2248-2251.

2. M. Friedman et. al. Distribution of catechins, theaflavins, caffeine, and theobromine in 77 teas consumed in the United States, Journal of food science, 2005 Nov-Dec, v. 70, no. 9, p. C550-C559.

3. Linus Pauling Institute's Page on Tea - Comprehensive and well-referenced summary of the health effects of tea.

4. R.F. Hurrell et. al, Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages., British Journal of Nutrition, April 1999, Vol. 81, No. 4, pp. 289-95.

5. C. Vijayakumar et. al., Addition of Milk Does Not Alter the Antioxidant Activity of Black Tea, Annals of Nutrition and Metabolism, Vol. 49, 2005.


Further Reading:

Antioxidants - On Wikipedia


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