An Italian study found that while green tea was six times more potent in inhibiting lipid peroxidation in vitro, when healthy human subjects ingested the same amount of either black or green tea, the plasma antioxidant capacity (expressed as TRAP, or total radical-trapping antioxidant parameter) was similar in both groups.

The increase in plasma antioxidant capacity was quite rapid, peaking at 30-50 minutes. This indicates that the most bioactive antioxidant compounds are quickly absorbed in the upper part of the gastrointestinal system.

Interestingly, while the addition of milk did not diminish antioxidant activity of either black or green tea in vitro, it completely abolished it in vivo, according to an Italian study. Some authors, however, think that skim milk does not significantly interfere with the absorption of polyphenols.

One study found green tea polyphenols to be comparable to BHT (a well-known synthetic phenolic antioxidant) in antioxidant power when it came to protecting canola oil; green tea outperformed BHT when the oil was heated.

Catechins have also been found to outperform Vitamin C and beta-carotene ten times in scavenging the alkyl peroxyl radical. One study found green tea polyphenols to be more potent antioxidants than Vitamin C, Vitamin E, rosemary extract, and even curcumin in some systems.

One of the antioxidant properties of green tea catechins is their ability to protect against the ravages of oxidized linoleic acid - an omega-6 fatty acid usually consumed in excess in the Western world, from sources such as margarine, safflower oil and corn oil; excess linoleic acid (most likely in the peroxidized form, which activates various pro-inflammatory enzymes) is involved in promoting inflammation, tumor growth, and most degenerative disorders. Few people are aware that when polyunsaturated cooking oils are heated, the result is carcinogenic peroxides. A recent study discovered that catechins synergize with alpha-tocopherol (Vitamin E) to protect against oxidative damage by oxidized linoleic acid before this fatty acid is incorporated into cell membranes.

Catechins also have a sparing effect on Vitamin E (alpha-tocopherol) and beta-carotene, resulting in overall greater antioxidant protection of the polyunsaturated fatty acids that are incorporated into cell membranes. The sparing effect on tocopherol may be due to the fact that catechins are effective scavengers of aqueous oxygen radicals, and thus prevent their entry into the lipids, where these radicals would oxidize alpha-tocopherol. It is also possible that catechins can regenerate the antioxidant power of alpha-tocopherol by donating a hydrogen molecule to its oxidized form. Thus it appears that green tea polyphenols, like flavonoids in general, boost the levels of other antioxidants.

Of special interest is the ability of catechins to inhibit the dangerous peroxynitrite free radical, a strong reactant oxidant formed when the potent free radical superoxide encounters nitrogen-containing compounds such as nitric oxide. Peroxynitrite destroys proteins, as well as vital antioxidants such as glutathione and Vitamin E. Green tea catechins were shown to be more effective than Trolox, a synthetic antioxidant, in protecting critical amino acids such as tyrosine and the amino acids of apolipoprotein B in LDL cholesterol against nitration damage by the peroxynitrite radical.

Another interesting recent discovery concerns one particular green tea polyphenol called epicatechin. This particular polyphenol appears to be metabolized to an anthocyanin-like compound that is also an antioxidant, offering particularly long-lasting protection.
Returning to the question of antioxidant properties of simple catechins in green tea versus the complex polymeric polyphenols in black tea, a new study found that in lipids the simple compounds were more effective, while in aqueous conditions polymers had more activity, but only up to a point. Glycosylation, meaning cross-linking with glucose, decreased the antioxidant properties of all compounds tested.