Green Tea Polyphenols Reduce Oxidative Damage in Ocular Tissues

Green Tea and its Catechin Components

Daily consumption of green tea has been linked to a number of potential health benefits. The results of several well-designed epidemiologic studies, for example, indicate that green tea consumption may be of modest benefit in preventing myocardial infarction or stroke.

The four primary polyphenols found in fresh green tea leaves are epigallocatechin gallate (EGCG), epigallocatechin, epicatechin gallate, and epicatechin. These tea catechins have been shown to have antioxidant activities in many biological systems, and have been shown to be protective against oxidative challenge in retinal photoreceptors in vitro. However, whether orally ingested catechins accumulate in eye tissues and actually exert a protective effect in vivo had not been examined until recently.

Researchers from the Chinese University of Hong Kong have now published the first paper to show distribution of individual catechins after ingestion of green tea extract and to evaluate their in vivo antioxidant effects in various compartments of the mammalian eye.

Study Design

Eleven groups of Sprague-Dawley rats of equal weight and age were fasted and fed 550 mg/kg of powdered green tea extract (GTE) suspended in water and administered via feeding tube. One group received water with no GTE, and served as controls.

The rats were sacrificed at various time points after GTE administration, and the aqueous and vitreous humors, cornea, lens, retina, and choroid-sclera were dissected out immediately and assayed for catechin content. 8-epi-isoprostane, a unique bioactive product of lipid peroxidation, was assayed in ocular tissues and fluids.

Results

Catechins were differentially distributed in eye tissues. Gallocatechin was present at the highest concentration in the retina, and epigallocatechin in aqueous humor. No catechin was detected in any of the tissues and fluids in the control group.

The time of maximum concentration of the catechins varied from 0.5 to 12.2 hours. Some catechins remained at high levels even after 20 hours of administration, for example gallocatechin in vitreous humor and in choroid-sclera.

Significant reductions in 8-epi-isoprostane levels were found in all compartments except the choroid-sclera or plasma, indicating antioxidant activities of catechins in these tissues (see graphs below).

Comments

The researchers concluded that green tea consumption can benefit the eye against oxidative stress. “Many studies on the antioxidative effect of green tea have focused on EGCG,” report the researchers. “However, in this study, we found its tissue level was not high. Gallocatechin, epigallocatechin, catechin, and epicatechin, on the other hand, sustained high levels in many ocular compartments. Although these compounds have a reducing power similar to or lower than that of EGCG, use of a mixture, such as green tea extract, was better than use of a single catechin because of synergistic antioxidant effects and bioavailability,” they added.

Reference

Chu KO, et al. Green tea catechins and their oxidative protection in the rat eye. J of Agricultural and Food Chemistry 58:1523-34, 2010.