What is Rich in Polyphenolic Antioxidants?

Green Tea

Tea (Camellia sinensis), especially green tea, is rich in polyphenolic antioxidants. The total antioxidant content of human plasma increases significantly within 30 min of ingesting green tea. Whether this effect relates to the modulation of oxidative stress in humans is unknown. Also, the potential health benefit of this effect is unknown. Hence, the purpose of this two-part study was to evaluate and compare the in vitro and clinical effects of different concentrations of green tea on human lymphocytic DNA. The specific teas (supplied by the Ying Kee Tea House, Hong Kong) were chosen based on their regional (Hong Kong) popularity.
In Vitro Assessment

Infusions (1% w/v) of Longjing green tea and screw-shaped green tea were infused in phosphate buffered saline (PBS) for 10 min and then filtered. Freshly prepared infusions were diluted in PBS to give 0.1%, 0.05%, 0.01%, and 0.005% (w/v) tea solutions. The total antioxidant content (as the ferric-reducing antioxidant power value) was measured. Lymphocytes were pooled from the venous blood of 5 healthy volunteers. The comet assay, most commonly performed on peripheral lymphocytes, was conducted. The comet assay is often used to evaluate potential genoprotective or genotoxic agents. It is used to study intervention-related changes in basal levels of oxidation-induced DNA damage and resistance of DNA to oxidant challenge.

Both teas had similar total antioxidant content. According to the comet assay, no detectable DNA damage was caused by either tea at any concentration tested. Cells that were pretreated with tea prior to an oxidant challenge had less DNA damage than cells not pretreated. Both teas offered similar levels of protection. There was no dose-response effect of the different concentrations of teas.

Clinical Assessment

Eighteen healthy men and women (aged 35-50 years) participated in this single-blind, crossover study conducted at The Hong Kong Polytechnic University, Kowloon, Hong Kong. Subjects ingested 150 ml 1% (w/v) freshly prepared Longjing green tea, screw-shaped green tea, or hot water (control) twice daily for 4 weeks. Subjects made their own tea following a set of standardized instructions and using the teabags and mugs provided. Subjects were non-smokers, without hypertension, not on regular medication, and with no previous history of stroke, myocardial infarction, angina, diabetes mellitus, cancer, or any illness that had required hospitalization during the previous 12 months. At entry, 6 subjects were allocated to each treatment. After a 6-week wash-out period, the subjects crossed-over into 1 of the other 2 treatments. Then, following a second 6-week washout, the subjects ingested the third treatment.

By the end of the study, all of the subjects had taken all 3 treatments. Compliance was assessed by inquiry and by counting the number of returned tea bags at the end of each intervention. Fasting venous blood and urine samples were collected before and after each supplementation phase. Blood was tested with the comet assay and urine was assayed for 7,8-dihydro-2-deoxyguanosine (8-oxodG). 8-oxodG is a biomarker for whole-body oxidative stress.

According to an oxidant challenge, cells collected from subjects after 4 weeks of tea supplementation had 22% less DNA damage after oxidant challenge than cells obtained after water ingestion (P < 0.05). According to the comet assay, DNA damage was 30% less in cells collected after Longjing tea consumption and 35% less after screw-shaped tea consumption compared with before consumption. This amount of DNA damage was significantly less than the amount of DNA damage after water treatment (P < 0.05). There were no significant differences in 8-oxodG. This may be attributed to the high biological variability in urine 8-oxodG.

The authors conclude that both types of tea provided significant genoprotection both in vitro and clinically, and both teas had a similar magnitude of effect and total antioxidant content. This implies that the genoprotective effects may be generalizable to other green teas containing similar total antioxidant content. The mechanism of action is unknown but may be related to the polyphenol content. Unfortunately, the authors did not measure plasma polyphenol content.

A dietary relevant dose of green tea was evaluated and produced a significant decrease in oxidation-induced DNA damage and significant increase in DNA resistance to oxidant challenge. Oxidation-induced DNA damage can cause cellular mutation, malignant (cancerous) changes in cells, altered homeostasis, and apoptosis (programmed cell death). This study supports the belief that green tea can promote health. The study should be repeated with additional types of green tea.

—Heather S. Oliff, PhD

http://cms.herbalgram.org/herbclip/414/111051-414.html

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