Author: Szeto, Yim-tong Savio
Title: Antioxidant potential of selected dietary and medicinal agents; implications for improving in vivo antioxidant status
Degree: Ph.D.
Year: 1999
Subject: Antioxidants
Hong Kong Polytechnic University -- Dissertations
Department: Department of Nursing and Health Sciences
Pages: xv, 206 leaves : ill. ; 30 cm
Language: English
Abstract: The main purpose of the study was to assess the potential for dietary agents, such as teas, fruits and vegetables, and selected drugs and medicinal agents, to increase the antioxidant status of the body, reduce oxidative stress and thereby, lower risk of diseases associated with increased oxidative stress. The specific aims of the study were: 1. To measure and compare the in vitro total antioxidant (reducing) power of various types of teas, drugs, fruits and vegetables, and selected medicinal agents. 2. To measure the bioavailability of the antioxidant power in selected beverages with high in vitro total antioxidant (reducing) power by monitoring post-ingestion changes in plasma and urine. 3. To measure the potential protective and/or genotoxic effects of selected antioxidants, e.g. tea polyphenolics and ascorbic acid (vitamin C). In vitro total antioxidant capacities and ascorbate content of more than 30 varieties of fruits and vegetables, including several Chinese fruits and vegetables, were studied by the FRASC assay. The database of antioxidant capacity and ascorbic acid level of fruits and vegetables is being developed and will hopefully form the basis of a long-term systematic study. Also, selected hypoglycaemic and lipid lowering drugs were tested for antioxidant power. The antioxidant power of different types of teas (Camellia sinensis) was also investigated. The ferric reducing/antioxidant power (FRAP) assay was used to measure the total antioxidant power of freshly prepared infusions of 29 types of teas including black, oolong and green teas. Results showed that different teas had widely different in vitro antioxidant power and that the antioxidant capacity was strongly correlated with the total phenolics content of the tea. Green tea was found to contain very high antioxidant power and was selected for the bioavailability study. Post-ingestion changes in the plasma and urine antioxidant power of 10 adults were measured at timed intervals. Results showed that absorption of tea antioxidants was rapid, with a peak increase in plasma around 4% at 40 minutes. Urinary excretion of absorbed polyphenolic antioxidant was also fast, peaking at 60-90 minutes. Purified polyphenols from tea and wine were purchased and tested for DNA protective and damaging effects on human lymphocytes. The single cell gel electrophoresis test (Comet assay) was used for determining DNA damage in individual cells before and after polyphenol treatment. DNA protective effect was tested after polyphenol incubation followed by oxidative stress induced by H2O2 treatment. Uric acid and antioxidant vitamins (ascorbic acid, α-tocopherol) were included for comparison. Results showed that quercetin, caffeic acid and α-tocopherol, were protective against oxidant stress. Ascorbic acid at low concentration (< 200 umol/1) showed a trend of protection but this did not reach a statistically significant level. Catechin, epicatechin and catechin gallate showed no apparent DNA protective or damaging effect. Uric acid and ascorbic acid at high concentration (> 200 umol/1), and epigallocatechin, epigallocatechin gallate, resveratrol, green tea, black tea and Lingzhi at all concentrations tested caused increased damage. Results indicate that various plant-based foods and some medicinal agents have high antioxidant potential. Results have shown that at least some of the antioxidant power in polyphenolic rich dietary agents is absorbed and enters the plasma. Results also show, however, that DNA damage may be increased in the presence of some antioxidants. Further study is needed to assess the clinical effect of increased antioxidant status.
Rights: All rights reserved
Access: open access

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