Methods available for the measurement of antioxidant capacity are reviewed, presenting the general chemistry underlying the assays, the types of molecules detected, and the most important advantages and shortcomings of each method. This overview provides a basis and rationale for developing standardized antioxidant capacity methods for the food, nutraceutical, and dietary supplement industries. From evaluation of data presented at the First International Congress on Antioxidant Methods in 2004 and in the literature, as well as consideration of potential end uses of antioxidants, it is proposed that procedures and applications for three assays be considered for standardization: the oxygen radical absorbance capacity (ORAC) assay, the Folin-Ciocalteu method, and possibly the Trolox equivalent antioxidant capacity (TEAC) assay. ORAC represent a hydrogen atom transfer (HAT) reaction mechanism, which is most relevant to human biology. The Folin-Ciocalteu method is an electron transfer (ET) based assay and gives reducing capacity, which has normally been expressed as phenolic contents. The TEAC assay represents a second ET-based method. Other assays may need to be considered in the future as more is learned about some of the other radical sources and their importance to human biology.

https://faculty.missouri.edu/~glaserr/3700s14/Antioxidant-Capacity_jf0502698.pdf

Standardized Methods for the Determination of Antioxidant Capacity and Phenolics in Foods and Dietary Supplements

There has been increasing interest in the research on flavonoids from plant sources because of their versatile health benefits reported in various epidemiological studies. Since flavonoids are directly associated with human dietary ingredients and health, there is need to evaluate structure and function relationship. The bioavailability, metabolism, and biological activity of flavonoids depend upon the configuration, total number of hydroxyl groups, and substitution of functional groups about their nuclear structure. Fruits and vegetables are the main dietary sources of flavonoids for humans, along with tea and wine. Most recent researches have focused on the health aspects of flavonoids for humans. Many flavonoids are shown to have antioxidative activity, free radical scavenging capacity, coronary heart disease prevention, hepatoprotective, anti-inflammatory, and anticancer activities, while some flavonoids exhibit potential antiviral activities. In plant systems, flavonoids help in combating oxidative stress and act as growth regulators. For pharmaceutical purposes cost-effective bulk production of different types of flavonoids has been made possible with the help of microbial biotechnology. This review highlights the structural features of flavonoids, their beneficial roles in human health, and significance in plants as well as their microbial production.

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Chemistry and Biological Activities of Flavonoids: An Overview

A novel automated direct measurement method for total antioxidant capacity using a new generation, more stable ABTS radical cation

Increasing interest in the health benefits of tea has led to the inclusion of tea extracts in dietary supplements and functional foods. However, epidemiologic evidence regarding the effects of tea consumption on cancer and cardiovascular disease risk is conflicting. While tea contains a number of bioactive chemicals, it is particularly rich in catechins, of which epigallocatechin gallate (EGCG) is the most abundant. Catechins and their derivatives are thought to contribute to the beneficial effects ascribed to tea. Tea catechins and polyphenols are effective scavengers of reactive oxygen species in vitro and may also function indirectly as antioxidants through their effects on transcription factors and enzyme activities. The fact that catechins are rapidly and extensively metabolized emphasizes the importance of demonstrating their antioxidant activity in vivo. In humans, modest transient increases in plasma antioxidant capacity have been demonstrated following the consumption of tea and green tea catechins. The effects of tea and green tea catechins on biomarkers of oxidative stress, especially oxidative DNA damage, appear very promising in animal models, but data on biomarkers of in vivo oxidative stress in humans are limited. Larger human studies examining the effects of tea and tea catechin intake on biomarkers of oxidative damage to lipids, proteins, and DNA are needed.

Tea catechins and polyphenols: health effects, metabolism, and antioxidant functions.

Natural antioxidants from residual sources.

This study aimed to compare in vitro antioxidant power of different types of tea (Camellia sinensis). The ferric reducing/antioxidant power (FRAP) assay was used to measure the total antioxidant power of freshly prepared infusions of 25 types of teas. Results showed that different teas had widely different in vitro antioxidant power and that the antioxidant capacity was strongly correlated (r = 0. 956) with the total phenolics content of the tea. Expressed as micromol of antioxidant power/g of dried tea leaves, values ranged as 132-654 micromol/g for black ("fermented") teas, 233-532 micromol/g for Oolong ("semifermented") teas, and 272-1144 micromol/g for green ("nonfermented") teas. One cup of tea of usual strength (1-2%), therefore, can provide the same potential for improving antioxidant status as around 150 mg of pure ascorbic acid (vitamin C).

Total antioxidant capacity of teas by the ferric reducing/antioxidant power assay.

Epidemiological evidence links high intake of ascorbic acid (AA) and other antioxidant micronutrients to health promotion. It would be useful to know the overall, or 'total' antioxidant capacity of foods, to establish the contribution of AA to this, and to assess how this information may translate into dietary intakes to meet the new US daily reference intake for AA. In this study, the total antioxidant capacity, as the ferric reducing-antioxidant power (FRAP) value, and AA content of thirty-four types of fruits and vegetables were measured using a modified version of the FRAP assay, known as FRASC. This measures AA (reduced form only) simultaneously with the FRAP value. Results covered a wide range: 880-15940 micromol/kg fresh wet weight and <20-540 mg/kg fresh wet weight respectively, for FRAP and AA, which comprised < 1-73 % and < 1-59 % total antioxidant capacity of fruits and vegetables respectively. We estimate that 100 mg AA is contained in one orange, a few strawberries, one kiwi fruit, 1-2 slices of pineapple, several florets of raw cauliflower or a handful of uncooked spinach leaves. Apples, bananas, pears and plums, the most commonly consumed fruits in the UK, contain very little AA. Results indicate also that the antioxidant capacity of vegetables decreases rapidly and significantly after fragmentation. Results of this, and future studies, using FRASC as a biomonitoring tool will be useful in food production, preparation, preservation, and aid dietary choices to increase antioxidant and AA intake. Furthermore, FRASC will facilitate bioavailability studies of antioxidants from different foods of known antioxidant capacity and AA content.

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Total antioxidant and ascorbic acid content of fresh fruits and vegetables: implications for dietary planning and food preservation.

Green tea contains polyphenolic antioxidants that have shown anticarcinogenic properties in animal and in vitro experimental studies. Current data regarding absorption and bioavailability of tea antioxidants in humans, however, are conflicting. In this study, plasma and urine antioxidant power after ingestion of green tea was measured using the ferric reducing/antioxidant power (FRAP) assay (US patent pending) to assess absorption, systemic distribution, and renal excretion of green tea antioxidants in healthy adults. Results showed that absorption of green tea antioxidants was rapid, with peak increase in plasma FRAP of around 4% at 40 minutes after ingestion: mean increase was 44 +/- 9 (SE) mumol/l. Excretion of polyphenolic antioxidants was also fast, peaking at 60-90 minutes, with significant correlation between urinary FRAP values and urinary total phenolic concentrations (r = 0.845, p < 0.001). In control studies, no increase in plasma or urine FRAP values was seen after intake of water. Although the amount of antioxidants absorbed was relatively small and the increase in plasma antioxidant power was of short duration, results demonstrate that some potentially anticarcinogenic polyphenolic antioxidants in green tea enter the systemic circulation soon after ingestion and cause a significant increase in plasma antioxidant status. This increase may, in turn, lower oxidative damage to DNA and so decrease risk of cancer.

Consumption of Green Tea Causes Rapid Increase in Plasma Antioxidant Power in Humans

Effects of a long-term vegetarian diet on biomarkers of antioxidant status and cardiovascular disease risk.

The protective effect of fruits and vegetables against cancer is well established. It is believed that this effect is mediated by antioxidants and decreased oxidative damage to DNA. However, the identity of the antioxidant(s) responsible is not clear. Moreover, a potentially damaging pro-oxidant effect of some antioxidants has been reported. In this study the ex vivo effects of several dietary antioxidants, including quercetin, various catechins, ascorbic acid and alpha-tocopherol, were investigated, at concentrations up to 200 microM, using the single cell gel electrophoresis (comet) assay for DNA damage. Lymphocytes from three healthy subjects were pre-incubated with these antioxidants, and the comet assay was performed on treated, untreated, challenged and unchallenged cells in parallel, oxidant challenge being induced by 5 min exposure to hydrogen peroxide (final concentrations H2O2: 30, 45, or 60 microM). Results using this ex vivo cellular assay showed protection by some antioxidants (quercetin, caffeic acid), no effect by some (catechin, epicatechin, catechin gallate, epicatechin gallate) and an apparently damaging effect by others (epigallocatechin, epigallocatechin gallate). Damage may have been caused by production of H2O2 from these polyphenolics. Neither ascorbic acid nor alpha-tocopherol protected or damaged DNA. Further study of the role of quercetin and caffeic acid in DNA protection is needed.

Yim Tong Savio Szeto

Papers

Total antioxidant capacity of teas by the ferric reducing/antioxidant power assay.

Total antioxidant and ascorbic acid content of fresh fruits and vegetables: implications for dietary planning and food preservation.

Consumption of Green Tea Causes Rapid Increase in Plasma Antioxidant Power in Humans

Effects of a long-term vegetarian diet on biomarkers of antioxidant status and cardiovascular disease risk.

Effects of dietary antioxidants on human DNA ex vivo.