Oxidative Damage By Oxygen Free Radicals Biology Essay


Oxidative damage by oxygen free radicals are known to be one of the mechanisms of chronic disorders such as atherosclerosis or cancer. Oxygen free radicals are created through aerobic metabolism and are mostly removed by antioxidants in vivo. These antioxidants can either be endogenous e.g. superoxide dismutase, glutathione peroxidase and metal-binding proteins or exogenous coming from foods or drink e.g. tea (Sung et al., 2000).

Tea is the world's second most consumed beverage after water and is a rich dietary source of

Flavonoids (Higdon and Frei, 2003). These are polyphenolic compounds, which in vitro, possess powerful antioxidant properties (Langley-Evans, 2000). A range of epidemiological studies have suggested that the consumption of flavonoids and particularly tea flavonoids (catechins, theaflavins and thearubigins) may be associated with reduced risk of coronary heart disease, stroke and cancer. This is due to the antioxidants inhibiting the free radicals (from foods, smoking and other environmental factors) oxidising and damaging the cell (Panglossi, 2006).

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About antioxidants

The term "antioxidant" is used widely in the marketing industry, it is defined by the Institute of Medicine of the National Academy of Sciences as "a substance in foods that significantly decreases the adverse effects of reactive species, such as reactive oxygen and nitrogen species, on normal physiologic function in humans." Therefore understanding antioxidants is the key to knowing why they are imperative to the human body remaining healthy. As shown in a review done by Catoni et al. (2008) there have been many explanations to how and why antioxidants achieve this. Antioxidants act in vivo but the research is being carried out in vitro, this difference has been analysed by Zhang and Shen (1997) with the intention of discovering if antioxidants react differently depending whether they are in or out of the body.

Benefits of antioxidants

Many scientists believe that green tea may lower the risk of heart disease and some types of cancer due to there being lower incidence of these in the Orient, where more tea is consumed than anywhere else in the world. Many studies have been conducted to discover whether this hypothesis is correct. The antioxidant affect of green tea was found to be stronger than that of other natural antioxidants such as vitamins C and E. Ingestion of tea was reported to have raised the global antioxidant power in human plasma by 41% to 48% (Asfar et al., 2003).

The research has uncovered several different ways in which antioxidants can reduce the risk of developing heart disease or some cancers. A study conducted by Nakagawa et al. (1999) shows that Catechin has a positive correlation with reducing cardiovascular disease by increasing the plasma antioxidant capacity in humans. Another study conducted by Zhang et al. (2004) shows that the polyphenols in green tea can induce apoptosis and cell cycle arrest in human carcinoma cells, therefore preventing the spread of cancer. This had also been shown in a study done by Asfar et al. (2003) that green and black teas have been found to have a protective effect on chemically induced experimental tumours of the small intestine, colon, liver, and mammary epithelium in rats. In addition, case control studies have associated consumption of large quantities of green tea with the prevention of chronic active gastritis and with decreased risk of gastric cancer in Japan and China. Shibata et al. (2000) attributed these effects of green tea to its antioxidant properties.

Antioxidants also had a positive effect on decreasing the risk of developing atherosclerosis by reducing the amount of circulating LDL in the blood. (Vinson, 2004). All studies have shown a positive correlation between drinking tea and reducing health risks which strengthens the rationale for doing the research.

Antioxidants in vitro

There are many studies that have looked at the antioxidant levels in tea as well as many other beverages, fruits, vegetables and wine such as Thiapong et al., (2006) and Seeram et al, (2008) whom studied the antioxidant power of guava fruit and pomegranate juice respectively. A lot of these studies investigate the antioxidant levels in vitro, which may not reflect the antioxidant power within the body. These studies are very accurate with the antioxidant potential of the foods and beverages but if these cannot be used within the body then they will not have any beneficial effect, such as being chemoprotective. In a study conducted by Jenson et al., (2008) antioxidants were examined to see whether the anti-inflammatory compounds known to be within them were present in the components of the unprocessed ingredients of the MonaVie Active juice blend of fruits and berries were in a form able to enter into and protect human calls in vitro. Furthermore, the study also investigated the bioavailability of these compounds following ingestion of the MonaVie Active juice blend and its effect on serum indicators of oxidative damage. The results found that antioxidants had a beneficial effect both in vitro and in vivo.

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Antioxidants in vivo

Although it is well established that certain foods and beverages have a high amount of antioxidants when measured in vitro it is not well established whether they are as potent in vivo. This has been addressed by my researchers that have looked at antioxidants within the bodies of mice (Koutelidakis et al., 2009), hamsters (Vinson et al., 2004; Rouanet et al., 2010) and human plasma (Nakagawa et al., 1999 and Sung et al., 2005). Considering that antioxidants have the potential to be beneficial in protecting against atherosclerosis, cardiovascular disease (CVD) and some cancers, it is essential that it is recognised whether these antioxidants are effective within the body. In a study done by Koutelidakis et al., (2009), on mice found that green and white tea and of the aromatic plant Pelargonium purpureum (little robin) increased the total antioxidant capacity of plasma and of selected organs which shows that antioxidants can be absorbed by the body but this does not mean that they prevent CVD or cancer. In another study conducted by Sung et al., (2005) this was addressed by evaluating the effects of green tea ingestion over four weeks on atherosclerotic biological markers. The study concluded that antioxidants in tea did have an effect on atherosclerotic biological markers in vivo; therefore suggesting that regular tea ingestion could reduce the risk of atherosclerosis. Nakagawa et al., (1999), in their study, also found that the ingestion of tea may protect against CVD or atherosclerosis. These studies were also carried out in humans, which gives stronger evidence of antioxidants being beneficial to humans and not just to mice.

Different teas and brewing time

The research done by Komes (2009) shows that the different methods of analysing antioxidants are more suitable than others, depending on what is being investigated. This study was done on green tea and also identifies how preparation time can also affect antioxidant levels. This helped me to decide what would be investigated and why as well as what method should be used, ultimately concluding that the FRAP method would be used to establish the amount of antioxidants in certain teas.

As tea is one of the most widely consumed beverages it could play an important role in preventing chronic disease, but not all tea is brewed in the same way. A study conducted by Langley-Evans (2000) considers the consequence of altering brewing time and temperature on the antioxidants released from green and black tea. It is concluded that brewing time and temperature do have an effect on antioxidant capacity and this is determined using the FRAP method. Different teas may have different levels of antioxidants and the way in which tea is prepared and brewed may also affect antioxidant levels, which forms the basis of my investigation. A study conducted by Komes et al., (2010) showed not only did the brewing time of green tea have an effect on the antioxidant capacity of the cup of tea but also whether the tea was in bags, loose or powdered, concluding that different forms of green tea need different brewing times to extract all the antioxidants, which could be a contributing factor to consumer choice. Although some work has already been done in this area, more research needs to be conducted to establish the optimal way to make green or black tea thus optimising antioxidant intake.


The data suggests that green tea drinking does have a beneficial effect, by reducing the development or the enhancement of oxidative stress and therefore protecting the individual from oxidative stress diseases (Asfar et al., 2003). Many studies have been conducted in vitro and in vivo to discover if antioxidants are beneficial to the human body in protecting it from certain diseases. Both methods seem to find that antioxidants especially in tea do have protective qualities (Jenson et al., 2008). Depending on the type or form of tea is the preparation method which should be used to brew the tea, as different methods can optimise the antioxidant extraction (Komes et al., 2010).


The hypothesis is that the longer time tea is brewed the larger the FRAP values indicating that more antioxidants have been released in both teas although, these values will be higher in green tea than black tea.

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