Antioxidants and free radicals are subject of great interest in both medical and nutritional circles. When reactive oxygen and nitrogen species are produced in vivo, they can cause harm to the human body. Superoxide dismutases, hydrogen peroxide removing enzymes, metal binding protein are endogenous antioxidant defences but cannot sufficiently prevent damage totally. It therefore follows that there is need to access antioxidants through other sources in other to maintain health. Plants abound in nature and many are a good source of antioxidant. Modern (western) lifestyle which most of us has adopted comes with some negative side effects from processed foods, reliance on medication and high exposure to chemical and environmental pollutants and these all lay a foundation for the proliferation of free radicals. Since we are exposed to such high rate of oxidative stress from a young age, we need more than ever to consume high antioxidant foods and therefore more research into plant capable of exerting these effects is necessary.


Title Page………………..i




Table of content………v


1.1    Introduction    -    -    -    -    -    -    -    -    -    1-3

1.2    Lifestyle and Human Health    -    -    -    -    -    -    -    3

1.2.1    Green tea consumption and human health    -    -    -    -    -    3-4

1.2.2    Polyphenol-type antioxidants in black tea    -    -    -    -    -    4-5

1.2.3    Red wine and human health    -    -    -    -    -    -    -    5-6


2.1    Free radicals    -    -    -    -    -    -    -    -    -    7-8

2.2    Types of free radicals    -    -    -    -    -    -    -    -    8

2.3    Oxidative Stress    -    -    -    -    -    -    -    -    8-9

2.3.1    Generation of free radicals    -    -    -    -    -    -    -    10-11

2.3.2    Damaging reactions of free radicals    -    -    -    -    -    -    11-12

2.4    Categories of Antioxidant    -    -    -    -    -    -    -    13

2.4.1    Based on their activity:     -    -    -    -    -    -    -    13

2.4.2    Enzymatic antioxidants     -    -    -    -    -    -    -    13

2.4.3    Non-enzymatic antioxidants    -    -    -    -    -    -    -    13

2.4.4    Based on solubility    -    -    -    -    -    -    -    -    13

2.4.5    Based on size    -    -    -    -    -    -    -    -    -    14

2.5    Definition of an Antioxidant     -    -    -    -    -    -    -    14

2.5.1    History of antioxidants    -    -    -    -    -    -    -    14-15

2.6       Sources of antioxidants in diet    -    -    -    -    -    -    15-16

2.6.1    Fruits/foods as good sources of antioxidants:        -    -    -    -    16-17

2.7        Mechanism of action of various natural antioxidant molecules    -    -    17-18

2.8    Natural non enzymatic antioxidants    -    -    -    -    -    -    18-19

2.8.1    Bioflavonoids        -    -    -    -    -    -    -    -    21-24

2.8.2    Carotenoids    -    -    -    -    -    -    -    -    -    24-26

2.8.3    Hydroxycinnamates    -    -    -    -    -    -    -    -    26-27   

 2.8.4    Other natural antioxidants    -    -    -    -    -    -    -    28-32

2.9    Mechanism of action of some antioxidants found in plants:    -    -    -    32   

2.9.1    Alpha tocopherol (Vitamin E):    -    -    -    -    -    -    32

2.9.2    Ascorbic Acid (Vit C)    -    -    -    -    -    -    -    -    32

2.9.3    Beta Carotene    -    -    -    -    -    -    -    -    -    32


3.1    Role of antioxidants in disease prevention    -    -    -    -    -    33

3.1.2    Neurological disorders    -    -    -    -    -    -    -    33-34

3.1.3    Diabetes     -    -    -    -    -    -    -    -    -    34

3.1.4    Immunological disorders    -    -    -    -    -    -    -    34-35

3.1.5    Cancer    -    -    -    -    -    -    -    -    -    -    35-36

3.2    Antioxidants from medicinal plants    -    -    -    -    -    -    36

3.3    Conclusion    -    -    -    -    -    -    -    -    -    40

References    -    -    -    -    -    -    -    -    -    41-47


1.1    Introduction

The human body has a complex system of natural enzymatic and non-enzymatic antioxidant defenses which counteract the harmful effects of free radicals and other oxidants. Free radicals are responsible for causing a large number of diseases including cancer (Kinnula and Crapo, 2004), cardiovascular disease (Singh and Jialal, 2006), neural disorders (Sas et al., 2007), Alzheimer’s disease (Smith et al., 2000), mild cognitive impairment (Guidi et al., 2006), Parkinson’s disease (Bolton et al., 2000), alcohol induced liver disease (Arteel, 2003), ulcer-ative colitis (Ramakrishna et al., 1997), aging (Hyun et al., 2006) and atherosclerosis (Upston et al., 2003). Protection against free radicals can be enhanced by ample intake of dietary antioxidants. Substantial evidence indicates that foods containing antioxidants and possibly in particular the antioxidant nutrients may be of major importance in disease prevention. There is, however, a growing consensus among scientists that a combination of antioxidants, rather than single entities, may be more effective over the long term. Antioxidants may be of great benefit in improving the quality of life by preventing or postponing the onset of degenerative diseases. In addition, they have a potential for substantial savings in the cost of health care delivery.

Any chemical compound or substance that inhibits oxidation is an antioxidant. Oxidation is a chemical reaction that can produce free radicals (an atom, molecule or ion with an unpaired valence electron) leading to chain reactions that may damage cells. Antioxidants terminate these chain reactions. Though human beings require oxygen for existence, oxygen is a highly reactive molecule that may damage living cells by producing Reactive Oxygen Species (ROS) like Superoxides, hydroxyl radicals, etc.

Having a biological context in mind, it is expected that ROS are formed as a natural byproduct of the normal metabolism of the oxygen and have significant roles in cell signaling and homeostatis (Devasagayam et al., 2004). The function of antioxidant systems is not to remove antioxidants entirely, but instead to keep them at an optimum level (Rhee, 2006).

ROS can actually damage cells by starting chemical chain reactions like lipid peroxidation (oxidative degradation of lipids) or by oxidizing DNA or proteins. Lipid peroxidation is simply the oxidative degradation of lipids where free radicals ‘steal’ electrons from lipids in cell membranes thus leading to cell damage. Most often lipids peroxidation involves polyunsaturated fatty acids because they contain multiple double bonds in between which lie methylene bridges (-CH2-) that have especially reactive hydrogen atoms. Just like any radical reaction, this reaction consists of three major steps: Initiation, Propagation and Termination. And its chemical products are called Lipid peroxides or Lipids Oxidation Products (LOPS).

Though oxidation is a normal chemical process that occurs every day in the human body, it can be accelerated by stress, unhealthy lifestyles like cigarette smoking, and alcohol. When there are disruptions in the natural oxidation process, the highly unstable and potentially damaging molecules called free radicals are created. Left uncontrolled, they can cause damage to body cells. It can be likened to a chemical reaction that creates rust on a bicycle thereby damaging the bicycle if left unchecked.

If we picture the body as a busy smooth road, we know that sometimes cracks and potholes can occur in the road. Free radicals are likened to the cracks while antioxidants are the materials used to fill those cracks to make the road normal again.  Also, as humans, when we grow older, we tend to become less active, our physiological systems function less optimally, and the probability of illness may increase. Antioxidants work by significantly slowing or preventing the oxidative damage from free radicals. These free radicals are hazardous and can damage body cells thus causing aging and disease. They can cause cell dysfunction and onset of problems like heart disease, diabetes, inflammation and cancer.

1.2    Lifestyle and human health

Smoking is one of the proposed major cause of cancer in the world. It has been implicated in over 30 % of cases. Improper diet (especially in industrialized nations) is another major factor in 35% cancer and degenerated diseases associated with aging. Inflammation due to chronic infection, especially in under developed countries has been cited as the cause for about 30% cases while occupational hazards (2%) and environmental factors (1%) are the other minor cause (Ames et al., 1993).

This indicates that if all external factors could be controlled and kept within reasonable limits, cancer and degenerative diseases associated with aging could be adequately regulated by the intake of healthy diet.

1.2.1    Green tea consumption and human health

Green tea has been valued as a powerful and miraculous medicine vital for the maintenance of good health. Epidemiological studies have shown that death rate from cancer is low in Shizuoka prefecture of Japan, a major green tea producing region (Oguni et al., 1989). Animal studies using mice have also proven the beneficial effects of components of green tea especially catechin in both inhibiting the initiation of cancer and suppressing the growth of tumor cells (Oguni et al., 1988). Green tea also has beneficial effects on the regulation of blood cholesterol levels, (Muramatsu et al., 1986). Green tea catechin is also believed to regulate blood pressure and can help to lower blood sugar levels (Shimizu, 1988). Evaluation of in vitro anti-oxidative activity of tea polyphenols showed that of the four catechins evaluated (-) epicatechin gallate and (-) epigallocatechin gallate shows the strongest protection against lipid peroxidation and were also found to be more active than the standard antioxidants α – tocophenol and propyl gallate (Osawa et al., 1990).

1.2.2    Polyphenol-type antioxidants in black tea

Black tea is richer than green tea in theaflavins which are dimers of catechins formed by enzymatic oxidation during the manufacture of black tea. During investigations theaflavin with two gallic acid moieties (theaflavin digallate) exhibited stronger antioxidative activity than that with just one gallic acid moiety (theaflavin monogallate) indicating that the gallic acid moiety is probably important for the antioxidative and anti mutagenic activities  of theaflavins (Ramarathmam et al., 1995; Shiraki et al., 1994).


Theaflavin monogallate                    Theaflavin digallate

1.2.3    Red wine and human health

Polyphenol oligomers namely procyanidines are widely distributed in the plant kingdom and have been used in the treatment of vascular disorders in humans because of the positive biochemical effects they have on blood vessels (Ames et al., 1993). Procyanidines are also effective as free-radical scavengers and inhibitors of oxidative enzymes such as xanthine oxidase (Ames et al., 1993). The vasorelaxing activity of wine and other grape products including grape juices and grape skin extracts has been documented. This effect is believed to be induced by quercitin and tannic acid, compounds known to be present in grape skin.

It appears that the consumption of red wine in moderate amounts may have a long term health benefit. Phytochemicals in wine products may play a critical role in reducing mortality from coronary heart diseases. In a study initiated by the World Health Organization (WHO), a marked difference in mortality and morbidity from coronary heart disease between French and US population showed that despite having a similar dietary intake of saturated fatty acids and comparable plasma cholesterol levels, the French subjects were less susceptible to coronary heart disease than the US subjects. It was postulated that the consumption of wine was the only dietary factor responsible for this discrepancy commonly referred to as the French paradox (NRCCDH, 1989).

Investigation has demonstrated that the non-alcoholic portion of red wine, particularly the phenolic substances inhibited the oxidation of human LDL in vitro (Kanner et al., 1994).

It is also noteworthy that the Mediterranean diet is rich in natural antioxidants and this leads to a limited incidence of cardiovascular diseases in people of this region (Sudhir et al., 1986).

The biggest danger of free radical to the human body is their potential to react with cellular components like DNA or the cell membrane, causing cells to function poorly or die. The best assurance of consuming adequate amounts of protective antioxidants is to eat between 5 and 9 servings of fruits and vegetables representing all the colors of the rainbow every day. Snacking on small amounting of nuts and consuming wine in moderation also contribute to antioxidant consumption.

Phenolic compounds in plants are recognized as important compounds that confer stability against oxidation. As antioxidants they maintain optimal cellular and human body health. Natural antioxidants increase the antioxidant capacity of the plasma and reduce the risk of certain diseases (Sudhir et al., 1986).




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