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Oxygen is a highly reactive atom that is capable of becoming part of potentially damaging molecules commonly called free radicals. Free radicals are capable of tacking the healthy cells of the body, causing them to lose their structure and function (Percival, 1996).
Free radicals are molecules produced when your body breaks down food, or by environmental exposures like tobacco smoke and radiation. Free radicals can damage cells, and may play a role in heart disease, cancer and other diseases.
Antioxidants are substances that may protect cells from the damage caused by unstable molecules known as free radicals (Hamid et. al., 2010). In other words, antioxidants are substances that neutralize free radicals or their actions (Sies, 1996). This is because antioxidants main characteristic is the ability to trap free radicals.
Oxidation is a chemical reaction that transfers electrons from a substance to an oxidizing agent. Oxidation reactions can produce free radicals, which start chain reactions that damage cells. Antioxidants terminate these chain reactions by removing free radical intermediates, and inhibit other oxidation reactions by being oxidized themselves.
The term oxidative stress is used to explain the shift towards the pro-oxidants in the pro-oxidant/antioxidant balance that can occur as a result of an increase in oxidative metabolism.
IDENTIFICATION AND CLASSES OF ANTIOXIDANTS
According to Hamid et. al. (2010), antioxidants are classified into two which are primary or natural antioxidants and secondary or synthetic antioxidants.
Primary or natural antioxidants
They are the chain breaking antioxidants which react with lipid radicals and convert them into more stable products. Antioxidants of this group are mainly phenolic in structure and include antioxidants minerals, antioxidants vitamins and phytochemicals.
Antioxidants minerals are the cofactor of antioxidants enzymes. Their absence will definitely affect metabolism of many macromolecules such as carbohydrate, protein and fat.
Antioxidants vitamins are needed for most body metabolic functions. The examples are vitamin B, vitamin C and vitamin E.
Phytochemicals are phenolic compounds that are neither minerals nor vitamins. An example of a phytochemical is flavonoids. Flavonoids have been demonstrated to have anti-inflammatory, anti-allergenic, anti-viral, anti-aging, and anti-carcinogenic activity (Percival, 1996).
Secondary or synthetic antioxidants
These are phenolic compounds that perform the function of capturing free radicals and stopping the chain reactions. The included compounds are Butylated hydroxyl anisole (BHA), butylated hydroxyrotoluene (BHT), propyl gallate (PG) and metal chelating agent (EDTA), tertiary butyl hydroquinone (TBHQ) and nordihydro guaretic acid (NDGA).
However, according to another author, antioxidants are classified into two groups which are the preventive and chain breaking antioxidants. Preventive antioxidants reduce the rate of chain initiation and include catalase and other peroxidases that react with the ROOH and chelators of metal ions such as EDTA and DTPA.
Chain breaking antioxidants are the ones that interfere with chain propagation and include superoxide dismutase and vitamin E.
Classifications of antioxidants are dependent on the way they act. There are three classifications of antioxidants. The first classifications are the preventive antioxidants. These antioxidants suppress the formation of free radicals for example such as superoxide dismutase, catalase, carotenoids and albumin.
The second classification is the radical-scavenging antioxidants. It scavenge radicals to inhibit chain initiation and break chain propagation for example such as vitamin A, vitamin C and vitamin E.
The third classification is the repair and de novo enzymes that repairs the damage and reconstitute membranes. For example DNA repair enzymes, lipase and transferase.
Vitamin C, vitamin E and beta-carotene are among the most abundant and cheap source of antioxidants.
Vitamin C is an important water soluble antioxidant in the extracellular fluids. It is capable of neutralizing ROS in the aqueous phase before lipid peroxidation is initiated. Vitamin C is capable of regenerating vitamin E (Sies et. al.,1992).
Vitamin E is the major lipid soluble antioxidant. It is most effective in chain-breaking antioxidant within the cell membrane where it protects membrane fatty acids from lipid peroxidation.
Beta carotene and other carotenoids are believed to provide antioxidant protection to lipid-rich tissue. Past studies suggested beta carotene may work synergistically with vitamin E (Jacob, 1995; Sies et. al.,1995).
Flavonoids have the ability of anti-inflammatory, anti-allergic, anti-viral, anti-aging and anti-carcinogenic activity. In addition to an antioxidant effect, flavonoid compounds may exert protection against heart disease through the inhibition of cyclooxygenase and lipoxygenase activities in platelets and macrophages (Havsteen, 1983).
Two principle mechanisms of action have been proposed for antioxidants. The first is a chain-breaking mechanism by which the primary antioxidants donate electrons t the free radicals. The second mechanism involves removal of reactive oxygen species (ROS) and reactive nitrogen species (RNS) initiator by quenching chain initiator catalyst.
The example below is given based on lipid radicals (Hamid et. al., 1996)
RH R.+ H.
R. R. O2 ROO.
2ROOH ROO. + RO. +H2O
R. + O2 ROO.
ROO. + RH ROOH + R.
RO. + RH ROH + R.
R. + R. R - R
R. + ROO. ROOR
ROO. + ROO. ROOR +O2
Antioxidants + O2 oxidized antioxidants
FACTORS INFLUENCE ANTIOXIDANT BIOAVAILABILITY
Antioxidants are found most in fruits and vegetables. It is also can be found in foods such as nuts, grains and some in poultry and fish. Vegetables that is rich in antioxidants such as beta carotene, lutein and lycopene.
There are four ways of people to consume and absorb higher levels of antioxidants as part of their daily diets. The four ways are to consume more fruits and vegetables and choose fruits and vegetables that contain relatively higher levels of antioxidants per serving. Moreover, improve the health of their gastrointestinal (GI) tracts in ways that improve absorption of ingested dietary antioxidants and consume foods that are grown, processed and prepared in ways that increase antioxidant levels in food at harvest.
Eating more servings of fruits and vegetables in among the best way to increase antioxidant intake; seeking out antioxidant-rich food is another step to increase it. The health of a person's GI tract can have a big impact on antioxidant bioavailability.
Besides that, a person needs to seek out for foods grown and processed in ways that maximize and retain antioxidant concentrations. They are advised to switch to organic farming methods because it may increase concentrations of antioxidants in fruits and vegetables without a proportional rise in calories.
EFFECTS OF IMBALANCE INTAKE OF ANTIOXIDANT
Oxidative stress is thought to contribute to the development of a wide range of diseases including Alzheimer's disease, Parkinson's disease, the pathologies caused bydiabetes, rheumatoid arthritis, and neurodegeneration in motor neuron diseases.
Indeed, a 2009 review of experiments in mice concluded that almost all manipulations of antioxidantsystems had no effect on aging. Diets high in fruit and vegetables, which are high in antioxidants, promote health and reduce the effects of aging, however antioxidant vitamin supplementation has no detectable effect on the aging process, so the effects of fruit and vegetables may be unrelated to their antioxidant contents.
One reason for this might be the fact that consuming antioxidant molecules such as polyphenols andvitamin E will produce changes in other parts of metabolism, so it may be these other effects that are the real reason these compounds are important in human nutrition.
Based on a past study, excess of free radicals may lead to cardiovascular disease. Oxidative damage to cholesterol component of low-density lipoprotein (LDL) leads to oxidised LDL by a series of consecutive events. This induces endothelial dysfunction, which promotes inflammation during atherosclerosis (Devasagayam et. al., 2004).
The figure below shows the mechanism by which oxidation of LDL may contribute to atherosclerosis.
Based on the same study, it was found that excess of free radicals may lead to neurodegenerative disorder such as Alzheimer's disease which was mention earlier. This is because the nervous tissue including brain is highly susceptible for free radical damage due to high content of lipids especially polyunsaturated fatty acids (Devasagayam et. al.,2004)
Free radical damage to DNA and cancer
DNA is a major target of free radical damage. These damages can result in mutations that are heritable change in the DNA which can lead to the yielding of cancel cells and tumor cells.
Free radicals and diabetes
The involvement of free radicals in the onset of diabetes had been proven by past study. It was also suggested that this leads to the development of diabetic complications (Lipinsky, 2001). Persistent hyperglycemia in the diabetic patients leads to generation of oxidative stress due to autooxidation of glucose, non-enzymatic glycosylation and polyol pathway.
Free radicals and ageing
Mitochondrial reactive oxygen species (ROS) production and oxidative damage to mitochondrial DNA results in ageing. Further increased lipid peroxidation in cellular membranes due to oxidative stress leads to fatty acid unsaturation (Barja, 2004).
ROLES OF ANTIOXIDANT IN EXERCISE PERFORMANCE
Antioxidants and exercise performance
Even though there is no solid conclusion in vitamin C supplementation in enhancing sports performance but Vitamin E has its benefits. Vitamin E had been shown to enhance oxygen utilization during exercise at altitude (Simon-Schnass et. al.,1994)
Ubiquinone also has a positive effect on sports performance. CoQ10 as it is known may improve oxygen uptake in the mitochondria of the heart. CoQ10 also has been used therapeutically for the treatment of cardiovascular disease. CoQ10 improved oxygen usage in the heart and skeletal muscles which could improve aerobic endurance performance (Williams, 2004).
Antioxidants and muscle tissue damage
Strenuous exercise may generate ROS to a level to overwhelm tissue antioxidant defense systems. This results in oxidative stress and it damages the muscle tissue. Preventing muscle tissue damage during exercise may help optimize the training effect and eventual competitive sports performance (Sen, 2001).
A study suggested that individuals performing regular heavy exercise should consume both antioxidants and vitamins supplement to assist the lipid peroxidation effect and reduce muscle damage (Dekkers et. al.,1996).
This study was supported by another study that reported Vitamin E have been shown to decrease the exercise-induced increase in the rate of lipid peroxidation, which could help prevent muscle tissue damage (Evans, 2000).
CONCLUSION AND RECOMMENDATION
Vigorous exercise, chronic inflammation and infections contribute to the increase of antioxidant load in the body. Plus other exposure to drugs and toxins such as cigarette smoke, pollutions and pesticides.
The human body utilizes an integrated antioxidant system composed of several players that work together as a team. The reducing potential of each individual member of the antioxidant defense team is enhanced when a full complement of players is available.
In order to counter balance the formation of free radicals, antioxidants are needed. Antioxidants are the first line of defense against free radical. Therefore, it is crucial to maintain the optimum health and well-being.