Free radicals exist as independent molecular species, generated by normal biological process (cellular metabolism) and environmental effects. Free radicals are extremely unstable, energy rich entities resulting from unpaired electrons. This is the reason that they are so quick to take part in chemical reactions and convert to different substances. This property of free radicals is responsible for the damage of biomolecules and forms the basis for several diseases.
Our body has natural defense system against these free radicals. Antioxidants are substances that protect cells from the damage caused by unstable free radicals. "Antioxidants interact with and stabilize free radicals and may prevent some of the damage free radicals might otherwise cause". Examples of antioxidants include beta-carotene, lycopene, antioxidant vitamins like A, C, E, reduced glutathione (GSH), thiols, and other antioxidant enzymes .
An antioxidant is a molecule capable of slowing or preventing the oxidation of other molecules. 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.
Higher intake of fruits, vegetables, and antioxidants may help in protecting from oxidative damage, thus lowering risk for cardiovascular disease and different types of cancer .
Epidemiologic studies have consistently found that a diet rich in fruits and vegetables is associated with reduced risk for cancer, cardiovascular diseases etc. Increased intake of fruits and vegetables may thus provide a defense against oxidative stress, a potential target for preventing diseases [3, 4].
The oxygen consumed is utilized by mitochondria for oxidative phosphorylation and is reduced to water in the electron transport chain (ETC). A small fraction of it is not used for this purpose instead it is converted into free radicals - which are harmful for the body when present in excess. Free radicals are harmful for the body because they contain an unpaired electron in their structure. These oxygen particles with an unpaired electron are called as reactive oxygen species (ROS) and are proven to cause cell and tissue injury. They are also responsible for certain diseases and to some extent the aging process. 'Reactive oxygen species' (ROS) of various types are formed in vivo and many are powerful oxidizing agents, capable of damaging DNA and other biomolecules. Increased formation of ROS can promote the development of malignancy, and the 'normal' rates of ROS generation may account for the increased risk of cancer development in the aged. Indeed, knockout of various antioxidant defense enzymes raises oxidative damage levels and promotes age-related cancer development .
When antioxidant system becomes ineffective, the body is said to be in oxidative stress. The antioxidant system can be weakened by a poor diet and a lack of nutrients, pathologic conditions or pharmacological intervention, including intake of certain medications.
SOURCES AND GENERATION OF ROS/FREE RADICALS
The major sources responsible for the generation of free radicals may be considered under two categories
i. Due to normal biological process (or cellular metabolism)
ii. Due to environmental effects
By definition, a free radical contains one or more unpaired electrons. Eg: O2-, OH-, ROO-. There are certain non radical derivatives of oxygen which do not contain unpaired electrons. Eg: H2O2, 1O2.The term reactive oxygen species is used in a broad sense to collectively represent free radicals and non free radicals (which are extremely reactive) of the biological systems . Radical formation is a spillover of the respiratory metabolism in the mitochondria of a cell . There are several mechanisms that can lead to the generation of free radicals. During the process of oxidative phosphorylation in the mitochondria, oxygen is used to form ATP. Little amount of oxygen can leak out and bind with free single electrons from the respiratory chain and later form super oxide radical (O2-). These superoxide radicals (O2-) can lead to the formation of hydrogen peroxide (H2O2) and the highly reactive hydroxyl radical (OH-) .
Another way of production of free radicals is through inhalation of environmental pollutants, such as NO2 and Ozone. The air can be a direct source of free radicals, for example nitrogen dioxide, or as an indirect source through the highly reactive pollutants such as ozone. Free radical formation is a normal biological process in cellular synthetic process, including formation of DNA and RNA and certain hormones.
Strenuous exercise induces free radical formation in muscles and in the liver which will lead to oxidative damage such as lipid peroxidation . This damage caused by oxidative stress can be reduced by taking dietary supplements of different antioxidants, such as vitamin E, vitamin C and beta carotenoids. It is estimated that about 1-4% of the O2 taken up by the body is converted to free radicals.
BENEFICIAL EFFECT OF FREE RADICALS
Free radicals play an important physiological role in humans. They participate in the metabolism of endogenous and exogenous lipids, in cellular respiration, in the production of prostaglandins and leukotrienes by arachidonic acid, in phagocytosis and in the immune response . Also, free radicals themselves play a role as scavengers for other free radicals. During the course of phagocytosis, inflammatory cells, particularly the macrophages produce superoxide (O2-), by a reaction catalyzed by NADPH oxidase. This superoxide (O2-) radical gets converted to H2O2 and then to hypochlorous (HClO). The superoxide (O2-) radical along with hypochlorous ions brings about bactericidal action. This truly represents the beneficial effects of the free radicals generated by the body. A large amount of O2 is consumed by macrophages during their bactericidal function, a phenomenon referred to as respiratory burst.
Perhaps free radicals (at least for most of our lifespan) do not pose a great threat to our wellbeing unless we expose ourselves to an excess of free radical-generating agents such as cigarette smoke or ionising radiation [11, 12].
HARMFUL EFFECTS OF FREE RADICALS
• Free radicals are highly reactive because of an unpaired electron present in their atomic structure. They are capable of damaging biomolecules - lipids, proteins, carbohydrates and nucleic acids. As already mentioned earlier, free radicals once formed in the biological system will continuously generate free radicals by chain reaction. They damage the membranes, cells and even tissues.
• Poly unsaturated fatty acids are highly susceptible to damage by free radicals.
Lipid peroxidation: Free radical induced peroxidation of membrane lipids occurs in three stages - initiation, propagation and termination.
• Initiation phase: This step involves the removal of hydrogen atom (H) from Poly Unsaturated Fatty Acids (LH), caused by hydroxyl radical
• Propagation Phase: Under aerobic conditions, the fatty acid radical (L-) takes up oxygen to form peroxy radical (LOO-). The Latter in turn, can remove H- atom from another PUFA (LH) to form lipid hydroperoxide (LOOH). The hydroperoxides are capable of further stimulating lipid peroxidation as they can form alkoxy (LO-) and peroxyl (LOO-) radicals.
• Termination Phase: lipid peroxidation proceeds as a chain reaction until the available PUFA gets oxidized.
Products of lipids peroxidation are unstable. Ex: carbonyls, esters, alkanes, alkenes, 2 - alkenal, 2,4- alkadienal, Maondialdehyde (MDA). Of these, is the most extensively studied, and is used as a biochemical marker for the assessment of lipid peroxidation. The estimation of serum MDA is often used to assess oxidative stress, and free radical damage to the body .
Free radicals can cause oxidation of sulfhydryl group of sulpher containing amino acids present in the biologically active proteins. ROS may damage proteins by fragmentation, cross linking and aggregation. Thus leading to conformational changes in the protein and it can become inactive.
Glycation of proteins (carbohydrate + proteins) present in cells increases the susceptibility of proteins to the attack by free radicals. This explains the reason for major complications involved with diabetes mellitus. Reactive species (RS) of various types are formed in vivo and many are powerful oxidizing agents, capable of damaging DNA and other biomolecules. Increased formation of RS can promote the development of malignancy, and the normal rates of RS generation may account for the increased risk of cancer development in the aged. Indeed, knockout of various antioxidant defense enzymes raises oxidative damage levels and promotes age-related cancer development in animals .
So excessive generation of free radicals in vivo is associated with various diseases, for example oxidized low density lipoproteins formed by the action of free radicals will promote atherosclerosis. It is also proved that free radicals are associated with certain inflammatory diseases like rheumatoid arthritis and respiratory disorders as well.
It is well known that in diabetes mellitus there is increased oxidative stress and accumulation of ROS can lead to the development of cataract in elderly people. Free radicals are also responsible for male infertility and aging process .
MAJOR ANTIOXIDANT NUTRIENTS:
Vitamin related antioxidants include vitamin E, C and carotenoids.
• Vitamin E: Vitamin E is a collective name for numerous different tocopherols and tocotrienols which share the same biological activity. Vitamin E is a fat soluble substance and is the major antioxidant in all cellular membranes and protects poly unsaturated fatty acids against oxidation.
• Vitamin C: Ascorbic acid is a water soluble substance. It is believed to be the most important antioxidant in cellular fluids, and it has many known intracellular activities and many preventive effects in other medical conditions as well.
• Carotenoids: The carotenoids are a group of red, yellow and orange pigments found in plant foods, particularly fruits and vegetables and in the tissues of animals which eat these plants. Some carotenoids can act as precursors of vitamin A.
MAJOR ENZYMATIC ANTIOXIDANTS
• Superoxide dismutase (SOD): It converts superoxide O2- to hydrogen peroxide and O2. Three forms of SOD are found in human tissues. They are a). Superoxide dismutase with copper and zinc, found in cytoplasm and organelles of almost all human cells. b). Superoxide dismutase with manganese, mainly distributed in mitochondria. c). Extracellular superoxide dismutase contains copper and zinc, synthesized by fibroblasts and endothelial cells.
• Catalase: Hydrogen peroxide produced by SOD is removed by catalase. It is mostly found in the peroxisomes.
• Glutathione peroxidase (GSH): It catalyze the oxidation of the reduced glutathione (G-SH) to oxidized glutathione (GS-SG). Activity of GSH depends on selenium.
Nutrition has a key role in maintaining the body's defense mechanism against free radicals. A proper diet with adequate intakes of antioxidants is important in the prevention of disease and promotion of optimal health and well being. The best known antioxidant vitamins are Vitamin E, vitamin C and carotenoids. Some foods contain substances with no nutritional function, but they are important to human health because of their antioxidant property for example resveratrol found in red wine.
ANTIOXIDANTS AND HEALTH BENEFITS
The most important factor in the development of cancer is the damage to the DNA. Much of this damage is oxidative in nature. According to research it is estimated that a human cells experiences about 10,000 hits to its DNA every day. These hits get repaired by DNA repair enzymes most of the time, but not always. Oxidative damage to the DNA accumulates with age and so does the risk of cancer.
When a cell containing damaged DNA starts to divide before it is repaired, a permanent genetic alteration occurs. This is the initial step in carcinogenesis. Antioxidants may be able to slow down or totally prevent this process from turning into cancer. Studies have shown that some antioxidants (beta carotene) may be beneficial in the treatment of oral leukoplakia, which may lead to oral cancer. Carotenoids enhance the immune function and there by act to prevent cancer .
The role of dietary factors, particularly micronutrients, in the prevention of the major chronic diseases has been the focus of an ever growing body of scientific investigation . Mounting evidence from laboratory and human studies points to the action of free radical species in the pathogenesis, and to the potential efficacy of vitamin C, vitamin E, and beta-carotene in reducing the risk of cancer, degenerative eye disorders, and cardiovascular diseases. For example, in both the Health Professionals Follow-up Study and the Nurses' Health Study supplementary intakes of vitamin E were associated with a reduced coronary heart disease risk of 4O% [17, 18]. Preliminary examination of the established populations for epidemiologic studies of the elderly (EPESE) revealed a reduction in coronary heart disease risk of 69% in older adults consuming supplements of vitamins C and E and of 58% in those taking vitamin E supplements only . Robertson et al found that people consuming supplements of vitamin C, vitamin E, or both appear to have a 40-75% reduction in the risk of developing cataracts . These data were reviewed during the First International Conference on Antioxidant Vitamins and beta Carotene in Disease Prevention, which was held in London in 1989 and revised again in late 90's. At that time, many scientists, confident in the strength of the existing evidence suggested that the results of the then-ongoing, large-scale epidemiologic and intervention studies would lead to clear and easily understandable recommendations for the general public regarding their intake of antioxidant micronutrients.
SUPPLEMENTATION OF ARTIFICIAL ANTIOXIDANTS VERSUS NATURAL ANTIOXIDANTS
Even though it is known that antioxidants can be obtained from food the consumption of antioxidant supplements in the general population is broad in extent. Many people are taking antioxidant supplements to supplement the natural antioxidant intake from their diet to improve the free radical scavenging activity in their bodies as a way to prevent health problems and prevent the development of disease states or otherwise to slow down the aging process or slow down the progression of disease conditions that are free-radical induced . A growing number of researchers have noticed that there is an industry-driven public obsession with antioxidants, which are equated to safe, health-giving molecules to be swallowed as mega-dose supplements or in fortified foods . More recent theories suggest that certain vitamins consumed as part of a healthy diet - and perhaps taken in supplement form - may be able to prevent damage to the body's tissues. This damage has been implicated in several major diseases including cancer and heart disease, yet the implication that vitamin supplements might protect people from these illnesses is controversial. As a defense against the detrimental effects of ROS/RNS a growing number of individuals use high doses of antioxidants (artificial supplements), and since there are few human studies, it is difficult to know the proper dosage to take. Selecting a dose too high may result in unacceptable safety problems, while selecting a dose too low may lead to ineffectiveness . Widespread use of antioxidant supplements/enzymes has failed to quell the current pandemic of cancer, diabetes, and cardiovascular disease or to stop or reverse the aging process .
It has been suggested that antioxidant supplements may show interdependency and may have effect only if given in combination . Synthetic vitamins use analogues or synthetics instead of the L-form molecules from food sources. They are like any other synthetic molecules but because of their antioxidant nature, they are able to donate one electron, after which they do not remain stable but are broken down in a metabolic process that yields hydrogen peroxide. Administering synthetic vitamins in persons with disease states can thus be counter-productive. These people already have a problem associated with or directly caused by excess free radicals, including hydroxyl radicals. Adding substances into their biological system that can lead to the formation or more hydroxyl radicals only exacerbates their free radical biochemistry. There are several studies that show that synthetic vitamins are harmful. For a therapeutic purpose, there is a need to enhance the free radical scavenging potential in patients with disease states that successfully converts all the hydroxyl radicals and hydrogen peroxide into water and oxygen as soon as they are formed - something that occurs during the prime of youth.
Foods rich in antioxidants include all fresh and seasonal fruit and vegetables (peppers, apples, onions, pineapple, dark leafy vegetables, flaxseeds, walnuts, pumpkin seeds, and olives) and olive oil . The combination of antioxidants in fruits and vegetables causes their regeneration and enhance their defense against reactive oxygen species. Consumption of vegetables and plant-derived foods and beverages has a positive impact on the prevention of age-related diseases such as heart disease, cancer and atherosclerosis as well as for longevity . Two main factors seem to be predisposing for the beneficial activities of plant foods: (a) the lower concentration of nutrients and non-nutrients in whole food natural food matrices and (b) the additive or synergistic effects of complex mixtures of phytochemicals and nutrients . Recent epidemiological studies suggest that the effects of generous antioxidant intake via supplementation may be of more than modest magnitude, particularly when relatively high doses are consumed .
High intakes or plasma concentrations of antioxidant nutrients may be serving as a marker for some other nutrient or non nutrient, dietary practice, or other lifestyle habits that have yet to be identified but that could prove to be the protective factor. However, available evidence suggests that, even if other protective factors are involved importantly in the disease risk reduction, it is unlikely that antioxidants serve exclusively as a marker and provide no beneficial action of their own. Nonetheless, the complexity of this situation emphasizes that, for small or modest effects, uncontrolled confounding in observational study designs could provide misleading findings. Cross sectional studies are limited by the inability to determine whether the antioxidant status is a risk factor or consequence of the disease; thus, prospective data should represent an important element of the totality of evidence.
DESIRABLE INTAKES OF VITAMIN C, VITAMIN E, AND I-CAROTENE
Recommended dietary allowances have been largely focused on the essentiality of nutrients for growth and development and health maintenance. However, the evidence that nutrients such as the antioxidant vitamins and I-carotene also play an important role in the reduction of risk of chronic disease has led to serious considerations about revising recommended dietary intakes to reflect these data . Although the actual preventive potential of essential antioxidants remains to be determined, the currently available data, although admittedly incomplete, may be sufficient to recommend prudent dietary regimes targeted to maintaining plasma concentrations correlated with the greatest reduction in chronic disease risk. In this regard, a substantial amount of useful information can be derived from published studies such as the WHO/MONICA project vitamin substudy, Edinburgh angina-control study, basel prospective study, euronut SENECA study on nutrition and the elderly, dietary and nutritional survey of British adults, and the VERA Study [37, 38].
Gey et al suggested that the complementary epidemiologic studies, particularly those associating increased risk of cardiovascular disease with suboptimal antioxidant status, provide a practical basis for establishing cutoffs for desirable intakes of vitamin C, vitamin E, and /3-carotene. Through analysis of the available data, Gey et al propose that decreasing ischemic heart disease risk through nutrition may be possible when plasma concentrations of vitamin C are >50 imol/L, when vitamin E is >30 imol/L (lipid standardized), and Î²-carotene is >4 p.mol/L. These values are also consistent with those reported by Riemersma et al as being associated with a reduced risk of angina pectoris . A poor concentration of any single one of these antioxidants appears to increase risk and the combination of suboptimal concentrations has additive or even synergistic effects on the risk of cardiovascular disease. In some cases, suboptimal antioxidant concentrations are stronger predictors of the several-fold regional differences of cardiovascular risk in Europe than classical risk factors such as hypercholesterolemia and hypertension. Although it is difficult to specify nutrient intake necessary to achieve a particular plasma concentration in an individual, attaining the plasma concentrations described by Gey et al is associated with reduced cardiovascular disease would suggest daily intakes of 150 mg vitamin C, 30 mg vitamin E, and 3 mg Î²-carotene . Interestingly, these intakes are greater than the currently recommended dietary allowances.
The overall positive effect of artificial supplementation of antioxidant vitamins on the risk for cancer is of borderline statistical significance. The present studies do not provide the type of incontrovertible evidence that would mandate a recommendation for the wide use of supplementation of antioxidant vitamins among adults, or among men specifically. That said, regular doses of antioxidant vitamin should not be harmful. If a patient elects to take an antioxidant vitamins supplementation, the physician should review the product label to ensure that its contents are in the standards for these preparations. Multivitamins containing very high doses of antioxidant vitamins, such as vitamin A, vitamin E may not be safe and should be exchanged for lower-dose preparations .
In conclusion, the health benefits of antioxidant supplementation are controversial and many a times confusing many clinicians because the results of some studies conflict with others. Hence making simple conclusions as to efficacy and safety is very difficult. An evidence-based approach should focus on the need to answer the myriad questions surrounding pro- and antioxidative mechanisms of action associated with antioxidant use, and what effects dose and environment have on these mechanisms before recommending nutritional or nutritional-pharmacologic interventions . The key to the future success of decreasing oxidative-stress-induced damage should thus be the suppression of oxidative damage without disrupting the well integrated antioxidant defense network . Finally, it is worthwhile to take a step back and consider that reversible health behaviors, such as smoking habit and overeating combined with a sedentary lifestyle, contribute far more to the risk for cancer than may be improved with any multivitamin. While lifestyle changes may not be as easy as regular use of a multivitamin, the benefits of smoking cessation and weight loss would have a much more profound effect on the risk for cancer. Supplementation with vitamin C, vitamin E, or beta carotene offers no overall benefits in the primary prevention of total cancer incidence or cancer mortality. There is widespread antioxidant availability, extensive antioxidant food advertising, and pervasive promotion of antioxidants in the media. In this circumstance, patient safety must remain the number one concern. Until scientifically tested, treatment modalities should always place patient safety first. Thus, based on current data and theoretical considerations, the excessive supplementations of antioxidants appear to pose unnecessary dangers and could harbor considerable potential for harm.
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About Author / Additional Info:
I am a research scholar working at Department of Biochemistry, Narayana Medical College, Nellore, Andhra Pradesh, India.