Oxidation is the process central to the functioning of many living organisms for the production of energy to fuel their biological processes. It is the primary provider of energy required for cellular anabolism, the reductive synthesis of metabolites, protein and nucleic acid synthesis or motility, oxidation of amino acids to form neurotransmitters, production of ATP in the form of oxidative phosphorylation, etc. These oxidation reactions also result in the production of free radicals.

Dual role of free radicals
Free radicals are produced from two major sources a) endogenous and b) exogenous. Free radicals are generally produced inside the cells by electron transfer reactions, which can be both enzymatically and non-enzymatically mediated. The production of free radicals can be either accidental in, which a small number of electrons prematurely leak from mitochondria and endoplasmic reticulum to oxygen, resulting in the formation of potentially damaging toxic superoxide radical or deliberately for specific, essential roles like superoxide generation during the process of intracellular killing of bacteria by phagocytic cells such as activated macrophages and granulocytes and in catalytic reactions. Exogenous sources include exposure to certain drugs (methotrexate, antibiotics etc.), electromagnetic radiation, tobacco, cigarette smoke, air pollution, etc.

Types of free radicals
Free radicals are categorized into two types:
a) Reactive oxygen species (ROS) include superoxide, hydrogen peroxide and hydroxyl radicals
b) Reactive nitrogen species (RNS) include nitric oxide, peroxynitrite radicals.
Apart from these, there are certain non-free radical species such as hypochlorous acid, singlet oxygen etc.

Potentially damaging effects of free radicals
These oxidative products possess a high degree of chemical reactivity owing to the presence of unpaired electrons in their outermost orbit/valence shell. Free radicals are continuously produced in vivo, induce alterations in the structures and functions of tissues by reacting with all classes of biological molecules namely lipids, DNA and proteins. Polyunsaturated fatty acids are particularly susceptible and prone to free radical attack because they contain multiple double bonds in between methylene -CH2- groups that possess especially reactive hydrogen atoms. This increased oxidative stress has been implicated in the pathogenesis of a host of chronic degenerative diseases videlicet cancer, arthritis, myocardial infarction, arteriosclerosis, Alzeihmer's disease, diabetes mellitus and in ageing.

Countermeasure to oxidative stress
There is a complete array of natural antioxidant defense system present inside the human body at various levels ensuring homeostasis through fundamental "housekeeping' activities. Sometimes, this innate defense system is not capable enough to prevent ongoing oxidative damage to biologically important macromolecules. Hence, a constant demand of exogenous antioxidants may serve as a possible preventive intervention for free radical mediated cellular damage and diseases. Antioxidants can prevent the damage caused by free radicals by reacting with them thus neutralizing their effects, by chelating catalytic metals and also playing as oxygen scavengers.

Natural vs. synthetic antioxidants
Within cells, there are abundant free radical 'scavenger' enzyme systems, such as superoxide dismutase (SOD), catalase and Glutathione peroxidase (GP). Besides, natural anti-oxidants include urate, vitamin C & E, lipoic acid, beta-carotene, bilirubin, melatonin, carnosine, glutathione and ubiquinone (conenzyme Q10). Additionally, selenium, a trace metal that is required for proper function of one of the body's antioxidant enzyme systems, is sometimes included in this category. The body cannot manufacture these micronutrients so they must be supplied in the diet.
Many synthetic antioxidants such as butylated hydroxyanisole and butylated hydroxytoluene are also available but in the past few years, there is an increased preference for natural antioxidants, especially of plant origin over the synthetic ones because of the low health risks and toxicity of the former.

Medicinal plants as a source of antioxidants
Literature survey ascertains that medicinal plants are a rich source of natural antioxidants. Most natural antioxidants are specifically localized in wood, bark, stem, leaf, fruit, root, flower and seed. Polyphenols, flavonoids and proanthocyanidins found in the plant extracts are mainly antioxidant in nature. There is abundant literature regarding the antioxidative potential of medicinal plants, for reference, I am listing a few:
1. Bacopa monniera, a medha rasayana drug of ayurveda acts as a powerful antioxidant and is effective in the treatment of schizophrenia, epilepsy, depression, and insomnia.
2. Turmeric, also contains antioxidant components, which are useful for the treatment of arthritis, inflammatory diseases and possibly cancer.
3. The polyphenolic compounds isolated from Ruta graveolens plant have been demonstrated to have antioxidant potential.
4. Gingko biloba extract also helps in providing antioxidants to overcome the continued oxidative stress.

In conclusion, it can be suggested that the antioxidant rich dietary supplement constitutes an important defense against a wide variety of chronic and debilitating diseases. One should include a plenty of fruits, nuts, vegetables etc. in the diet to lead a healthy lifestyle. Moreover, I believe that a combination supplement of antioxidants of plant origin will be more beneficial rather than produced synthetically.

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Written by Shikha Sharma and Debasis Sahu