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Ascorbic acid
Ascorbic acid or “vitamin C” is a monosaccharide antioxidantfound in both animals and plants. As it cannot be synthesized in humans and must be obtained from the diet, it is a vitamin.[58] Most other animals are able to produce this compound in their bodies and do not require it in their diets. In cells, it is maintained in its reduced form by reaction with glutathione, which can be catalyzed by protein disulfide isomerase and glutaredoxins.[59] Ascorbic acid is a reducing agent and can reduce and thereby neutralize ROS such as hydrogen peroxide.[60] In addition to its direct antioxidant effects, ascorbic acid is also a substrate for the antioxidant enzyme ascorbate peroxidase, a function that is particularly important in stress resistance in plants.[61]
Glutathione
Glutathione is a cysteine-containing peptide found in mostforms of aerobic life.[62] It is not required in the diet and is instead synthesized in cells from its constituent amino acids. Glutathione has antioxidant properties since the thiol group in its cysteine moiety is a reducing agent and can be reversibly oxidized and reduced. In cells, glutathione is maintained in the reduced form by the enzyme glutathione reductase and in turn reduces other metabolites and enzyme systems as well as reacting directly with oxidants.[63] Due to its high concentration and central role in maintaining the cell's redox state, glutathione is one of the most important cellular antioxidants.[33] In some organisms, glutathione is replaced by other thiols, such as by mycothiol in the actinomycetes, or by trypanothione in the kinetoplastids.[64]
Melatonin
Melatonin, also known chemically as N-acetyl-5-methoxytryptamine,[65] is a naturally occurring hormone found in animals and in some other living organisms, including algae.[66] Melatonin is a powerful antioxidant that can easily cross cell membranes and the blood–brain barrier.[67] Unlike other antioxidants, melatonin does not undergo redox cycling, which is the ability of a molecule to undergo repeated reduction and oxidation. Melatonin, once oxidized, cannot be reduced to its former state because it forms several stable end-products upon reacting with free radicals. Therefore, it has been referred to as a terminal (or suicidal) antioxidant.[68]
Tocopherols and tocotrienols (Vitamin E)
Vitamin E is the collective name for a set of eight related tocopherols and tocotrienols, which are fat-soluble vitamins with antioxidant properties.[69] Of these, α-tocopherol has been most studied as it has the highest bioavailability, with the body preferentially absorbing and metabolizing this form.[70] It has been claimed that the α-tocopherol form is the most important lipid-soluble antioxidant, and that it protects membranes from oxidation by reacting with lipid radicals produced in the lipid peroxidation chain reaction.[71] This removes the free radical intermediates and prevents the propagation reaction from continuing. This reaction produces oxidized α-tocopheroxyl radicals that can be recycled back to the active reduced form through reduction by other antioxidants, such as ascorbate, retinol, or ubiquinol.[72]
Uric acid
Uric acid accounts for roughly half the antioxidant ability of plasma. In fact, uric acid may have substituted for ascorbate in human evolution.[73] However, like ascorbate, uric acid can also mediate the production of active oxygen species.
PLANTS AS SOURCE OF ANTIOXIDANTS
Synthetic and natural food antioxidants are used routinely in foods and medicine especially those containing oils and fats to protect the food against oxidation. There are a number of synthetic phenolic antioxidants, butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA) being prominent examples. These compounds have been widely uses as antioxidants in food industry, cosmetics, and therapeutic industry. However, some physical properties of BHT and BHA such as their high volatility and instability at elevated temperature, strict legislation on the use of synthetic food additives, carcinogenic nature of some synthetic antioxidants, and consumer preferences have shifted the attention of manufacturers from synthetic to natural antioxidants.[74] In view of increasing risk factors of human to various deadly diseases, there has been a global trend toward the use of natural substance present in medicinal plants and dietary plats as therapeutic antioxidants. It has been reported that there is an inverse relationship between the dietary intake of antioxidant-rich food and medicinal plants and incidence of human diseases. The use of natural antioxidants in food, cosmetic, and therapeutic industry would be promising alternative for synthetic antioxidants in respect of low cost, highly compatible with dietary intake and no harmful effects inside the human body. Many antioxidant compounds, naturally occurring in plant sources have been identified as free radical or active oxygen scavengers.[75] Attempts have been made to study the antioxidant potential of a wide variety of vegetables like potato, spinach, tomatoes, and legumes.[76] There are several reports showing antioxidant potential of fruits.[77] Strong antioxidants activities have been found in berries, cherries, citrus, prunes, and olives. Green and black teas have been extensively studied in the recent past for antioxidant properties since they contain up to 30% of the dry weight as phenolic compounds.[78]
Apart from the dietary sources, Indian medicinal plants also provide antioxidants and these include (with common/ayurvedic names in brackets) Acacia catechu(kair), Aegle marmelos (Bengal quince, Bel), Allium cepa (Onion), A. sativum (Garlic, Lahasuna), Aleo vera (Indain aloe, Ghritkumari),Amomum subulatum (Greater cardamom, Bari elachi),Andrographis paniculata (Kiryat),Asparagus recemosus (Shatavari),Azadirachta indica (Neem, Nimba),Bacopa monniera (Brahmi), Butea monosperma (Palas, Dhak), Camellia sinensis (Green tea), Cinnamomum verum (Cinnamon), Cinnamomum tamala (Tejpat), Curcma longa(Turmeric, Haridra), Emblica officinalis (Inhian gooseberry, Amlaki), Glycyrrhiza glapra(Yashtimudhu), Hemidesmus indicus(Indian Sarasparilla, Anantamul),Indigofera tinctoria, Mangifera indica (Mango, Amra), Momordica charantia (Bitter gourd), Murraya koenigii (Curry leaf), Nigella sativa(Black cumin), Ocimum sanctum(Holy basil, Tusil), Onosma echioides(Ratanjyot), Picrorrhiza kurroa(Katuka), Piper beetle, Plumbago zeylancia (Chitrak), Sesamum indicum, Sida cordifolia,Spirulina fusiformis (Alga), Swertia decursata, Syzigium cumini (Jamun),Terminalia ariuna (Arjun),Terminalia bellarica (Beheda),Tinospora cordifolia (Heart leaved moonseed, Guduchi), Trigonella foenum-graecium (Fenugreek),Withania somifera (Winter cherry, Ashwangandha), and Zingiber officinalis (Ginger).[79]
ANTIOXIDANT POTENTIAL OF FUNCTIONAL FOODS
Concepts of functional foods and nutraceuticals
In the last decade, preventive medicine has undergone a great advance, especially in developed countries. Research has demonstrated that nutrition plays a crucial role in the prevention of chronic diseases, as most of them can be related to diet. Functional food enters the concept of considering food not only necessary for living but also as a source of mental and physical well-being, contributing to the prevention and reduction of risk factors for several diseases or enhancing certain physiological functions.[80] A food can be regarded as functional if it is satisfactorily demonstrated to affect beneficially one or more target functions in the body, beyond adequate nutritional effects, in a way which is relevant to either the state of well being and health or reduction of the risk of a disease. The beneficial effects could be either maintenance or promotion of a state of well being or health and/or a reduction of risk of a pathologic process or a disease.[81] Whole foods represent the simplest example of functional food. Broccoli, carrots, and tomatoes are considered functional foods because of their high contents of physiologically active components (sulforaphen, B-carotene, and lycopene, respectively). Green vegetables and spices like mustard and turmeric, used extensively in Indian cuisine, also can fall under this category.[82] “Nutraceutical” is a term coined in 1979 by Stephen DeFelice.[83] It is defined “as a food or parts of food that provide medical or health benefits, including the prevention and treatment of disease.” Nutraceuticals may range from isolated nutrients, dietary supplements, and diets to genetically engineered “designer” food, herbal products, and processed products such as cereals, soups, and beverages. A nutraceutical is any nontoxic food extract supplement that has scientifically proven health benefits for both the treatment and prevention of disease.[84] The increasing interest in nutraceuticals reflects the fact that consumers hear about epidemiological studies indicating that a specific diet or component of the diet is associated with a lower risk for a certain disease. The major active nutraceutical ingredients in plants are flavonoids. As is typical for phenolic compounds, they can act as potent antioxidants and metal chelators. They also have long been recognized to possess anti-inflammatory, antiallergic, hepatoprotective, antithrombotic, antiviral, and anticarcinogenic activities.[85]
Indian dietary and medicinal plants as functional foods
Ingredients that make food functional are dietary fibers, vitamins, minerals, antioxidants, oligosaccharides, essential fatty acids (omega-3), lactic acid bacteria cultures, and lignins. Many of these are present in medicinal plants. Indian systems of medicine believe that complex diseases can be treated with complex combination of botanicals unlike in west, with single drugs. Whole foods are hence used in India as functional foods rather than supplements. Some medicinal plants and dietary constituents having functional attributes are spices such as onion, garlic, mustard, red chilies, turmeric, clove, cinnamon, saffron, curry leaf, fenugreek, and ginger. Some herbs as Bixa orellana and vegetables like amla, wheat grass, soyabean, and Gracinia cambogia have antitumor effects. Other medicinal plants with functional properties include A.marmelos, A. cepa, Aloe vera, A. paniculata, Azadirachta india, and Brassica juncea.[86]
CONCLUSION
Free radicals damage contributes to the etiology of many chronic health problems such as cardiovascular and inflammatory disease, cataract, and cancer. Antioxidants prevent free radical induced tissue damage by preventing the formation of radicals, scavenging them, or by promoting their decomposition. Synthetic antioxidants are recently reported to be dangerous to human health. Thus the search for effective, nontoxic natural compounds with antioxidative activity has been intensified in recent years. In addition to endogenous antioxidant defense systems, consumption of dietary and plant-derived antioxidants appears to be a suitable alternative. Dietary and other components of plants form a major source of antioxidants. The traditional Indian diet, spices, and medicinal plants are rich sources of natural antioxidants; higher intake of foods with functional attributes including high level of antioxidants in antioxidants in functional foods is one strategy that is gaining importance.
Newer approaches utilizing collaborative research and modern technology in combination with established traditional health principles will yield dividends in near future in improving health, especially among people who do not have access to the use of costlier western systems of medicine.
