Antioxidants is the name of the groups which has the anti-oxidative by preventing free-radical, for example:
- Vitamin A
- Vitamin C (ascorbic acid)
- Vitamin E (mostly alpha-tocopherol)
- Carotenoids (mostly beta-carotene)
- Anthocyanins
- Catechins
- Lutein
- Lycopene
- Selenium
- Manganese
- Ellagic acid
- Resveratrol
- Zeaxanthin
- ...
Antioxidants are present in large amounts in several foods. However, the actual amount of antioxidants in several plant products may differ due to several factors. These include:
- soil type and chemistry
- available nitrogen and levels of other plant nutrients
- moisture levels
- temperature
- pests
It has been seen that plants which are exposed to stress are driven to synthesize antioxidants and are richer in these polyphenols and flavonoids.
Food rich in antioxidants are shown here:
To protect the cells and organ systems of the body against reactive oxygen species, humans have evolved a highly sophisticated and complex antioxidant protection system. It involves a variety of components, both endogenous and exogenous in origin, that function interactively and synergistically to neutralize free radicals. These components include:
• Nutrient-derived antioxidants like ascorbic acid (vitamin C), tocopherols and tocotrienols (vitamin E), carotenoids, and other low molecular weight compounds such as glutathione and lipoic acid.
• Antioxidant enzymes, e.g., superoxide dismutase, glutathione peroxidase, and glutathione reductase, which catalyze free radical quenching reactions.
• Metal binding proteins, such as ferritin, lactoferrin, albumin, and ceruloplasmin that sequester free iron and copper ions that are capable of catalyzing oxidative reactions.
• Numerous other antioxidant phytonutrients present in a wide variety of plant foods.
DIETARY ANTIOXIDANTS
Vitamin C, vitamin E, and beta-carotene are among the most widely studied dietary antioxidants. Vitamin C is considered the most important water-soluble antioxidant in extracellular fluids. It is capable of neutralizing
ROS in the aqueous phase before lipid peroxidation is initiated. Vitamin E, a major lipid-soluble antioxidant, is the most effective chain-breaking antioxidant within the cell membrane where it protects membrane fatty acids from lipid peroxidation. Vitamin C has been cited as being capable of regenerating vitamin E [3].
Beta-carotene and other carotenoids are also believed to provide antioxidant protection to lipid-rich tissues. Research suggests beta-carotene may work synergistically with vitamin E. A diet that is excessively low in fat may negatively affect beta-carotene and vitamin E absorption, as well as other fat-soluble nutrients. Fruits and vegetables are major sources of vitamin C and carotenoids, while whole grains and high quality, properly extracted and protected vegetable oils are major sources of vitamin E.
PHYTONUTRIENTS
A number of other dietary antioxidant substances exist beyond the traditional vitamins discussed above. Many plant-derived substances, collectively termed “phytonutrients,” or “phytochemicals,” are becoming increasingly known for their antioxidant activity. Phenolic
compounds such as flavonoids are ubiquitous within the plant kingdom: approximately 3,000 flavonoid substances have been described. In plants, flavonoids serve as protectors against a wide variety of environmental stresses while, in humans, flavonoids appear to function as “biological response modifiers.” Flavonoids have been demonstrated to have anti-inflammatory, antiallergenic, anti-viral, anti-aging, and anti-carcinogenic activity.
The broad therapeutic effects of flavonoids can be largely attributed to their antioxidant properties. 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.
The best way to ensure an adequate intake of phytonutrients is to eat a diet rich in a wide variety of fresh fruits and vegetables. Phytonutrient supplements are also now widely available
ENDOGENOUS ANTIOXIDANTS
In addition to dietary antioxidants, the body relies on several endogenous defense mechanisms to help protect against free radical-induced cell damage. The antioxidant enzymes – glutathione peroxidase, catalase, and superoxide dismutase (SOD) – metabolize oxidative toxic intermediates and require micronutrient cofactors such as selenium, iron, copper, zinc, and manganese for optimum catalytic activity. It has been suggested that an inadequate dietary intake of these trace minerals may compromise the effectiveness of these antioxidant defense mechanisms. Research indicates that consumption and absorption of these important trace minerals may decrease with aging. [4] Intensive agricultural methods have also resulted in significant depletion of these valuable trace minerals in our soils and the foods grown in them.
Glutathione, an important water-soluble antioxidant, is synthesized from the amino acids glycine, glutamate, and cysteine. Glutathione directly quenches ROS such as lipid peroxides, and also plays a major role in xenobiotic metabolism. Exposure of the liver to xenobiotic substances induces oxidative reactions through the upregulation of detoxification enzymes. When an individual is exposed to high levels of xenobiotics, more glutathione is utilized for conjugation (a key step in the body’s detoxification process) making it less available to serve as an antioxidant. Research suggests that glutathione and vitamin C work interactively to quench free radicals and that they have a sparing effect upon each other.[5]
Lipoic acid, yet another important endogenous antioxidant. Lipoic acid and its reduced form, dihydrolipoic acid (DHLA), are capable of quenching free radicals in both lipid and aqueous domains and as such has been called a “universal antioxidant.”[6] Lipoic acid may also exert its antioxidant effect by chelating with pro-oxidant metals. Research further suggests that lipoic acid has a sparing effect on other antioxidants.[6,7] Animal studies have demonstrated supplemental lipoic acid to protect against the symptoms of vitamin E or vitamin C deficiency.[6] Additional physiological antioxidants are listed here:
Endogenous Antioxidants
• Bilirubin
• Thiols, e.g., glutathione, lipoic acid, N-acetyl cysteine
• NADPH and NADH
• Ubiquinone (coenzyme Q10)
• Uric acid
• Enzymes:
– copper/zinc and manganese-dependent superoxide
dismutase (SOD)
– iron-dependent catalase
– selenium-dependent glutathione peroxidase
Dietary Antioxidants
• Vitamin C
• Vitamin E
• Beta-carotene and other carotenoids and oxy carotenoids, e.g., lycopene and lutein
• Polyphenols, e.g., flavonoids, flavones, flavonols, and proanthocyanidins
Metal Binding Proteins
• Albumin (copper)
• Ceruloplasmin (copper)
• Metallothionein (copper)
• Ferritin (iron)
• Myoglobin (iron)
• Transferrin (iron)
Each of antioxidants nutrients has the different role in preventing free-radicals. Foods probably contain other antioxidants that are still undiscovered. Eating a wide variety of foods will help you get the full benefit of these antioxidants.
References
[1] Antioxidants, Dr.Mark Percivals, Clinical Nutrition Insights, 1996
[2] Antioxidants Handouts, American Dietetic Association, 2010
[3] Halliwell, B., Free Radicals, Antioxidants, and Human Disease: Curiosity, Cause, or Consequence?
Lancet 1994;344:721-724.
[4] Duthie, G.G., and Brown, K.M., Reducing the Risk of Cardiovascular Disease, ch 2, p. 19-38, In: Functional Foods, ed. Goldberg, I. Chapman and Hall: New York 1994.
[5] Jacob, R.A., The Integrated Antioxidant System. Nutr Res 1995;15(5):755-766.
[6] Packer, L. and Witt, E.H., Antioxidant Properties and Clinical Implications of Alpha-Lipoic Acid. in Packer L. and Cadenas E. eds. Biothionls in Health and Disease. New York: Marcel Dekker, Inc, 1995, 479-516.
[7] Kagen, V.E., et al., Dihydrolipoic Acid–a Universal Antioxidant Both in the Membrane and in the Aqueous Phase. Biochem Pharmacol 199244:1637-1649.
