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What are antioxidants?

Antioxidants are chemicals that can block the activity of other chemicals known as free radicals, which can damage the cells or cause cancer [1].

What is oxidation?

Oxidation is a reaction with molecular oxygen, or the process in which molecules give electrons to other molecules, which lack electrons. Every molecule that lacks electrons is an oxidant.

What are free radicals or oxidants?

Free radicals are nonmetallic atoms or molecules with one or more unpaired (odd) electrons on their outer shells, what makes them unstable and highly reactive. Radicals tend to steal electrons (to get paired electrons and thus becoming stable) from the nearby molecules in the body cells and thus damage them [3].

Oxygen radicals or reactive oxygen species (ROS), such as superoxide anion (O2·¯) or hydrogen peroxide (H2O2), which are created during normal cell respiration have a beneficial role in the metabolism until they are under the control [3]. These and other radicals, such as nitric oxide (NO) or hypochlorite (OCl¯), may become harmful when created in excess, which may happen due to hypoxia (lack of oxygen delivery to tissues, for example, due to a clogged artery), hyperoxia (inhaling more than 60% oxygen for prolonged periods), high consumption of fried foods or heavy metals (lead, mercury), ionizing radiation (ultraviolet light during sun exposure, cosmic rays, medical X-rays, nuclear power plants), certain drugs and anesthetics, or inhaling ozone, cigarette smoke, pesticides, vapors of petrochemical paints, solvents or automobile exhaust [1,3] and emotional stress (anxiety) [32].

Free radicals may damage the body cells, which can result in chronic disorders, such as cancer, atherosclerosis, senile cataract, age-related macular degeneration of the eye, retinopathy (damage of the eye retina), rheumatoid arthritis, alcohol-induced liver damage, diabetic neuropathy, nephropathy (kidney damage), wrinkled skin, premature aging and Alzheimer’s disease.

How can your body fight against free radicals?

Enzymes in your body, such as glutathione peroxidase, superoxide dismutase (SOD) and catalase, can destroy free radicals [3]. These enzymes need selenium, copper, manganese and zinc from food to work properly.

Antioxidants can neutralize free radicals.

  • Antioxidants synthesized in your body: glutathione, coenzyme Q10, alpha- lipoic acid, uric acid and bilirubin [3].
  • Antioxidants obtained from foods, such as vitamin A, C and E (see below)

Antioxidants in Foods

Main antioxidants found in foods include [1,4]:

  • Vitamin A (in carrots, sweet potatoes)
  • Beta-carotene (in sweet potatoes, carrots, cantaloupe, squash, apricots, pumpkin, mangos)
  • Lutein (in dark green leafy vegetables: spinach, kale, collard greens)
  • Lycopene (in tomatoes, watermelon, papaya, apricots, pink grapefruit, red oranges)
  • Vitamin C (in cranberries, strawberries, kale, red bell peppers, Brussels sprouts, broccoli, collards, cabbage, citrus fruits, tomatoes, artichokes, Swiss chard, strawberries)
  • Vitamin E (in nuts, seeds and soybeans and their unrefined oils, in brown rice, oats, fresh wheat germ, eggs from free-range chickens)
  • Selenium (in wheat, rice). Selenium by itself is not an antioxidant but is required for proper action of certain enzymes that destroy free radicals.

In general, dark-colored (dark green, orange, red, blue) plant foods are high in antioxidants.

Oxygen Radical Absorbance Capacity (ORAC) of Foods

In the laboratory, the antioxidant power of foods can be determined and expressed as Oxygen Radical Absorbance Capacity (ORAC) [6]. Foods high in ORAC do not necessary have high antioxidant power in your body, though.

Possible Benefits of Antioxidants from Foods

There is INSUFFICIENT EVIDENCE about the effectiveness of green tea [8] or black tea [24], which contain antioxidants catechins in prevention or treatment of arthritis, cancer, heart disease, high cholesterol levels, inflammation or any other condition [9,10,11,12,13,14]. Bottled tea may contain only negligible amounts of antioxidants.

Polyphenols, such as flavonoids [15,16], curcumin (in turmeric) [17,18,19,20,21,22,23] and resveratrol  [25,26,27,28,29,30,31] do not likely have significant antioxidant power in the human body.

Antioxidant Supplements

Antioxidants for Therapy

Oxidants or free radicals are considered to be a cause of several chronic disorders, so the idea about how antioxidant supplements could help to prevent or treat these disorders became popular in recent years. However, to date, antioxidant supplements have not been proven to have beneficial effects to human health and are currently not recommended for treatment of heart disease, cancer, cystic fibrosis or other conditions [1,5].

Possible exceptions include:

  • A combination of vitamin C, vitamin E, beta-carotene, and zinc supplements has decreased the risk of the development of advanced age-related macular degeneration (AMD).
  • A combination of vitamin E, beta-carotene and selenium supplements has lowered the risk of gastric cancer and other cancers in Chinese men and women, especially at those with borderline low blood selenium levels.

It is possible that antioxidants work optimally only when combined with minerals, fiber and other nutrients in fruits, vegetables and whole grains [5].

Possible Toxic Effects

In certain circumstances, antioxidant supplements may be harmful [5]:

  • In two studies, beta-carotene supplements have increased the risk of lung cancer in heavy smokers and former asbestos workers [5]. In one study, a combination of antioxidant supplements has increased the risk of skin cancer in women [5].
  • Antioxidant supplements may prevent health benefits of exercise.

  1. Antioxidants and Cancer Prevention  National Cancer Institute
  2. Antioxidants: Beyond the Hype  Harvard School of Public Health
  3. Free radicals and reactive oxygen  Colorado State University
  4. Antioxidants and cancer prevention  National Cancer Institute
  5. Antioxidants: beyond the hype  Harvard T.H. Chan
  6. Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 (2010)  US Department of Agriculture
  7. Arts IC et al, 2000, Catechin contents of foods commonly consumed in The Netherlands. 2. Tea, wine, fruit juices, and chocolate milk  PubMed
  8. Chacko CM et al, 2010, Beneficial effects of green tea: A literature review  PubMed Central
  9. Tea  Linus Pauling Institute
  10. Green tea  WebMD
  11. Scalon C et al, 2013, Flavonoids for treating venous leg ulcers  Cochrane
  12. Hartley R et al, 2013, Green and black tea to prevent cardiovascular disease  Cochrane
  13. Jurgens TM et al, 2012, Green tea for weight loss and weight maintenance in overweight or obese adults  Cochrane
  14. Boehm K et al, 2009, Green tea for the prevention of cancer  Cochrane
  15. Flavonoids  Linus Pauling Institute
  16. Jin H et al, 2012, Dietary flavonoid for preventing colorectal neoplasms  Cochrane
  17. Curcumin  Linus Pauling Institute
  18. Curcumin  Food and Agriculture Organization
  19. Garg SK et al, 2012, Curcumin for maintenance of remission in ulcerative colitis  Cochrane
  20. Henrotin Y et al, 2013, Curcumin: a new paradigm and therapeutic opportunity for the treatment of osteoarthritis: curcumin for osteoarthritis management  PubMed Central
  21. Chainani-Wu N, 2003, Safety and anti-inflammatory activity of curcumin: a component of tumeric (Curcuma longa)  PubMed
  22. Dong-wei Z et al, 2013, Curcumin and Diabetes: A Systematic Review  PubMed Central
  23. Sahebkar A et al, 2014, A systematic review and meta-analysis of randomized controlled trials investigating the effects of curcumin on blood lipid  PubMed Health
  24. Goldbohm RA et al, 1996, Consumption of black tea and cancer risk: a prospective cohort study  PubMed
  25. Resveratrol uses, side effects  WebMD
  26. Resveratrol  Linus Pauling Institute
  27. Sahebkar A, 2013, Effects of resveratrol supplementation on plasma lipids: a systematic review and meta-analysis of randomized controlled trials  PubMed
  28. Hausenblas HA et al, 2015, Resveratrol treatment as an adjunct to pharmacological management in type 2 diabetes mellitus–systematic review and meta-analysis  PubMed
  29. Vang O et al, 2011, What is new for an old molecule? Systematic review and recommendations on the use of resveratrol  PubMed
  30. Sahebkar A et al, 2015, Lack of efficacy of resveratrol on C-reactive protein and selected cardiovascular risk factors–Results from a systematic review and meta-analysis of randomized controlled trials  PubMed
  31. Aluyen JK et al, 2011, Resveratrol: potential as anticancer agent  PubMed
  32. Bouayed J et al, 2009, Oxidative stress and anxiety  PubMed Central

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