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Understanding Antioxidants, Free Radicals & Oxidative damage

Quick Hit Summary

The words antioxidant, free radicals and oxidative damage are commonly thrown around by the media. However, few individuals actually understand what they mean. Free radicals are produced when ones break down fat, protein or carbohydrates for energy. They are also introduced into our system via environmental pollutants. Unfortunately, free radicals are highly reactive and tend to damage objects they come into contact with such as DNA and the cell walls (ie- oxidative damage). This destruction can be limited via antioxidants which are obtained from the diet or naturally produced by the body. These special molecules “quench” the reactivity of free radicals; thus, preventing them from harming DNA, etc. Although they do have some drawbacks, free radicals are necessary to human existence. Our immune system uses them to destroy foreign invaders (bacteria, viruses, etc). Additionally they are needed post exercise to prevent excessive inflammation. As you can see, it’s all about balance when it comes to antioxidants, free radicals and oxidative damage.

Free Radicals, Oxidative Damage & Antioxidants

Figure 1 Are you be filling your plate with antioxidant rich fruit and vegetables?

While walking down the aisles of the grocery store, listening to the news and/or looking at newspaper ads do you hear about a certain product (food or supplement) being promoted for its high antioxidant levels? I’m guessing that if I asked this question to a large group of individuals, many hands would go up. However, I’m guessing that fewer hands would rise if I asked the same group of individuals if they truly understood how antioxidants functioned within the human body. The purpose of this article is to explain the functions of antioxidants within the human body and why they are beneficial.

An Analogy

Along with some of your friends, imagine that you’re playing “hot potato” with a smoldering coal from the barbeque pit. (Yes, I realize that playing with a smoldering coal is not the smartest thing to do, but necessary for the sake of the analogy!) As soon as you touch the coal, your hands are scorched and you quickly pass the coal onto the next person. Upon getting burned, you have to stop playing because your hands are “non-functional.” This process repeats itself until only a few people are left. Cleverly, one of your friends puts on protective gloves that prevent the coal from burning their hands. Thus, upon catching the coal, they simply hold it until it cools off and no one else gets burned.

Back to Science

Now let’s relate this back to free radicals, oxidative damage and antioxidants. Think of the burning coal as a free radical, the burned hands as cellular structures affected by oxidative damage, and the protective gloves as antioxidants. As the free radical gets passed from one structure to the next, oxidative damage occurs, rendering it useless. Eventually though, the free radical comes into contact with the antioxidant. Instead of being damaged by it, the antioxidant is able to cool the unstable radical off, preventing further damage.

To better understand how free radicals are formed, we must do a little chemistry 101. The smallest fundamental unit of existence is the atom1. There are many various types of atoms (oxygen atoms, carbon atoms, nitrogen atoms, etc) that compose everything on earth. Although there are various types of atoms, they all contain the same basic components. The center of an atom is the nucleus which has positively charged protons and neutral charged neutrons. Orbiting the nucleus are high energy/speed electrons which have a negative charge. Atoms are most stable when the total number of protons equals the total number of electrons.

When this is not the case and the atom has a net positive or negative charge, it is said to be a free radical. Similarly, a molecule, which consist of multiple atoms, can be a free radical too if it has a net positive or negative charge. These free radicals are highly reactive. In order to become "balanced", free radicals will drop an electron or pick one up from another molecule, altering the structure of the 2nd molecule in a process referred to as a redox reaction as one molecule is oxidized and the other molecule is "reduced". Like a chain reaction, this 2nd molecule will then try to acquire an electron to stabilize itself, creating yet another free radical in the process. This will repeat itself until the free radical comes into contact with an antioxidant or an antioxidant enzyme system. Please see Figure 1 Although the chain reaction has been stopped, each molecule that has been affected in the process (starting with the original free radical) will usually undergo a shape change, preventing it from properly functioning. This is generally referred to as oxidative damage.

One of the main targets of free radicals in the human body are lipids (fats), which are located in all cell membranes, and circulating lipoproteins (LDL/HDL cholesterol). Damages to these lipids result in cardiovascular disease and various neurodegenerative problems such as Alzheimers2. Free radicals can also alter DNA, leading to the development of cancer3. For these reasons and more, individuals are encouraged to eat foods with high antioxidant content.

Free radicals are generated from many environmental sources including pollution, smoke, radiation, and oxygen. Please see Figure 1 Yes, the same oxygen we breathe is a source of free radicals. Via oxygen, we are able to extract energy from the food we eat. Unfortunately, this process isn’t 100% efficient; 2-4% of the time, oxygen molecule end up with an unpaired electron, and are referred to as reactive oxygen species (ROS)4. The ROS will then cause oxidative damage, as it tries to acquire an extra electron (as discussed above). I know what you’re thinking, “Wait a minute….We use more oxygen when we exercise, and wouldn’t this cause more free radicals to form? Thus, aren’t we’re risking DNA damage every time we exercise… how can exercise be healthy?” Before you refuse to exercise again and start trying to hold your breath, continue to read on!!

Figure 1 The relationship between free radicals, oxidative damage and antioxidants.

Exercise and Free Radicals

When exercising, especially at moderate to hard intensity, one consumes more oxygen, leading to greater ROS formation5. However, the formation of increased ROS, via exercise, increases the body’s production of natural antioxidant enzymes. These naturally produced antioxidants work slightly different than those that obtained from our diet. Regardless of how antioxidants work though, the same end result is achieved; free radicals are stabilized. Studies have indicated that the highest concentrations of antioxidants are found in muscle tissue that consumes the greatest amount of oxygen6. Thus, the additional amount of ROS produced during "typical" exercise (ie- going at average workout intensity; not exhaustive) will not cause excessive, life-altering, oxidative damage.

There is evidence in trained endurance athletes that exhaustive exercise will outstrip our body's natural antioxidant system, causing some oxidative damage9. Thus, the body does rely to some degree on dietary antioxidants. Remember though, oxidants stimulate the body's ability to naturally make antioxidant enzymes. With the increased level of antioxidant enzymes present in their system, I hypothesize that habitual exercisers have a greater capacity to handle environmental free radicals vs. those who don’t engage in regular physical activity. In other words, if two individuals get exposed to an environmental based free radical, the sedentary will have greater cellular oxidative damage vs. the habitual exerciser.

Necessity of Free Radicals

Up to this point, I’ve painted a somewhat gloomy picture of free radicals. However, free radicals have important functions within the human body. Most notably, they are critical for healthy immune responses7. When a foreign particulate (bacteria or virus) enters the body, immune cells such as neutrophils (non specific immune cells which are responsible for breaking down in damaged tissue or foreign bacteris/viruses) are summoned. These cells encapsulate the foreign particulate, and then produce ROS which destroy the intruding virus or bacteria. Also, following the attack of specific bacteria/viruses, ROS play a role in shutting off antigen specific T-lymphocytes (cells that respond to specific viruses/bacteria)8. Therefore, autoimmune diseases caused by various antigens are prevented.

Free radicals are also required to some degree to preserve muscle tissue. ROS help to prevent chronic inflammation following muscle breakdown (such as seen in exercise). When skeletal tissue is injured, neutrophils are attracted to the area, breaking down damaged structures. Once the dead tissue is cleared away, ROS trigger reactions that cause the neutrophils to die8. Without this “death” signal, neutrophil activity would continue, breaking down healthy tissue.

Bottom Line

As you can see, a delicate balance of ROS must be maintained within the cell. On one hand, excessive free radicals cause accelerated aging, cancer and cardiovascular disease. Yet, on the other hand, free radicals are necessary for healthy immune responses. In upcoming articles, we’ll examine specific dietary antioxidants and look at the research regarding there effectiveness with regards to both chronic disease and athletic performance.


1 John C. Kotz, Paul M. Treichel , and John Townsend. Chemistry and Chemical Reactivity, Enhanced 7th Edition. Belmont, CA: Brooks/Cole,2009.Print.

2 Sachdev S, Davies KJ. Production, detection, and adaptive responses to free radicals in exercise. Free Radic Biol Med. 2008 Jan 15;44(2):215-23.

3 Schippling S, Kontush A, Arlt S, Buhmann C, Stürenburg HJ, Mann U, Müller-Thomsen T, Beisiegel U. Increased lipoprotein oxidation in Alzheimer's disease. Free Radic Biol Med. 2000 Feb 1;28(3):351-60.

4 Pan JS, Hong MZ, Ren JL. Reactive oxygen species: a double-edged sword in oncogenesis. World J Gastroenterol. 2009 Apr 14;15(14):1702-7.

5 Ji LL. Modulation of skeletal muscle antioxidant defense by exercise: Role of redox signaling. Free Radic Biol Med. 2008 Jan 15;44(2):142-52.

6 Ji, L. L.; Fu, R. G.; Mitchell, E. W. Glutathione and antioxidant enzymes in skeletal muscle: Effects of fiber type and exercise intensity. J. Appl. Physiol. 73:1854–1859; 1992.

7 Valko M, Leibfritz D, Moncol J, Cronin MT, Mazur M, Telser J.Free radicals and antioxidants in normal physiological functions and human disease. Int J Biochem Cell Biol. 2007;39(1):44-84.

8 Seifried HE, Anderson DE, Fisher EI, Milner JA. A review of the interaction among dietary antioxidants and reactive oxygen species. J Nutr Biochem. 2007 ep;18(9):567-79.

9 Watson TA, Callister R, Taylor R, Sibbritt D, MacDonald-Wicks LK, Garg ML. Antioxidant restricted diet increases oxidative stress during acute exhaustive exercise. Asia Pac J Clin Nutr. 2003;12 Suppl:S9.p.

10 Accessed June 13, 2010 from: www.public-domain-photos.com/food/fruits-and-vegetables

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Written on October 14, 2009 by Sean Casey
Last Updated: August 07, 2013

This information is not intended to take the place of medical advice.Please check with your health care providers prior to starting any new dietary or exercise program. CasePerformance is not responsible for the outcome of any decision made based off the information presented in this article.

About the Author: Sean Casey is a graduate of the University of Wisconsin-Madison with degrees in both Nutritional Science-Dietetics and Kinesiology-Exercise Physiology. Sean graduated academically as one of the top students in both the Nutritional Science and Kinesiology departments.
Field Experience: During college, Sean was active with the UW-Badgers Strength and Conditioning Department. He has also spent time as an intern physical preparation coach at the International Performance Institute in Bradenton, FL. He also spent time as an intern and later worked at Athletes Performance in Tempe, AZ. While at these locations he had the opportunity to train football, soccer, baseball, golf and tennis athletes. Sean is also active in the field of sports nutrition where he has consulted with a wide variety of organizations including both elite (NFL’s Jacksonville Jaguars) and amateur athletic teams. His nutrition consultation services are avalable by clicking on the Nutrition Consultation tab.