Free radicals are unstable, highly reactive molecules which are generated both in the body and outside the body. They are described as electron hungry molecules produced when oxygen is metabolized or burned by the body. They travel via cells disrupting the structure of molecules causing cellular damage. They do serve useful purposes in the human body and under certain situations have several deleterious effects in the body. The changes induced by free radicals are believed to be a major cause of aging, disease development and death. For optimal health, a balance must be maintained between free radical formation and antioxidant production. This balance results in reduction and oxidation of free radicals which is called redox. If there is an imbalance between free radicals and antioxidants, oxidative stress occurs that may lead to a number of diseases. Dietary antioxidant nutrients which include vitamin C, E and A (including lycopene) serve as bodyguards giving the body protection against free radical damage; since the body cannot provide enough antioxidants needed.

The alarming rate at which degenerative diseases such as cancer, arthritis, diabetes, hypertension, and heart failure are prevalent is an important issue of public interest. The basic concept of free radicals and antioxidants are of utmost importance in order to know how to combat these diseases.

Free radicals are formed when weak bonds split. Most biological molecules contain paired electrons in the outer orbit but in the case of free radicals, an unpaired electron is alone. They serve useful purposes in the human body and under certain situations have several deleterious effects in the body, Our bodies create free radicals naturally as they process food, and additional free radicals develop when we’re exposed to environmental toxins (for example, pesticides and radiation), pollutants (cigarette smoke, air pollution), ultraviolet rays, and even stress .While you can't entirely avoid free radicals, it may be possible to reduce their presence in your body by eating foods that are high in antioxidants and limiting your exposure to environmental toxins and pollution whenever possible [7]. Although free radical formation is a normal consequence of a variety of essential biochemical reactions without which we could not live but due to their relatively unstable nature, they have a tremendous potential to damage cells and tissues. Once formed these highly reactive radicals can start a chain reaction like dominoes. Free radicals damage cell proteins, DNA (deoxyribonucleic acid) and lipids. Cells may function poorly or die if this occurs without control [8].

Free radicals are essential to life. We need them and they fight infections i.e. our cells gives a burst of these free radicals to kill the germs when we become infected with bacteria, fungi and parasites.

Generation of Free Radicals
One of the prevalent theories in the current literature revolves around free radicals, which are molecules containing unpaired, highly reactive electrons, as causal agents in the process of aging. In the 1950s, Harman proposed the “free radical theory,” postulating that damage to cellular macromolecules via free radical production in aerobic organisms is a major determinant of life span [6]. Free radicals are natural by-product of aerobic cell metabolism. They are produced by a number of actions including infections, diseases and lifestyle. The generation can be endogenous (within the body) or exogenous (outside the body). The exogenous source of free radical is from the environment. These include oxidizing agent i.e. Ionizing radiations (from industry, sun exposure, cosmic rays, medical x-ray); Ozone and nitrous oxide (primarily from automobile exhaust); Heavy metals (such as mercury, cadmium and lead); Cigarette smoking (both active and passive); Alcohol; Unsaturated fat (this may create a strain on the natural antioxidants of the body) and other chemicals (pesticides) and compounds from food, water, drugs and air. Endogenous free radical formation occurs continuously in the cells as a consequence of both enzymatic and non-enzymatic reactions. Endogenous free radicals are produced in the body by four different mechanisms [8].

First, from the normal metabolism of oxygen requiring nutrients: Mitochondria-the intracellular powerhouse which produces the universal energy molecules Adenosine triphosphate (ATP) normally consume oxygen in the process and convert it to water. However unwanted by-products such as hydrogen peroxide and hydroxyl radical are inevitably produced due to incomplete reduction of the oxygen molecule. It has been estimated that more than 20 billion molecules of oxidants are produced per day by each cell during normal metabolism. Imagine what happens with insufficient cell metabolism. It is really ironic that oxygen, an element indispensable for life can under certain situations have several harmful and damaging effects on the human body. This notwithstanding, humans need to breathe air-containing oxygen in order to sustain life. This can be likened to the burning of wood in a fireplace where smoke is a by-product. Likewise when food is metabolized into energy, oxygen oxidizes (or burns) the food in order to produce energy. This process does not create smoke like burning wood in a fire place, but it does produce dangerous by-products known as free radicals [2]. Oxygen is vital, yet it has a dark side. Oxygen free radicals occur just by breathing but excessive amounts of these free radicals occur when we are exposed to air pollution and cigarette smoke.

Secondly, white blood cells destroy parasites, bacteria and viruses by using oxidants (free radicals) such as nitric oxide, super oxide and hydrogen peroxide. This is done by the release of free radicals to destroy invading pathogenic microbes as part of the body’s defense mechanism against diseases but with electrons unhinged, free radicals roam the body wrecking havoc. The free radical in an effort to achieve stability attacks nearby molecules in order to obtain another electron, and once this is done the attacked molecules become damaged. When the attacked molecule losses its electron, it becomes a free radical itself, beginning a chain reaction. Once the process is started it can cascade finally resulting in the disruption of living cells, which in turn rip through the tissues and ultimately results in tissue damage [3].

Thirdly other cellular components called peroxisomes produce hydrogen peroxide as a by-product of the degradation of fatty acids and other molecules. In contrast to the mitochondria which oxidizes fatty acids to produce ATP and water, peroxisomes oxidizes fatty acids to produce heat and hydrogen peroxide. The peroxide is then degraded by an enzymatic antioxidant called catalase. Under certain conditions some of the hydrogen peroxide escapes to wreak havoc in other compartments in the cell. Polyunsaturated fats contribute immensely to the over production of free radicals because they have multiple double bonds which become oxidized and cause an increased production of free radicals in the form of lipid peroxide radicals [1].

Finally an enzyme in the cells called cytochrome P450 is one of the body’s primary defenses against toxic chemicals ingested with food. The induction of these enzymes to prevent damage by toxic foreign chemicals like drugs and pesticides also result in the production of oxidant by-products. Virtually all organs and tissues in the body are under constant attack by these free radicals behaving like biological terrorists ripping through our bodies and they need to be stopped quickly. To prevent free radical damage, the body has a defense system called antioxidants. Antioxidants are molecules which can safely interact with free radicals and terminate the chain reactions before vital molecules are damaged [2].

Reactions of Free Radicals in the Body
Free radical reactions are expected to produce progressive adverse changes that accumulate with age throughout the body. Fortunately the body is naturally equipped with antioxidants defense system to detoxify these dangerous agents but unfortunately the body’s defense system becomes less effective as we get older leading to accumulation of oxidative damage and the development of chronic degenerative diseases Antioxidants are clearly important to human life. Supplementation with excessive quantities of antioxidants can shift the oxidant - antioxidant balance toward the antioxidant side [8]. However there are situation in which this common trend or pattern are influenced by genetic and environmental differences. If the generation of free radical is much more than can be handled by the antioxidant enzymes or antioxidant nutrients, then trouble arises, and they create damage in the cell membrane.

Irrespective of how and why free radicals are produced, they are harmful and can impair vital molecules within the body such as lipids, protein and DNA. These result to oxidative stress. It also affect cellular calcium metabolism if uncontrolled, and this can result in cell injury or death [11]. Oxidative stress causes damage to three major structures: DNA, Lipids and Proteins. The damage to DNA strands can occur directly by free radical in close proximity to the DNA or indirectly, by impairing production of protein needed to repair DNA. Alteration in DNA is a major factor in the development of cancer [3]. The oxidative damage to DNA overtime could cause changes to both the structure and functions of chromosomes, which can lead to cancer and chronic diseases. Free radical could attack fatty acid side chains of intracellular membranes and lipoproteins. A chain reaction known as lipid peroxidation ensues. The product of lipid peroxidation can further damage membrane proteins making the cell membrane ‘leaky’ and eventually leading to loss of membrane integrity. Lipid peroxidation is implicated in the development of arteriosclerosis [10].

The last structures damaged by oxidative stress are cellular proteins. This plays a role in the causation of cataracts. Free radicals can interfere with protein function leading to irregular, abnormal metabolism and rapid aging . Oxidative stress (free radical reactions) occurs in most human diseases. This is not to say that oxidative stress is the cause of most diseases. The increase in free radicals may be secondary to the disease process. Clinical conditions associated with free radical damage include the following diseases stated below: Alzheimer, Diabetes, Amyotropic lateral sclerosis, Coronary artery diseases, Arthritis, Heart failure, Arteriosclerosis, Hypertension and many other diseases. In fact free radicals are believed to play a role in more than 60 different health conditions [9].

Counteracting Free Radicals Damage
As soon as these free radicals are created, they are checked by enzymes in the body, which are known as antioxidant enzymes or nutrients in our food that are known as antioxidant nutrients (vitamin A, vitamin C, vitamin E, beta-carotene, flavonoid are common examples). Free radicals are referred as oxidants while the defense mechanisms against these free radicals are called antioxidants. Antioxidants are substances that prevent tissue damage due to free radical action [7]. In the dictionary of Food and Nutrition, antioxidants are defined as food additive or chemical that opposes oxidation or inhibit or retard reactions promoted by oxygen (or peroxides). In another definition, antioxidants are defined as compounds that help protect against free radical damage. They are able to disarm free radicals and break the vicious cycle. Biological systems antioxidants could work in various ways including catalytic removal of free radical, as scavengers of free radicals or in the form of proteins that minimize the availability of pro-oxidants such as metal ions [2]. Some antioxidants including glutathione, ubiquinol and uric acid are produced during normal metabolism in the body. Glutathione can neutralize free radicals many times before it (glutathione) is oxidized; it also restores vitamin C and vitamin E to their reduced form so that they can continue scavenging free radicals. Glutathione also helps repair DNA and prevents free radical damage to DNA. Other lighter antioxidants are found in the diet. Although about 4000 antioxidants have been identified, the best known are vitamin E the best known are vitamin E, vitamin C and the carotenoids [4]. Vitamin C is an oxygen scavenger. It transfers its hydrogen atoms to oxygen, making the oxygen unavailable for further reactions. Other dietary vitamin antioxidants includes vitamin E and A (including lycopene) which serve as bodyguards giving the body protection against free radical damage since the body cannot provide enough antioxidants needed. Vitamin E when present in food acts as an antioxidant and protects other nutrients against oxidation for example vitamin A is preserved by vitamin E [10]. Coenzyme Q10 is an antioxidant, which can regenerate vitamin E from its radical form and is capable of scavenging oxygen radicals and preventing disruption of lipid cell membranes.

Another defense mechanism is a system of antioxidant enzymes. These enzymes are endogenous (produced by the body) and these include superoxide dismutase (SOD), which removes O₂, catalase which convents H₂O₂ to water (H₂O) and O2and glutathione peroxidase which helps with H₂O₂ removal and prevents hydroxyl radical (OH-) formation [1]. Hence if the nutritional supply of these nutrients is inadequate, enzymatic defenses against free radicals may be impaired.

The body can handle free radicals normally. When antioxidants are unavailable or if the free radical production becomes excessive, damage may occur. It is important to note that free radicals accumulate with age. Life modification is very important in order to minimize over production and maximize the availability of antioxidant .These modifications include smoking cessation, minimizing sun exposure, avoidance of air pollution and inhalation of toxic chemicals such as gasoline fumes and moderating alcohol intake. Fruits, grains, legumes as well as vegetables are rich antioxidants containing accompanying substances known as phytochemical element that act as true natural pharmaceutical [5]. Antioxidants found in fruits and vegetables protect the cells from damage caused by metabolic by-product (free radicals) as well as toxic substances from food and the environment. The USDA 2000 Dietary guidelines encourage one to enjoy daily a minimum threshold of 7 servings of fruits and vegetables: fruit 2-4 servings and vegetables a minimum of 5 servings. One serving of fruit represents a medium sized fruit or ½ cup of natural fruit juice. A serving of vegetables represents 125 ml s to 250 ml s of vegetables [12].