Antioxidants are specific organic compounds that are active in the prevention of very rapid harmful chemical chain reactions with oxygen or nitric oxide, that is, oxidation reactions. In the body, oxidation reactions generally involve highly reactive molecules called free radicals. Free radicals reside primarily in the mitochondria of cells. When free radicals are released from the mitochondria in numbers sufficient to overwhelm the protective biochemical systems of the body, they become a threat to some cellular structures such as lipids, proteins, carbohydrates, and nucleic acids in cell membranes. Compromised cellular structure alters cellular function, and may lead to the initiation of the disease process. In severe oxidative stress, cell death may occur. Antioxidants react with the free radicals before they are able to react with other molecules, thus providing protection from oxidation reactions (Cross et al.).
How and Why Cells and Other Molecules Interact
The human body is made up of many different types of cells that are composed of multiple diverse types of molecules. Molecules are put together in such a way that one or more atoms of one or more elements are joined by chemical bonds. Atoms have a nucleus of neutrons and protons which is surrounded by electrons. It is the number of protons (positively charged particles) in the nucleus of the atom that determines the number of orbiting electrons (negatively charged particles). Electrons are involved in chemical reactions and are the substances that bond atoms together to form molecules. Electrons orbit the atom in one or more of the atom's shells. The innermost shell is full when it has two electrons. When the first shell is full, electrons begin to fill the second shell. When the second shell has eight electrons, it is full, and electrons begin to fill the third shell, and so on. The electrons surrounding antioxidants react with the electrons surrounding free radicals, causing them to become much less reactive. Antioxidants may be more effective when one antioxidant is used in combination with another. This synergistic relationship between several antioxidants occurs when, for example, vitamin E donates an electron from its outer shell to a free radical and vitamin C donates an electron to vitamin E, maintaining the ability of vitamin E to continue donating electrons to free radicals. Vitamin C may then receive an electron from glutathione that would enable vitamin C to remain active as an antioxidant. Therefore in this type of situation, an attack on membranes by a free radical results in the participation of three different antioxidants.
In What Forms Are Antioxidants Found and How Are They Metabolized?
Antioxidants are found in many forms. The principal vitamins with antioxidant properties are vitamins E and C, and beta-carotene. Vitamin E (d-alpha tocopherol) is a fat-soluble antioxidant, which means it is stored in body fat and works within the lipid portion of cell membranes to provide an alternative binding site for free radicals, preventing the oxidation of polyunsaturated fatty acids (Chow). Vitamin E is a family of eight compounds synthesized by plants in nature: four tocopherols (alpha, beta, gamma, delta) and four tocotrienols (alpha, beta, gamma, delta). Each has different levels of bioactivity in the body over quite a wide range, but generally speaking, alphatocopherol has greater bioactivity than beta-tocopherol, which has greater bioactivity than gamma-tocopherol, which has greater bioactivity than delta-tocopherol. Only alpha-tocotrienol has bioactivity of any significant amount, which is slightly less than that of beta-tocopherol. Digestion and absorption of vitamin E is greatly improved when consumption is accompanied with dietary lipids or fats. Absorption of vitamin E ranges from 20 to 50 percent, but may be as high as 80 percent, with absorption decreasing as intake increases (Bender, 1992). Dietary vitamin E absorption requires bile and pancreatic enzymes in the small intestine, where it is incorporated into micelles within the lumen of the small intestine. The micelles carry the vitamin E across the brush border of the small intestine and the vitamin E is then taken up by chylomicrons, which are transported by the lymph system to tissues and the liver. Vitamin E may be stored in the liver, adipose tissues, and skeletal muscle. When needed, vitamin E places itself in cell membranes. Excretion of vitamin E is by way of urine, feces, and bile (Wardlaw and Kessel).
Vitamin C (ascorbic acid) is a water-soluble antioxidant and is found in the water compartments of the body where it interacts with free radicals. It has been shown that short-term supplementation of vitamin C lasting two to four weeks can significantly reduce the level of free radicals in the body (Naidoo and Lux). Dietary vitamin C is absorbed primarily by active transport in the small intestine, with absorption decreasing as intake increases. Approximately 70 to 90 percent of vitamin C is absorbed when dietary intake is between 30 and 180 mg/day. The kidneys excrete excess dietary vitamin C in urine, but excrete virtually no vitamin C when intake of the vitamin is very low (Wardlaw and Kessel). After absorption in the small intestine, vitamin C is transported in the blood to cells in its reduced form, ascorbic acid or ascorbate. The concentration of vitamin C varies in different tissues in the body. For instance, vitamin C concentrations are highest in the adrenal and pituitary glands, intermediate in the liver, spleen, heart, kidneys, lungs, pancreas, and white blood cells, and lowest in the muscles and red blood cells (Olson and Hodges). This vitamin may also possess some prooxidant properties, meaning it can participate in oxidizing other molecules such as iron in the blood stream (Alhadeff et al.).
Beta-carotene is a precursor to vitamin A (retinol). Beta-carotene is the most widely known compound in a group known as carotenoids, which are pigment materials in fruits and vegetables that range from yellow to orange to red in color. Carotenoids are also called proformed vitamin A because they can be made into vitamin A by the body when necessary. Carotenoids are pigments that are responsible for the orange color of many fruits and vegetables such as oranges and squash. Other carotenoids present in foods include antheraxanthin, lutein, zeaxanthin, and lycopene. Dietary retinol is usually found bound to fatty acid esters, which are in turn bound to proteins, and must undergo a process called hydrolysis that frees the retinol from the esters, enabling the retinol to then be absorbed in the small intestine. Proteolytic enzymes in the small intestine, such as pepsin, hydrolyze the retinol from the proteins. Approximately 70 to 90 percent of dietary retinol is absorbed provided there is adequate (10 grams or more) fat in the meal consumed (Olson). Carotenoids are absorbed at much lower levels, sometimes at levels as low as 3 percent, with absorption decreasing as intake increases (Brubacher and Weisler). Retinol and the carotenoids are carried through the absorptive cells of the small intestine by micelles for transport through the lymph system to the liver, which then can "repackage" the vitamins to send to other tissues, or act as the storage facility for the vitamins until needed by the body.
There are also enzymes that possess antioxidant properties. Glutathione peroxidase, superoxide dismutase, and catalase are the most well known. Glutathione peroxidase breaks down peroxidized fatty acids, converting them into less harmful substances. Peroxidized fatty acids tend to become free radicals, so the action of glutathione peroxidase serves to protect cells. The activity of glutathione peroxidase is dependent on the mineral selenium, which is the functional part of this enzyme, or the part of the enzyme that makes it have antioxidant activity. Therefore, selenium is considered to have antioxidant properties. Superoxide dismutase and catalase react with free radicals directly, reducing their ability to oxidize molecules and cause cellular damage.
A class of compounds termed isoflavones, which are derived from soy, also have antioxidant activity. Genistein, daidzein, and prunectin are all able to prevent the production of free radicals. Isoflavone activity as an antioxidant plays an important role in the aging process and cancer prevention primarily due to having estrogenrelated biologic activities in humans (Shils et al.).
The polyphenols (epicatechin, epicatechin-3-gallate, epigallocatechin, and epigallocatechin-3-gallate) found in jasmine green tea also possess natural antioxidant properties. Studies have shown that these polyphenols are able to protect red blood cells from destruction upon attack by free radicals (Shils et al.). The polyphenols present in red wine have also been found to be protective against the oxidation of low-density lipoproteins and high-density lipoproteins, which are very important factors in the prevention of the development of atherosclerosis or coronary artery disease (Ivanov et al.).
A final group of compounds, synthetic antioxidants, are often added to foods to prevent discoloration and delay oxidation of the foods after exposure to oxygen. They also help protect fats from rancidity. Rancidity causes fats to develop an unappealing flavor and odor. Most of the antioxidants used in foods are phenolic compounds. There are four antioxidants that are approved for use in foods, particularly fats. They are propyl gallate (PG), tertiary butylhydroquinone (TBHQ), butylated hydroxyanisole (BHA), and butylated hydroxytoluene (BHT) (Charley and Weaver). Sulfites, which are sulfur-based chemicals, are also used as antioxidants in foods. However, because some people may be very sensitive to sulfites and have adverse reactions to them in foods, the Food and Drug Administration has required that labels on foods containing sulfites alert the public to their presence.
Dietary Sources of Antioxidants
Vitamin E is found in egg yolks, milk, plant and vegetable oils (including margarine and to a lesser extent butter), nuts, seeds, fortified whole-grain cereals, flatfish, halibut, shrimp, canned tuna in oil, asparagus, peas, tomatoes, apples, canned apricots in light syrup, blueberries, grapefruit, oranges, peaches, and pears. The milling process of whole grains causes most of the dietary vitamin E to be lost. The Recommended Dietary Allowance (RDA) as established by the U.S. Department of Agriculture currently is 15 International Units (IU) per day for men and 12 IU/day for women. In order for toxic effects to be produced, the amount of vitamin E consumed from foods would have to be 15 to 100 times the amount recommended for humans and this is extremely unlikely to occur (Wardlaw and Kessel). Symptoms and effects of toxicity are discussed in detail in Signs and Symptoms of Antioxidant Deficiency and Toxicity, below.
Vitamin C is present in large amounts in broccoli, asparagus, cabbage, cauliflower, potatoes, tomatoes, apples, applesauce, apricots, bananas, blueberries, cherries, grapefruit, lemons, oranges, peaches, strawberries, kiwi, pineapples, pears, cranberries, and the juices and jams made from these fruits. The Recommended Dietary Allowance for vitamin C currently is 60 mg/day for both males and females. Vitamin C obtained from foods rarely can be consumed in amounts large enough to be toxic to humans (Wardlaw and Kessel).
Beta-carotene is found in liver (primary storage organ in animals for vitamin A), egg yolk, fortified milk, butter, spinach, carrots, squash, sweet potatoes, broccoli, tomatoes, peaches, mangoes, apricots, papaya, cantaloupes, and fortified breakfast cereals. Because beta-carotene is converted to vitamin A by the body, there is no set requirement. However, the RDA for vitamin A is set in Retinol Equivalents (RE) at the level of 625 μg/day RE for men and 500 μg/day RE for women (Wardlaw and Kessel).
Diets High or Low in Antioxidants
Diets that are rich in antioxidants focus on high intakes of a variety of foods, especially large amounts of fruits, vegetables, and foods made from whole grains. Vegetarian diets, especially vegan diets (diets that exclude all foods from animal sources), are made up primarily from fruits, vegetables, whole grains, and legumes, and are an example of the types of diets that incorporate high levels of antioxidants. Another example of a diet that provides optimal levels of antioxidants is the Mediterranean diet. The Mediterranean diet is based on traditional eating habits in Greece, southern Italy, and Crete. This diet is rich in olive oil, foods from whole grains, and tomatoes, and minimizes the daily intake of poultry, eggs, sweets, and red meat. Red wine often accompanies meals in the Mediterranean diet and possesses some antioxidant activity (Murcia and Martinez-Tome). Furthermore, many of the spices used in Mediterranean cooking also have been observed to have some level of antioxidant properties (Martinez-Tome et al.). Asian-American diets also focus primarily on fruits, legumes, nuts, seeds, vegetables, and whole-grain food products, with liberal use of vegetable oils, while a minimum of meat is eaten. The traditional healthy Latin American diet provides beans, whole grains, nuts, fruits, and vegetables at every meal, with fish or shellfish, milk products, plant oils, and poultry being optional for daily intake.
Unfortunately, the typical American diet does not involve adequate intakes of fruits, vegetables, and whole-grain food products. This is not due to the lack of foods that fall into those categories but rather to the fact that too many Americans prefer fast foods and processed foods that are not rich sources of antioxidants. The Food Guide Pyramid developed by the United States Department of Agriculture recommends that six to eleven servings of bread cereal, rice, and pasta be consumed daily; three to five servings of vegetables per day; two to four servings of fruit per day; two to three servings of milk products per day; two to three servings of meat, poultry, fish, dry beans, eggs, and nuts per day; and that the use of fats, oils, and sweets be sparse (Wardlaw and Kessel). Most Americans do not adhere to the guidelines of the Food Guide Pyramid and therefore do not receive adequate amounts of foods that provide large quantities of antioxidants.
Certain disease states make it difficult to obtain adequate amounts of fat-soluble vitamins due to an inability to digest foods with fat properly. The digestion and absorption of fat in foods is required for digesting and absorbing fat-soluble vitamins such as vitamins A and E. Individuals with cystic fibrosis, celiac disease, and Crohn's disease absorb fat very poorly, which also means that the fat-soluble vitamins are poorly absorbed. As the unabsorbed fat passes through the small and large intestine, it carries the fat-soluble vitamins along with it, and is eventually excreted in the feces (Wardlaw and Kessel). Chronic alcoholics are also at risk for not obtaining adequate amounts of antioxidants due to a marked decrease in food intake in favor of the consumption of alcohol. Alcoholism may also result in liver disease, which leads to an inability of the liver to store the fat-soluble antioxidants.
Signs and Symptoms of Antioxidant Deficiency and Toxicity
Obtaining dietary intakes of vitamin E, vitamin C, and vitamin A from foods to meet the recommendations of the Food Guide Pyramid will prevent most healthy individuals from experiencing any deficiencies of these antioxidants. However, in diets that do not provide adequate amounts of fruits, vegetables, and whole grains, deficiencies may occur. It takes longer to develop a deficiency of the fat-soluble antioxidants, vitamins E and A, than it does to develop a deficiency of the water-soluble vitamin C.
Failure to obtain adequate vitamin E in the diet may cause certain medical conditions. Hemolytic anemia is caused by vitamin E deficiency, with an increased breakdown of red blood cells or hemolysis. Premature infants are most susceptible to vitamin E deficiency due to very small stores of the vitamin at birth and the frequently required use of oxygen to accommodate immature lungs. Premature infants are also growing very rapidly and need increased intakes of vitamin E. Special formulas are used to provide vitamin E to help prevent deficiency (Wardlaw and Kessel).
The disease caused by vitamin C deficiency is scurvy. The symptoms of scurvy are fatigue and small, purple spots or hemorrhages (petechiae) that appear around hair follicles on the back of the arms and legs. There are also bleeding gums and joints, impaired wound healing, pain in the bones, fractures, and diarrhea. Consuming a vitamin C–free diet for as little as 20 days may cause scurvy, but resuming vitamin C intake for one week can cause the reversal of the disease and accompanying symptoms (Wardlaw and Kessel).
Vitamin E toxicity may result from intakes of more than 1,500 IU/day of vitamin E isolated from natural sources and 1,100 IU/day for synthetic vitamin E for adults nineteen years or older. It is only possible to acquire such high doses of either form of vitamin E via supplementation. Use of supplemental vitamin E at such high doses in persons with a compromised health status may lead to complications such as hemorrhaging in individuals who are taking anticoagulants or are vitamin K-deficient (vitamin K is important in blood coagulation) (Wardlaw and Kessel).
Vitamin C toxicity may occur at intakes of 2 g/day or higher. The symptoms of vitamin C toxicity are nausea, abdominal cramps, and osmotic diarrhea. Because vitamin C is a water-soluble vitamin, much of excess vitamin C obtained from supplemental megadoses is excreted in urine (Wardlaw and Kessel).
Small children who do not eat enough vegetables are at an increased risk for vitamin A deficiency. In fact, individuals with very low incomes and the elderly are also at risk for deficiency due to an inability to obtain adequate intakes of foods that are good sources of vitamin A and to the decreased gastrointestinal function that may occur with age. Night blindness is a symptom of vitamin A deficiency, causing the rod cells in the eye to take a longer period of time to recover from flashes of light. Another symptom of vitamin A deficiency is dry eyes caused by deterioration of the mucus-forming cells in the body. In an individual with dry eyes, dirt and other contaminants are not washed away, and this may lead to eye infections. If vitamin A deficiency is not corrected, the condition of the eyes worsens, leading to more serious disorders of the eye; eventually irreversible blindness may result. The skin is also affected by a compromised vitamin A status. Primary symptoms are very dry skin and rough and bumpy texture of the skin surface. When vitamin A supplements are taken long-term at three times the RDA a condition called hypervitaminosis A may develop. This condition can cause spontaneous abortions in pregnant women or birth defects in infants and therefore women of child-bearing age wishing to become pregnant should avoid using high doses of vitamin A supplements (Wardlaw and Kessel).
Maintaining Antioxidant Content in the Foods You Eat
Antioxidants in foods are a valuable addition to a healthy diet and steps can be taken to preserve the antioxidant content of foods until they are ready to be ingested. Keeping fruits and vegetables refrigerated or in a cool, dry place helps to slow down the natural breakdown by enzymes that begins to occur as soon as the foods are picked. Fruits and vegetables should not be trimmed or cut until they are ready to be consumed to prevent unnecessary exposure to oxygen. Cooking by steaming, microwaving, or stir-frying in small amounts of fat for short amounts of time also helps to preserve the vitamin content of foods. If liquids are used to cook fruits or vegetables, do not add fat while cooking if you are planning to discard the liquid before eating the fruits or vegetables, to avoid losing the fat-soluble vitamins that may be in the liquids. Finally, it is important to remember that the skin of some fruits and vegetables contains a higher vitamin content than the inner parts, such as the skin of an apple (Wardlaw and Kessel).