Antioxidants are molecules capable of inhibiting the oxidation of other molecules therefore they can protect cells from the damage caused by unstable molecules known as free radicals (the natural by-product of normal cell processes). Antioxidants are widely used as ingredients in dietary supplements and have been investigated for the prevention of diseases such as cancer, coronary heart disease and even altitude sickness.
Overview of Antioxidants
Antioxidants are classified into two groups which are based on whether specific antioxidants are soluble in water (hydrophilic) or in lipids (hydrophobic). In brief, water-soluble antioxidants react with oxidants in the blood plasma and the cell cytosol whereas lipid-soluble antioxidants protect cell membranes from lipid peroxidation. Antioxidants may be synthesised in the body or obtained from the diet. They can present in body fluids and tissues, with some such as glutathione or ubiquinone mostly present within cells.Examples of other antioxidants include beta-carotene, lycopene, vitamins C, E, and A. Selenium and zinc are also commonly referred to as antioxidant nutrients, but these chemical elements have no antioxidant action themselves and are instead required for the activity of some antioxidant enzymes.
| Antioxidant metabolite | Solubility | Concentration in human serum (μM) | Concentration in liver tissue (μmol/kg) |
|---|---|---|---|
| Ascorbic acid (vitamin C) | Water | 50 – 60 | 260 (human) |
| Glutathione | Water | 4 | 6,400 (human) |
| Lipoic acid | Water | 0.1 – 0.7 | 4 – 5 (rat) |
| Uric acid | Water | 200 – 400 | 1,600 (human) |
| Carotenes | Lipid | β-carotene: 0.5 – 1 | 5 (human, total carotenoids) |
| α-Tocopherol (vitamin E) | Lipid | 10 – 40 | 50 (human) |
| (coenzyme Q) | Lipid | 5 | 200 (human) |
Which foods are rich in antioxidants?
Antioxidants can be found in abundance in fruits and vegetables as well as in some meats, poultry and fish. The list below describes food sources of antioxidants.
Vitamin A is found in three main forms: retinol (Vitamin A1), 3,4-didehydroretinol (Vitamin A2), and 3-hydroxy-retinol (Vitamin A3). Foods rich in vitamin A include liver, sweet potatoes, carrots, milk, mozzarella cheese and egg yolks.
Vitamin C (ascorbic acid) can be found in high abundance in many fruits and vegetables and is also found in cereals, beef, poultry and fish.
Vitamin E (alpha-tocopherol) is found in almonds, in many oils including wheat germ, safflower, corn, and soybean oils. It is also found in mangos, nuts and broccoli.
Beta-carotene is found in many foods that are orange in color, including sweet potatoes, carrots, cantaloupe, squash, apricots, pumpkin, and mangos. Some green, leafy vegetables, including collard greens, spinach, and kale, are also rich in beta-carotene.
Lutein, best known for its association with healthy eyes, is abundant in green, leafy vegetables such as collard greens, spinach and kale.
Lycopene is a potent antioxidant found in tomatoes, watermelon, guava, papaya, apricots, pink grapefruit, blood oranges, and other foods. Estimates suggest 85 percent of American dietary intake of lycopene comes from tomatoes and tomato products.
Selenium is a mineral, not an antioxidant nutrient. However, it is a component of antioxidant enzymes. Plant foods like rice and wheat are the major dietary sources of selenium in most countries. The amount of selenium in soil, which varies by region, determines the amount of selenium in the foods grown in that soil. Animals that eat grains or plants grown in selenium-rich soil have higher levels of selenium in their muscle. In the United States, meats and bread are common sources of dietary selenium. Brazil nuts also contain large quantities of selenium.