Vitamin A is a fat-soluble vitamin essential to the maintenance of health. It plays many roles in the body, but it is especially important for the health of epithelial cells. These cells cover the surface of the body (skin and eyes) and body cavities (such as lungs, intestines, mouth, and stomach), serving as barriers to infection. Many of these cells secret mucus, a thick fluid that acts as a protective lubricant, and vitamin A is fundamental to this process. Vitamin A also participates in other processes such as growth, body development and reproduction. Vitamin A deficiency, rare in the US, can lead to blindness, and may impair immune function, increasing the risk for infections.
Vitamin A is found in foods in two forms: preformed vitamin A (retinoids) and provitamin A (carotenoids). Retinoids come from animal sources such as fish, organ meats, and eggs, but they can also be found in fortified milk, yogurt, and margarine. Carotenoids are phytochemicals (plant pigments) that can be transformed into vitamin A in the body. Beta-carotene is the most potent form of provitamin A and is present in carrots, sweet potatoes, and squash. Lutein and zeaxanthin are also examples of carotenoids which are found in high concentration in green, leafy vegetables such as spinach and kale. Carotenoids are also antioxidants, protecting body cells from damage caused by free-radicals. This means that they may play an important role in preventing cardiovascular disease, reducing the risk for cancer, or even improving immune function.
You can get all the vitamin A your body needs from food sources such as colorful fruits and vegetables, liver, fish, eggs, and fortified foods. Supplementation is usually unnecessary, and may even be harmful, if the intake is too high. That’s because vitamin A is stored in the body for long periods, and megadoses can cause liver toxicity.
 Free-radicals are by-products from normal metabolism but also come from smoking, pollution, poisons and fried foods. They are unstable atoms that steal electrons from other molecules in order to stabilize. Their action can damage body cells leading to several diseases.
 Megadose is an exceptionally large dose.