Why We Need Vitamin A in our Life?
For centuries and centuries, people has knowing that there is a relationship between diet and some diseases; it was only in XIX century when research really begin about what in our food that prevent certain diseases. And in 1913 the American biochemists Elmer McCollum and Marguerite Davis discovered the first vitamin, dubbed vitamin A.
In the beginning when vitamin A was discovered -it was called the “anti-infective agent.”- laboratory animals that were having a diet low in animal foods, fruits, and vegetables quickly got eye infections; these infections disappeared as soon as these foods were put back into their feed. The mysterious “agent” in the foods turned out to be a fat-soluble item that was named Vitamin A. 
Vitamin A forms:
Vitamin A comes from two various sources. From vegetables and fruits, we find Provitamin A carotenoids most notably alpha-carotene, beta-carotene and beta cryptoxanthin, but beta-carotene (sometimes written b-carotene) was the most provitamin A that received considerable attention from researchers because of its health benefits, and because beta-Carotene is the most common form of carotenoids in plants. And from animal products and supplements especially the liver we find the other source of vitamin A called retinoid.
Into the body vitamin A is existing in 5 forms: retinoic, retinal acid, retinol, retinyl esters(commonly retinyl acetate or retinyl palmitate), and beta-carotene, as seen in Figure-1, all of these forms of vitamin A do functions that none of the others forms can achieve.
But why is it preferable to get vitamin A from the beta-carotene in plant foods much than getting him from animal products and supplements? because there are a number of good reasons.
Beta-Carotene And retinoid Benefits:
- Foods that rich in beta-carotene are the most powerful antioxidants sources. Antioxidants counteract the effect of free radicals to reduce the risk of age-related macular degeneration, cardiac disease, cancer, and Cerebrovascular accident (ischemic stroke). About 60 percent of the beta-carotene that you eat from plant foods work as powerful antioxidants and the rest are converted into vitamin A in the liver and small bowel only as your body needs it.
- Up to now, no one has reported any toxic effects of carotene. Because if someone take an excessive intake of beta-carotene(not beta-carotene supplements), the only side effect is a discoloration of stratum corneum -the outermost layer of the skin- to an orange-yellow color (technically, Carotenosis or carotenodermia), but this discoloration is innocuous and does not require treatment, it goes away in a several weeks. And this is what makes beta-carotene a safe source of vitamin A.
- Almost every vegetable and fruit contains carotene, Five servings a day is enough to getting all the Vitamin A we need, along with a lot of other vitamins, minerals, fibres, and antioxidants.
On the other hand, retinoid is essential for healthy eyes, growth and development, especially for children and teens to help them develop and grow healthy and build strong teeth and bones.
Vitamin A Deficiency:
To hold off infections and diseases, Vitamin A assists you to set up powerful front-line barriers to infection, and that is done by helping epithelial tissues in your body todevelop and reform themselves. But without sufficient Vitamin A, these cells become stiff, dry, and much more probable to let their protection down. When that happens, microbes can easily pass through them and inside your body.Sometimes even if your body has an abundance of Vitamin A, those microbes can pass through your external defences. In that time, your immune system goes stronger by getting help from vitamin A.
 The Complete Idiot's Guide to Vitamins and Minerals, 3rd Edition.
 A substance that can be converted into a vitamin.
 the cells that constitute your skin and line your eyes, mouth, nose, throat, lungs, digestive tract, and urinary tract.