Vitamin A has been studied as one of the vital nutrient belonging to the fat-soluble retinoid. They are the retinols, the retinal, and the retinyl esters. These compounds are part of an important protein known as rhodopsin, therefore are need for clear vision. The nerve receptors of the eyes are very rich in this protein as they perform their function of absorbing light which is necessary for the growth and physiological functions of the membranes lining the conjunctiva. Again, Vitamin A enables growth of the body cells. Vitamin A interferes with the production of some types of protective secretion and cells by the immune system. The various forms of Vitamin present in the foods we eat have been grouped into two. These include;
Preformed vitamin A: Foods containing this form of vitamin A are mainly extracted from animal sources. Amongst these are the dairy products, meat and fish.
The Provitamin A carotenoids they are sufficient in plant products is carotenoids which are pigments. Beta-Cryptoxanthin and Alpha-carotene are other components of plant carotenoids.
However, in digestion, these vitamin A forms are first dissolved to become micelles which now become absorbable by the cells lining the mucosa of the duodenum, a part of the lumen. Both are transformed into retinol. The retinol becomes oxidized into retinal. Further oxidation results to the formation of retinoic acid. Of all the vital organs in the body, the liver stores a greater part of vitamin A. Several kinds of foods have been studied to contain, at varying concentrations, vitamin A. These range from animal to plant products.
Food sources from animals have been studied to almost contain only the preformed vitamin A, the retinol. These products and the varying concentrations of retinol are highlighted thus;
Fish: Generally, products from fish are not rich in vitamin A. this is because just a few species have been associated with this vitamin in concentrations of about 20 to 30 μ g in 100 g.
Liver: retinol is richly contained in liver meat. Concentration is about9,000 μ g to 16,000 μ g in 100 g of chicken liver.
Meanwhile, animal products, especially the animal meats, have been studied to serve as medium sources, with levels about 30 μ g in a100 g of chicken, mutton, meat or duck.
Milk: contained in the commercially sold milk is this retinol at concentrations of about 30 μ g in a 100 ml solution. The vitamin concentration is high in full cream milk powder, containing about400 μ g/100 g. Market brand also affects the concentration.
Egg: an entire egg, with its albumen and yolk, is rich in this vitamin, with concentration estimated to be 350 μ g/100 g. But the yolk still contains the greater quantity estimated to be about a 570μg of retinol in100 g.
On the other hand, studies have shown that foods that take their sources from plants are very rich in the provitamin A. provitamin A can be gotten from fruits such as orange, vegetables, and oil containing foods.
Other Dietary supplements
Other than the natural food sources, vitamin A is formulated in multivitamins and as a sole vitamin source in some supplement. This is usually in the form of these compounds; retinylpalmitate or retinyl acetate. Other supplements may contain only preformed vitamin A or only beta-carotene. The amounts of vitamin A in pure vitamin supplements vary widely as seen multivitamin supplements which typically contain about 2,500 to 10,000 IU vitamin A. This is usually in the form of both retinol and beta-carotene. In addition, about 28% to 37% of the general population uses supplements containing vitamin A.
Excessive intake; rarely does toxicity occur from ingesting food sources of preformed vitamin A. But there has been reported cases of some disorders associated with the increased consumption liver products.
Deficiencies; when the level of vitamin A in food becomes very low or not present at all, physiological disorders are bound to occur. The disease symptom, xerophthalmiais attributed to a deficiency in vitamin A. poor vision in low light is one of the earlier signs of this disease. Temporary blindness seen in children have been linked with A vitamin deficiency. Those with vitamin A deficiency (often, xerophthalmia with its characteristic Bitot’s spots) tend to have low iron status, which can lead to anemia. Vitamin A is important to the human health in the form of aiding sight as well as the immune system in fighting diseases. Its presence at normal concentrations can be maintained through feeding on nutrient rich diets/ foods.