Vitamins are essential nutrients found in foods. They are required in limited amounts. Basically, the name ‘Vitamin’ is given to an organic chemical compound that cannot be synthesized by an organism in sufficient quantities and can only be obtained through the diet. Hence, the term vitamin is conditional upon the circumstances and the particular organism. A good example is ascorbic acid (one form of vitamin C), which is an important vitamin for humans, but not for most other animal organisms. Vitamins perform certain physiological functions which aid in the regulation of metabolism, prevention of chronic diseases (as seen in heart disease and cancer), and maintenance of normal appetite, mental health, and immunity. There are thirteen universally recognized vitamins at present. These vitamins have been classified according to their biological and chemical activity into the following two categories:
- Fat-soluble vitamins Include Vitamins A, D, E and K; dissolve in fats; and are absorbed with the help of fats that are in the diet into the bloodstream to carry out their functions. Excessive fat-soluble vitamins are stored in the liver.
- Water-soluble vitamins Include Vitamins B and C, and dissolve in water Excessive amount of water-soluble vitamins are excreted through urine and sweat. Hence, continuous daily supplies of these vitamins are required in our diet.
The amount of vitamins in food is affected by the ways in which food is cooked or the storage condition. For example, Vitamins A and C and some Vitamins B may be destroyed under strong light. This is the reason it is important to store food rich in those vitamins in dim places or in the fridge. Similarly, Vitamin C and some Vitamins B which are soluble in water can be destroyed under heat. Therefore it is important that we should avoid; Washing them too much or cooking them too long time. Vitamins are majorly obtained from various foods, plant and animal products. Examples include Leafy vegetables, many fruits, Meat, poultry, fish, eggs, milk, mushrooms, tree nuts etc. A few other vitamins are obtained by other means. For example, vitamin K and biotin which are produced by microorganisms inhabiting the gastrointestinal tract, commonly known as the gut flora; and vitamin D synthesized by the skin when exposed to ultraviolet radiation from the sun.
Vitamin A comprises a group of unsaturated nutritional organic compounds belonging to the fat-soluble vitamins. This includes retinol, retinal, retinoic acid, and several pro-vitamin a carotenoids (most notably beta-carotene).Vitamin A has many physiological functions described thus;
Vision; The retinal, a form of vitamin A is a very important molecule required for the maintenance of normal vision. In the eyes, this compound, retinal, combines with a protein called opsin to give the bimolecular, rhodopsin, an essential light absorbing molecule that enables color vision and sight in dim light. The absence or deficiency in vitamin A can lead to impaired vision.
Immune system: Vitamin A aids the activities of the immune system in their fight against infections hence, offers protection to the body. Reduced levels/ deficiency may lead to decreased immune response to infections.
Gene transcription and protein formation: Retinoic acid which is a form of Vitamin A is essential for gene transcription. The body cells take up and oxidize retinol toretinaldehyde (by retinol dehydrogenases). This aldehyde is further oxidized to give retinoic acid. Studies have shown that the conversion process, from retinal to retinoic acid is irreversible and is therefore tightly regulated. This is because retinoic acid functions as a legend for nuclear receptors. Retinoic acid, however, binds to these nuclear receptors in order to regulate gene transcription, a pre-requisite for protein formation.
Cell growth: Retinoic acid functions as a vital hormone-like growth factor for epithelial cells and other cell types in the body.
Skin health: Retinoic acid has been shown to play a role in healthy skin maintenance by activating genes that triggers immature skin cells to develop into mature epidermal cells. The actual mechanism behind this is currently under study to help in the development of treatments for dermatological diseases. Currently, the retinoic acid drug isoretinoin has become the most commonly prescribed agent in the treatment of acne. This drug decreases the size of sebaceous glands and reduces their secretions (sebum). Retinoic acid also reduces the amount of bacteria present in the ducts and surface of the skin, which occurs as a result of reduced sebum, utilized by certain bacteria species as a source of nutrients.
Dietary sources of vitamin A include; Beef, calf, and chicken liver, Eggs, Fish liver oils, dairy products, including whole milk, whole milk yogurt, whole milk cottage cheese, butter, and other cheeses
There are two forms of vitamin A available in the human diet. These include;
- Preformed vitamin A: This is found in foods from animal sources, including dairy products, fish, and meat (especially liver). It comprises the retinol and its esterified form, retinyl ester)
- Provitamin A carotenoids: These are plant pigments with Beta-Carotene about the most important provitaminA carotenoid. Other provitamin A carotenoids includes alpha-carotene and beta-cryptoxanthin.
However, all forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group. These structural features are essential for vitamin activity. The orange pigments in carrots, the compound beta-carotene, are made up of two connected retinyl groups, which are used in the body to contribute to vitamin A levels. None of the other carotenes have vitamin activity.
General deficiencies; Low levels or absence of vitamin A in diets result to some physiological disorders. Its deficiency increases the risk of diarrhea. Infants and pregnant women suffer xerophthalmia as the most common symptom of vitamin A deficiency. One of the early signs of this disorder is night blindness indicated by the inability to see in low light or darkness. Vitamin A deficiency has been implicated as one of the top causes of preventable blindness in children. 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 deficiency also increases the severity and mortality risk of infections (particularly diarrhea and measles) prior to the onset of xerophthalmia.