Vitamins are small organic molecules that are needed for optimal body functioning. Vitamin A occurs in two forms in nature. It exists as retinolesters in foods of animal origin, whereas in plants it is found in a provitamin form known as carotenes. There exist alpha, beta and gamma carotenes. Notably, vitamin A is one of the essential vitamins. This implies that it is not synthesized in the body and therefore, obtained through diet. Vitamin A comprises a wide array of fat-soluble retinoids, namely, retinol (Vitamin A alcohol), retinal (Vitamin A aldehyde) and retinoic acid (Vitamin A acid) (Office of Dietary Supplements – Vitamin A. (n.d.). Notably, retinal and retinoic acid are derived from retinol. Additionally, retinal and retinol are interconvertible. However, retinoic acid cannot be converted to either retinal or retinol.
All the three forms of Vitamin A that exhibit vitamin A activity each playing a unique function in the body. Retinal plays a role in facilitating normal color and vision. Its role in vision is facilitated by one of its components known as rhodopsin (A visual pigment). Rhodopsin has the ability to absorb light in the retinal receptors since it supports the normal differentiation and functioning of the conjuctival membranes and the cornea (Rao, 2006). Therefore, a diet deficient in vitamin A inhibits its production thus disrupting the perception of vision color by the brain. In early stages of Vitamin A deficiency, one is unable to clearly perceive vision in dim light due to the blockage in rhodopsin resynthesis.
Retinol, on the other hand, is crucial for cellular growth and differentiation as well as reproduction. Retinol supports spermatogenesis, oogenesis and placental development as well. As such, it takes part in the formation of vital body organs such as the heart, lungs, and kidneys. It also facilitates the maintaining of epithelial integrity of systems such as the gastrointestinal tract, the skin, respiratory tract, urinary tract and the salivary glands. Additionally, vitamin A plays a role in growth and differentiation of erythrocytes. Therefore, deficiency may result in the development of vitamin A deficiency anemia.
Sources of preformed Vitamin A include the liver, milk, eggs and fish oils. However, the main source of provitamin A are fruits, carrots, and leafy green vegetables. Animal sources such as marine fish oils, liver oil, cod liver oil and shark liver oil are excellent animal sources. In plants, plant oils like red palm is an excellent source. Leafy vegetables such as amaranth leaves, curry leaves, drumstick leaves, spinach, coriander leaves and cabbage are good sources as well.
Indeed, Vitamin A plays a significant role in immune function, vision, reproduction and cellular communication. The recommended daily allowance (RDA) varies with age and sex. However, excessive intake (up to ten-twenty times the RDA) might lead to Vitamin A toxicity. Hypervitaminosis A characterized by general body weakness, headache, and muscle stiffness. Fortunately, the symptoms usually disappear within a week after stopping the excessive intake.