Vitamin A is important for continuous eye health. It facilitates better vision by influencing the synthesis of the pigmented rhodopsin found in the retina. It promotes good night vision or helps one see better in dim light. In addition, it ensures the cornea is kept moist by maintaining the integrity of related membranes.
It should be understood that vitamin A is not a single unit. Rather, it is made of several non-saturated hydrocarbons. These hydrocarbons include retinoic acid, retinol, retinal and other precursors known as carotenoids. Beta carotene is the most important of these carotenes. The retinal hydrocarbon reacts with a protein known as opsin to form rhodopsin. The resultant molecule helps in the achievement of scotopic (low light) and colour vision.
A deficiency in vitamin A may lead to various eye problems ranging from mild, moderate to severe manifestation depending on the chronicity of the deficiency. Lack of this crucial vitamin can either be primary or secondary. A primary deficiency is due inadequate intake of either provitamin A carotenoids (vegetables and fruits) or the preformed vitamin from dairy and animal products. This is the commoner form.
Secondary deficiency may be seen in a number of circumstances. For instance, it is seen when there is malabsorption of the vitamin. This frequently occurs when there is decreased uptake of lipids and reduced release of bile. The vitamin requires fat to dissolve and a deficiency of the fats will therefore result in decreased absorption. Other risk factors for the same include prolonged smoking and alcohol intake.
Night blindness is one of the commonest and earliest symptoms of deficiency. One of the causes of reduced vision is drying of the conjunctiva which takes place as the condition progresses. The normal secretory system is progressively replaced with hardened keratinised tissue. As keratin plaques are eroded progressively, more keratinisation takes place.
Vitamins C and B6 play a minor role but are still very important. Vitamin C is an antioxidant that protects the eye against the effects of irradiation. It also maintains capillary and collagen integrity and normal pressure. B6 vitamin ensures that the red cell structure is intact and this in turn contributes to proper functioning of the retina. The mineral zinc is involved in the conversion of beta carotene to active vitamin A.
While it may not be practical to take all vision vitamins every day, it is advisable to take food supplements that contain the maximum number of essential vitamins in them. The amount of intake is also limited by factors such as age, sex, pregnancy and lactation. It is important to ensure that one takes just the right amount of nutrients to avoid toxicity and related complications.