Vitamin A is a generic term that covers a group of liposoluble antioxidants capable and necessary for many functions in the body. There are two forms of vitamin A, a functional vitamin A known as retinol and provitamin A or carotenoids which are more prevalent in plants. Although carotenoids are a completely different group of compounds, in certain situations (as in deficiency) they can be converted into an active vitamin A, hence the name provitamin.
These antioxidants are absorbed in the upper small parts of the intestine in the presence of fat (liposoluble vitamins), and supplies are stored in the liver in the form of esters. The reserves are sufficient for one to two years in most adults.
The retinal, i.e. the oxidized metabolite of retinol is required for the process of visualization. The receptor cells in the retina of the eye, rods and cones, contain photosensitive pigment or photoreceptive protein rhodopsin whose composition includes the retinal. In case of vitamin A deficiency, i.e. retinal deficiency, rhodopsin can't be produced and subsequently the state of blindness occurs.
Vitamin A is also important for maintaining the moisture of the conjunctiva which protects the eye from infections. Therefore, the vitamin A eye drops have been proven to be an effective treatment for specific eye inflammations.
In combination with other antioxidants, like vitamins C and E, vitamin A plays an important role in the prevention of visual loss caused by old age. According AREDS (Age-Related Eye Disease Study) study, patients who received the recommended daily dose of the aforementioned antioxidants had a 25% less chance of old-age loss of vision.
Daily intake and food sources
And as in all other cases, the best intake of this vitamin is through regular diet. Vitamin A is an integral part of liver, red peppers, carrots, lettuce, pumpkin and dried apricots.
The daily intake of this vitamin is 5,000 IU (international units), and the best animal source of this vitamin is the liver that contains 100g on average, even 75,300 IU which is an outstanding amount. Vegetarians can turn to plant foods rich in beta-carotene to satisfy the need for vitamin A.