Vitamin A is an easily overlooked vitamin, yet it may have important functions in the body. This vitamin is fat soluble and comprises retinoids such as retinols, retinals, retinoic acids and retinyl esters.
Vitamin functions to improve vision and is an important component of rhodopsin, a protein which is capable of absorbing light in the retinal receptors. This vitamin contributes to the normal functioning of the conjunctival membranes and the cornea. It also supports normal functioning of other organs in the body such as the heart, kidneys, lungs and others.
There are two forms of Vitamin A in our diet: preformed Vitamin A and provitamin A carotenoids. Pre-formed vitamin A is found in foods from animal sources such as fish, meat, dairy products and liver. Provitamin A carotenoids include beta-carotene, alpha-carotene and beta-cryptoxanthin. The body is capable of transforming these carotenoids into Vitamin A.
The two forms of Vitamin A are metabolized inside cells to retinal and retinoic acids which are active forms of Vitamin A so that its biological functions are easily carried out. Other forms of carotenoids in food are lutein, lycopene and zeaxanthin; however they are not converted into vitamin A. The different forms of vitamin A are solubilised into micelles in the lining of the intestines and are absorbed by the cells of the duodenum so that these carotenoids and esters are converted into retinol which are oxidized to retinoic acid. Vitamin A is stored in the body as retinyl esters.
There are many sources of Vitamin A. Most Vitamin A can be derived from liver and fish oils; other good sources include eggs and milk which may also contain provitamin A. Vitamin A can also be obtained from leafy green vegetables, orange and green vegetables, tomato and tomato products, vegetable oils and fruits. Dairy products, fish, liver, fortified cereals, carrots, cantaloupe, broccoli and squash are also good sources of vitamin A. Vitamin A can also be taken in from dietary supplements such as supplements that contain retinyl acetate or retinyl palmitate. Vitamin D deficiency is rare in developed countries but may be common in developing countries who may have limited access to foods that contain preformed vitamin A. Infants may start having Vitamin A deficiency when they do not receive vitamin A from the breast milk or colostrums. In children, diarrhoea may predispose to vitamin A deficiency. In young children and pregnant mothers, lack of vitamin A gives a manifestation known as xeropthlamia or night blindness. Xeropthalmia is a condition wherein a person fails to see in darkness or in low light. Blindness can be prevented by receiving adequate amounts of vitamin A. Other manifestations of Vitamin A deficiency include low iron levels and diarrhoea.
Pregnant women need extra Vitamin A for proper growth and development of fetuses and for their own growth and metabolism. Vitamin A deficiency during pregnancy may predispose to increased maternal and infant deaths and morbidity, increased risk for anemia and slow growth and development of infants.
Vitamin A Deficiency and Asthma
A recent study has shown that there is a link between prenatal vitamin A deficiency and postnatal airway hyperresponsiveness, a hallmark of asthma. In this study done by researchers from the Columbia University Medical Center, mice were studied and the results showed that short-term deficiency of this vitamin while the fetal lungs are forming can cause changes in the smooth muscles surrounding the airways. Later on in adult life, these offspring have lungs that respond to various environmental and pharmacological stimuli with excessive airway narrowing. These are characteristics of asthma. The findings were published in the Journal of Clinical Investigation.
To know more about Vitamin A, you can check out our other articles on this site.