Vitamin A and Vision

Vitamins are a diverse group of organic substances that are essential in the proper functioning of the metabolic processes in the body. Many vitamins, especially those found in the B group, work as coenzymes. Vitamin A is a name given to a group of fat-soluble retinoids, including retinol, retinal, and retinyl esters. Vitamin A is involved in immune function, vision, reproduction and cellular communication.

After numerous chemical modification some retinoids are very potent drugs, used mainly in the treatment of psoriasis and acne. It is important to note that retinoids are drugs related to vitamin A and should not be thought of as vitamins and vice versa.  The scientific name for vitamin A is retinol, a name that gives a clue as to one of its functions. Vitamin A is an alcohol needed for the normal functioning of the retina. Vitamin A also supports cellular growth and differentiation, plays a very important role in the normal formation and maintenance of the heart, lungs, kidneys and other organs (Office of Dietary Supplements – Vitamin A, 2015).

Vitamin A is needed for the formation of visual purple, this is a light-sensitive pigment of the retina.  In the retina, retinol is converted into an aldehyde form. Retinal then combines with proteins called opsins, which results in the formation of four coloured compounds called the visual pigment. The retina is dark due to the presence of these pigments. These pigments change their chemical nature when they are excited by sunlight in a series of complicated chemical reactions.

One of the pigments found in the rods of the retina is purple or rhodopsin. The pigment is sensitive to low-intensity sunlight as is found in semi-darkness. Photochemical changes happen in graded potential when low-intensity light acts on rhodopsin. These changes stimulate the release of neurotransmitters and the message is recorded via the optic nerve in the visual centres of the brain as a picture.  If the pigment rhodopsin is lacking, this series of events do not happen, this leads to a condition called night blindness (NYCTALOPIA) (Rhodopsin – an Overview | ScienceDirect Topics, 2009).

Vitamin A is also needed for the differentiation and growth of the epithelial tissue. Other studies indicate that it is probably needed for the differentiation of all tissue. Vitamin A is mainly involved in the proper functioning of the mucus-secreting cells. In the absence of vitamin A, these cells become keratinised. Since the mucus-secreting cells of the respiratory tract and the genitourinary tract are part of the defence mechanism. Deficiency in vitamin A will lead to an individual being prone to infection (Vitamin A Deficiency – Nutritional Disorders, 2020).

The cornea can also become keratinised which may lead to dry eyes (xerophthalmia), this, in turn, can lead to permanent blindness.  The mechanism of action of retinal and retinoids is at the nuclear level, as such it may affect gene expression. Other signs and symptoms of vitamin A deficiency include dry skin, delayed growth in children, poor wound healing, acne breakouts and (Polcz & Barbul, 2019).

The main source of vitamin A in the diet is in the Liver, dairy products, egg yolk and fatty fish. Retinol is not found in most plant products, but fortunately for vegetarians and vegans, most plants contain a substance called carotenoid. Carotenoids act as provitamins that can be converted into retinol in the intestinal wall and the liver. The main carotenoid in the plant kingdom is beta-carotene, which gets its name from carrots. Any vegetable or fruit that is orange in colour will have this provitamin, Beta-carotene is also found in green vegetables with it colour being hidden or masked by chlorophyll. For example, next to carrots spinach has the highest concentration of Beta-carotene in all the consumed fruit and vegetables.

Vitamin A deficiency is usually caused by prolonged dietary deprivation, it is endemic in Southern and Eastern Asia, where rice, that does not have beta-carotene is in staple food.  Xerophthalmia as a result of a primary deficiency in vitamin A is the main cause of blindness among young children in developing countries.  Secondary deficiency in vitamin A may be due to reduced bioavailability of provitamin A carotenoids and interference in the absorption, storage or transport of vitamin A.

People at risk of vitamin A deficiency include; pregnant mums, people with cystic fibrosis, and premature babies. In developed countries the amount of vitamin A in breast milk is enough to meet the baby’s needs for six months. But in women with vitamin A deficiency, breast milk volume and vitamin content are minimal and not enough to meet the needs of an infant who is solely relying on breast milk (Polcz & Barbul, 2019) .

Sources 

Amengual, J., Zhou, F., Barrett, T. J., von Lintig, J., & Fisher, E. A. (2017). Role of beta-carotene conversion to vitamin a in atherosclerosis. Arteriosclerosis, Thrombosis, and Vascular Biology37(suppl_1), A168-A168.

Cabezas-Wallscheid, N., Buettner, F., Sommerkamp, P., Klimmeck, D., Ladel, L., Thalheimer, F. B., … & Przybylla, A. (2017). Vitamin A-retinoic acid signaling regulates hematopoietic stem cell dormancy. Cell169(5), 807-823.

Gundra, U. M., Girgis, N. M., Gonzalez, M. A., San Tang, M., Van Der Zande, H. J., Lin, J. D., … & Fisher, E. A. (2017). Vitamin A mediates conversion of monocyte-derived macrophages into tissue-resident macrophages during alternative activation. Nature immunology18(6), 642.

Office of Dietary Supplements—Vitamin A. (n.d.). Retrieved May 18, 2020, from https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional

Polcz, M. E., & Barbul, A. (2019). The Role of Vitamin A in Wound Healing. Nutrition in Clinical Practice, 34(5), 695–700. https://doi.org/10.1002/ncp.10376

Rhodopsin—An overview | ScienceDirect Topics. (n.d.). Retrieved May 18, 2020, from https://www.sciencedirect.com/topics/neuroscience/rhodopsin

Vitamin A Deficiency—Nutritional Disorders. (n.d.). MSD Manual Professional Edition. Retrieved May 18, 2020, from https://www.msdmanuals.com/en-au/professional/nutritional-disorders/vitamin-deficiency,-dependency,-and-toxicity/vitamin-a-deficiency

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