Vitamins are essential organic nutrients required in minute amounts for the body's optimal functioning. They could be water soluble or fat soluble. Water soluble vitamins include; vitamins Band C, while the fat soluble vitamins are; Vitamins A, D,E,K. Vitamin A was the first fat soluble Vitamin to be discovered in 1913.
Vitamin A's structure is depicted to the right. Retinol is the precursor of of retinal and retinoic acid. Both are active metabolites of retinol. Retinal plays an important role in vision while retinoic acid is the intracellular messenger which affects the transcription of some gene( Bates C.J,1995)
Beta- carotene is a Carotenoid capable of being converted to Vitamin A in some body tissues and the intestine. Beta-Carotene is contained in many plants as Vitamin A does not occur in plants. (Bates C.J,1995).
PROPERTIES OF VITAMIN A
Retinol is a light yellow crystalline compound which is fat soluble and highly heat resistant. It can be stored in fatty tissue like the Kidney and Liver for long periods of time. Also, it is not affected by weak acids and bases.
FUNCTIONS OF VITAMIN A
Vitamin A and its metabolites play a lot of role in the human body. This is evidenced by the number of disorders that accompany its deficiency and sometimes its excess. The following are some of it’s the functions:
VISION: Retinal is an important structural component of a pigment in the eye. Rhodopsin is found within the rod and cone cells in the Retina. It is known as visual purple, the light sensitive segment in the retina.
RESISTANCE TO INFECTIOUS DISEASES: Vitamin A deficiency has been observed in almost all infectious diseases studied. The reason for this is not yet clear, but it could be Vitamin A's activity in immune responses. Also, Vitamin A serves as an anti-oxidant in the body, mopping up several free radicals that could be toxic and injurious to the body.
EPITHELIAL CELL INTEGRITY: Vitamin A is required for proper maintenance and differentiation of epithelial cells. Lack of it results in scaly and keratinized skin with reduced mucous secretion.
REPRODUCTION: Vitamin A is required for normal reproductive cycles and development in men and women. It is essential for sperm production and normal female cycles.
FOOD SOURCES OF VITAMIN A
Retinol or Preformed Vitamin A is found in food such as: Egg yolk, Liver, whole milk, cheese, butter and oily fish such as, Sardines, Tuna, Herring and Pilchards.
Other sources of Beta Carotene are: Apricots, Pumpkins, Mango, Sweet potatoes, Melon, Carrots, Spinach, broccoli, Guava, Red pepper and papaya.
REQUIREMENTS
Consumption of a wide array of fresh fruits and vegetables is the best way to prevent Vitamin A deficiency. The Institute of Medicine's Food and Nutrition Board recommends 400 micrograms per day for infants under 6 months and 500 micrograms for 7 months to a year old. A child aged 1-3 is 300 microgram, 4-8 years is 400 microgram while 9-13 years is 600 microgram daily. For adult men, 900 microgram is recommended while 700 microgram is recommended daily for adult females. Vitamin A deficiency causes skin rashes and impaired body immunity and also other eye problems like night blindness and dry eyes.
HYPOVITAMINOSIS A
Its deficiency usually emanates from malnutrition. Fat deficient diets could be a reason for this. Also, poor intestinal absorption of Vitamin A, fats and Carotenoids could predispose to its deficiency. Disease states like chronic diarrhea, cystic fibrosis, pancreatic enzyme deficiency, surgical removal of the stomach and liver diseases have been implicated. Also, chemicals like dioxin enhance the breakdown of Vitamin A in the liver thereby preventing its absorption from the intestine. Additionally, utilization and transport of Vitamin A depends upon several binding proteins. In essence, a low protein intake invariably results in Vitamin A deficiency. Though, Zinc aids utilization and absorption of Vitamin A.
Symptoms of the deficiency do not appear immediately when enough Vitamin A is not consumed. This is as a result of the liver being a massive store for the Vitamin. Symptoms may take several weeks to months before appearing. Night blindness, hyperkeratosis, scaly skin, hair loss, increased incidence of infectious disease, poor bone growth, epithelial cell damage and epithelial cell dysfunction are all effects of hypovitaminosis A.
HYPERVITAMINOSIS A
Though hypervitaminosis A does not occur frequently as Hypovitaminosis A, there is little evidence to support the notion that carotenoids could be toxic at high levels in the body. It is however associated with long bones in infants and premature epipyseal closing. Also, there could be kidney damage due to high calcium, liver damage, hair loss and headaches.
NIGHT BLINDNESS
Night blindness is also known as Nyctalopia/Nyctanopia. It is a medical condition that affects an individual’s vision at night or in any area with no light. Apart from having problems seeing at night, such a person also finds it difficult seeing especially when moving from a brightly lit place to a dimly lit place. Consequently, individuals afflicted with night blindness find it difficult driving in the evening and night. It occurs a result of a problem in the Retina. The Retina is made up of Cones and Rods. The rod helps the eye see clearly in areas that have poor/bad lighting. Night blindness occurs as a symptom of many inherited and congenital retinal diseases. Also, it results from Vitamin A deficiency.
Congenital night blindness that is associated with or without nearsightedness is either a dominant, sex-linked hereditary trait or a recessive trait and is present throughout life. Night blindness that develops during adolescence or childhood may be a sign of Retinitis Pigmentosa. This is an hereditary disorder where continued worsening of vision because rod cells are destroyed leads to severe impairment of vision. Deficiency of Vitamin A causes a reduced rhodopsin photosensitivity. This also results in night blindness, but not the severe type. Vision improves tremendously when adequate amounts of Vitamin A is administered.
CAUSES OF NIGHT BLINDNESS
RETINITIS PIGMENTOSA
Generally, this is the commonest cause of night blindness. It occurs when the rods and cone cells in the Retina loose part or all of their ability to respond to light. This disorder is genetically linked, therefore a cure for it is almost nonexistent.
RETINOL DEFICIENCY
This is the biggest cause of night blindness after Retinitis Pigmentosa. It occurs due to inadequate consumption of Vitamin A or its precursors. It can be corrected by increasing the ingestion of Vitamin A.
CATARACTS
Cataracts occur when cloudy areas are formed around one’s eyes as one ages. Usually, it is seen more in individuals over 50 years. The person’s vision progressively deteriorates, eventually causing night blindness.
MYOPIA
Also known as Nearsightedness is inability to focus. Objects that are far off appear blurred. Myopic individuals find it difficult seeing at night.
SIDE EFECTS OF MEDICATION
Certain medications that inhibit the absorption of Vitamin A can cause night blindness. Example is dioxin.
BIRTH DEFECTS
Some newborn are born with congenital diseases of the retina. Such will lead to night blindness if not corrected.
THE ROLE OF VITAMIN A IN VISION
The retina has four types of photo pigments where the Vitamin A is stored. One of them is Rhodopsin. It is an important structural and sensitive pigment which allows detection of light in small amounts within the cones and rod cells. It plays an essential role in situations where low light or no light prevail. The other three pigments are called iodopsins which are found in the cone cells of the Retina. They are responsible for daytime vision.
SYMPTOMS OF NIGHT BLINDNESS
The symptoms of night blindness include Bitots Spots, Keratomalacia, Corneal ulceration and corneal scar.
Bitots spots are cheesy, foamy materials that accumulate in the eye
TREATMENT
Treatment is divided into the treatable and untreatable types.
The treatable types consists of the one caused by cataracts, myopia and Vitamin A deficiency.
Extracapsular or intracapsular surgical excision of the cataract is carried out when the night blindness is caused by cataracts. Also, corrective lenses are used for myopic patients, while Vitamin A supplements and increased ingestion of foods rich in Vitamin A is increased. In addition, diseases such as chronic diarrhea, chrohns disease and cystic fibrosis are also taken care of.
Night blindness caused by Retinitis Pigmentosa and other birth defects affecting the retina are not treatable. Though sunglasses and Vitamin A supplements are used as palliatives.
In conclusion, Night blindness is caused by both hereditary and acquired causes. The hereditary causes which include Retinitis Pigmentosa is untreatable while the acquired one caused majorly by Vitamin A deficiency can be effectively managed with the ingestion and administration of Vitamin A and its precursors. Ingestion of fresh fruits and vegetables would go a long way in preventing night blindness caused by hypovitaminosis A.
References
(2012) http://www.chm.bris.ac.uk/webprojects2002/schnepp/vitamina.html
C.J Bates,(1995) Vitamin A. Lancet 345:31,
http://www.vivo.colostate.edu/hbooks/pathphys/misc_topics/vitamina.html
(2012) http://www.livestrong.com/article/359938-what-vitamin-deficiency-can-cause-night-blindness/
C. Jacoby (2012) Causes of Night Blindness, Health Guidance forbetter Health, http://www.healthguidance.org/entry/14216/1/Causes-of-Night-Blindness.html
(2012) http://www.lvpei.org/resources/eye-faq/vitamina.html
