Vitamin A is a fat soluble vitamin which is derived from beta carotene which plays a significant role in the process of vision and other important metabolic pathways pertaining to cell division and genetic expression. The significant forms of vitamin A include retinol, retinal, retinoic acid and retinyl esters. There are approximately six hundred derivatives of beta carotenes and the most important form is retinol.
Functions of Vitamin A
Vitamin A is essential for many metabolic pathways in the body. It is the chief requirement for the function of the rhodopsin protein located in the retina to absorb light and to differentiate functions of the cornea and the conjunctival membranes. Vitamin A is essential for normal functioning of the retina. Apart from this, vitamin A plays a significant role in immune system functions, cell signaling and cellular communication and reproduction. The functions and pathways associated with vitamin A are directly related to the functionality of vital organs such as heart, brain, lungs, liver and kidneys. Hence vitamin A is also known as an important antioxidant. Besides it is required for the growth and differentiation of epithelial tissue, normal growth of bone and embryonic development. Most of our body's Vitamin A is stored in the liver in the form of retinyl esters.
Vitamin A Deficiency: Poor adaptation of vision to darkness or what is known as night blindness is an early symptom that may be followed by degenerative changes in the retina. Degenerative changes in eyes and skin are commonly observed in vitamin A deficiency. The predominant form of vitamin A deficiency is Nyctalopia or night blindness. This occurs as result of retinol imbalance which is the chief derivative of vitamin A. In third-world, vitamin A deficiency is the primary cause of blindness. Pregnant and lactating women, premature children, children living in rural areas of developing countries and patients who have a history of liver diseases such cirrhosis and cystic fibrosis are most susceptible to Vitamin A deficiency. Severe or prolonged deficiency may lead to dry eye or Xerophthalmia (dryness in conjunctiva and cornea of the eye) that can result in corneal ulcers, inflammation, ridge formation, scarring and eventually blindness. Xerophthalmia is due to lacrimal gland dysfunction. Other associated conditions include keratomalacia and follicular hyperkeratosis. Another important consequence of Vitamin A deficiency is acquired immunodeficiency disease, with an increased incidence of death related to infectious diseases. Vitamin A deficiency is associated with increased disease progression and mortality in HIV patients.
World Health Organization (WHO) Recommendations for Vitamin A:
Supplementation may be required in cases where the blood Vitamin A level falls below 20 µg/dL.
Severe deficiency is < 10 µg/dL
Food sources and recommended dietary allowance
Vitamin A is naturally available in dairy products such as milk, cheese, curd, cream. Meat products like liver and fish oil and leafy vegetables are excellent sources of vitamin A. Other sources include pumpkin, potatoes, broccoli, cereals, beans and cow peas. Studies indicate that the intensity of the fruit or vegetable color is directly proportional to the amount of vitamin A present in it. The recommended intake of vitamin A per day for children 500 micrograms, males 1000 micrograms and females 800 micrograms respectively.
RDAs (recommended dietary allowance) for vitamin A are given as mcg of retinol activity equivalents (RAE) to account for the different bioactivities of retinol and provitamin A carotenoids. FDA may introduce new labeling regulations in the near future which may result in listing Vitamin A with RAE values rather than in IU.
The following table shows conversion rates of mcg of RAE (retinol activity equivalents):
Essentially all dietary sources of vitamin A are converted into retinol by the body: 1 mcg of physiologically available retinol is equivalent to the following amounts from dietary sources:
Hypervitaminosis A: Vitamin A in excess can be toxic. According to WHO, values in excess of 120 µg/dL is Hypervitaminosis A. Chronic vitamin A over dosage may be a serious issue in normal adults who take more than 15 mg per day and in children who take more than 6 mg per day of vitamin A over a period of several months. Symptoms can include :
Other than that some symptoms such as pain, vomiting, alopecia and bone demineralization may result due to excessive intake of Vitamin A. In pregnant women, an over dose of Vitamin A over a period of time may result in spontaneous abortions or Congenital malformations, craniofacial abnormalities and valvular heart disease in the baby.
However, unlike preformed Vitamin A, beta-carotene is not known to be teratogenic (reproductive toxicity). Even a relatively large supplemental doses of beta carotene or eating carotenoid rich food for long duration need not result in toxicity always. Rarely a reversible condition known as carotenodermia - where the skin turns yellow/orange might be the result of long term over dosage of beta carotene.
In Xerophthalmia or dry eye syndrome, the eye doesn't produce enough tears or the tears have a chemical composition that causes them to evaporate quickly. Dry eye syndrome is common fallout of increasing age since the body produces less oil over time. The oil deficiency has a bearing on the tear film since the water layer over the eye dries faster. This condition is more pronounced in women than men. Women suffering from dry skin or menopausal symptoms may notice dry eyes too.Causes for dry eyes
A condition of dry eyes is determined by a physician who will measure the production, evaporation rate and quality of tear film. A thin strip of filter paper placed at the edge of the eye, called a Schirmer test, is one way of measuring this. An ophthalmologist can look at the film of tears on your eye using a slit lamp (bio-microscope). Treatment for dry eyes involves trying to preserve as much of the eye's natural moisture as possible. Eye drops act as artificial tears and provide temporary relief. Anti-inflammatory agents are prescribed for more severe cases of dry eyes.
While topical steroids can be used for inflammation, they can cause side effects after prolonged use. Some forms of dry eye syndrome benefit from the placement of tiny plugs in the ducts that drain tears from the eye. These special plugs trap the tears on the eye, keeping it moist. This may be done on a temporary basis with a dissolvable collagen plug or permanently with a silicone plug. For severe cases, special goggles called moisture-chamber spectacles can be worn.
Sjogren syndrome is an autoimmune disorder causing conditions such as xerostomia and Xerophthalmia. It was first discovered by Henrick Sjogren. The symptoms associated with this disorder are often chronic and occur due to damage or impairment of the exocrine glands caused due to the lymphocytic infiltrates and hypersensitivity reactions. The onset of Sjogren syndrome predominantly occurs in the region of salivary glands and lacrimal glands. The primary form of Sjogren syndrome is generally associated with external glandular symptoms such as dry eyes and dry mouth without any connective tissue damage. The secondary form of the Sjogren syndrome usually occurs because of preexisting autoimmune disorders.
Clinical manifestations of Sjogren syndrome
Sjogren syndrome usually occurs in the older age group; however exceptions have been reported on it affecting younger persons. The factors causing Sjogren syndrome are predominantly genetical or environmental. Neuro-endocrine impairment plays a significant role in the onset of the Sjogren syndrome. HLA-DR genotype is the predominant factor in the genetic propagation of the disease.
Sjogren syndrome is associated with a cascade of symptoms related to many organ systems and their functions. The affected regions are ocular, oral, ontological, laryngeal, vascular, neurologic and endocrine systems respectively. The effect of the Sjogren syndrome is mainly on the head and neck region and hence the sense organs are affected to a larger extent. Other clinical conditions include loss of hearing, arthralgia, myalgia and also frequent epistaxis. Sjogren syndrome if not treated, can lead to serious conditions such as lymphoma and renal failure.
Diagnosis and treatment of Sjogren syndrome
Sjogren syndrome is diagnosed by using radiological, pathological and serological analysis. Schirmer's test is used in the diagnosis of Xerophthalmia. It is used to estimate the tear secretion level of patients suffering with dry eye conditions. The decreased levels or absence of tear secretion indicates probability of Sjogren syndrome. Salivary biopsy is advised for patients suffering xerostomia. The pathological analysis of the salivary glands also reveals underlying infiltration and damage caused to the exocrine glands. Serological analysis of the Sjogren syndrome are generally associated with low WBC counts, increased erythrocyte sedimentation rates, elevated levels of protein and hyper-gammaglobulinemia of IgM. Careful clinical study of the diagnostic parameters along with symptoms is necessary to rule out false positive results.
The treatment of Sjogren syndrome is generally a slow process because it is an autoimmune disorder. Immunosuppressive drugs and salivary substitutions are some of the methods administered to manage Sjogren syndrome. Pilocarpine an FDA approved tear stimulating drug is widely used for the treatment of dry eyes.
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Bibliography / Reference
Collection of Pages - Last revised Date: November 22, 2019