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.
Osteomalacia is a condition in which the bones are softened due to vitamin D deficiency. This condition happens due to improper mineralizations of the bone matrix. Disorders of vitamin D metabolism and lack of its absorption plays a significant role in resulting osteomalacia.
Clinical manifestations of osteomalacia include skeletal deformities which are identified post fractures. Bone density is reduced to a larger extent which is associated with the loss of trabeculae and thinning of cortices. Significant X- ray determination includes radiolucent bands referring to pseudo fractures perpendicular to the femur, pelvis and scapula. Muscle weakness and achy bones are also associated with osteomalacia. The level of serum calcium and phosphorus is altered depending upon the cause of the disease. Osteomalacia in turn causes secondary hyperparathyroidism.
The symptoms of osteomalacia include loss of muscle density and strength, sharp pain or achy bones , gait and decreased physical activity. These symptoms are often noticed in the regions of hip, arms, legs, lower back and also ribs. Although osteomalacia is associated with vitamin D absorption disorder in the body, there are other conditions such as gastrectomy, celiac disorders which play a major role in the disease progression. The predominant reason for osteomalacia among patients who have undergone gastrectomy or having autoimmune disorders of the small intestine is due to the impairment of vitamin D absorption in these parts of the body. Other than these causes, osteomalacia can also be associated with drugs such as phenobarbital.
Since osteomalacia is associated with vitamin D deficiency, the treatment includes oral administration of ergocalciferol 50,000 international units for a period of eight weeks. This treatment is followed by continuous intake of 800 IU of vitamin D associated with maintenance therapy. Osteomalacia caused due to malabsorption requires higher doses of ergocalciferol ranging from 100,000 IU biannually. The intake of vitamin D should be supported by calcium supplementation. Subsequently in order to monitor the progress of prophylaxis, the serum and urinary calcium levels are monitored. The normal value for the 24 hour urinary calcium is 100-250mg/24hr.
Known popularly as the sunshine vitamin, Vitamin D is the only vitamin that the body is capable of producing on its own on being exposed to sunshine. Vitamin D, also called cholecalciferol is produced when the body is exposed to sunlight before it can produce this vitamin. Vitamin D is fat soluble and is stored in the body's fatty tissue. Vitamin D helps build strong bones as it helps the body use calcium from the diet. Vitamin D works in tandem with other nutrients and hormones in the body and supports healthy bone renewal. Recent studies have concluded that Vitamin D protects against a host of health ailments.
These include diabetes type 2, hypertension or high blood pressure, glucose intolerance and multiple sclerosis. Adequate levels of Vitamin D are needed for reducing the risk of hypertension. Those who do not have adequate exposure to sunlight might suffer Vitamin D deficiency which leads to osteoporosis in adults or rickets in children. Excessive levels of Vitamin D can lead to higher calcium levels that can lead to vomiting and kidney stones. Egg yolks, fish oils and fortified foods like milk contain Vitamin D. The role of Vitamin D in overall good health is often ignored. It is crucial in the immune system regulation as well as maintaining brain function well into later years. Vitamin D might have a bearing on your risk to cancer. In a study carried out by Cancer Treatment Center of America, it was noticed that cancer patients showed Vitamin D deficiency. This could result because the active form of Vitamin D is instrumental in inhibiting proliferation and plays a role in cellular differentiation.
Test for Vitamin D deficiency
The 25-hydroxy Vitamin D test accurately measures the quantity of Vitamin D in your body. This test changes Vitamin D into an active form of the vitamin which is measured through the blood. It is the active form of Vitamin D that helps control calcium and phosphate levels in your body. The normal range is between 30.0 and 74.0 nanograms per milliliter (ng/mL).
Vitamin D helps absorbs calcium. Since calcium and Vitamin D have other health benefits, they both are essential. For healthy bones, Vitamin D and calcium go hand in hand, as Vitamin D must be present for calcium to be absorbed from the digestive tract.
Studies reveal that Vitamin D can regulate stress hormone production and regulate glucocorticoid, and either too high or too less stress hormone is associated with poorer mental health. Vitamin D has significant immune enhancing benefits and therefore it may play a role in improving mental health through its anti inflammatory effects.
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Bibliography / Reference
Collection of Pages - Last revised Date: December 4, 2023