Vitamin A deficiency
Vitamin A deficiency or hypovitaminosis A is a lack of vitamin A in blood and tissues. It is common in poorer countries, especially among children and women of reproductive age, but is rarely seen in more developed countries. Nyctalopia is one of the first signs of VAD. Xerophthalmia, keratomalacia, and complete blindness can also occur since vitamin A has a major role in phototransduction. The three forms of vitamin A include retinols, beta-carotenes, and provitamin A carotenoids.
Vitamin A deficiency is the leading cause of preventable childhood blindness, and is critical to achieving Millennium Development Goal 4 to reduce child mortality. About 250,000 to 500,000 malnourished children in the developing world go blind each year from a deficiency of vitamin A, around half of whom die within a year of becoming blind. The United Nations Special Session on Children in 2002 set a goal of the elimination of VAD by 2010.
The prevalence of night blindness due to VAD is also high among pregnant women in many developing countries. VAD also contributes to maternal mortality and other poor outcomes in pregnancy and lactation.
VAD also diminishes the ability to fight infections. In countries where children are not immunized, infectious diseases such as measles have higher fatality rates. As elucidated by Alfred Sommer, even mild, subclinical deficiency can also be a problem, as it may increase children's risk of developing respiratory and diarrheal infections, decrease growth rate, slow bone development, and decrease likelihood of survival from serious illness.
VAD is estimated to affect about one-third of children under the age of five around the world. It is estimated to claim the lives of 670,000 children under five annually. Around 250,000–500,000 children in developing countries become blind each year owing to VAD, with the highest prevalence in Southeast Asia and Africa. According to the World Health Organization, VAD is under control in the United States, but in developing countries, VAD is a significant concern. Globally, 65% of all children aged 6 to 59 months received two doses of vitamin A in 2013, fully protecting them against VAD.
Signs and symptoms
The most common cause of blindness in developing countries is Vitamin A deficiency. The WHO estimated in 1995 that 13.8 million children had some degree of visual loss related to VAD.Night blindness and its worsened condition, xerophthalmia, are markers of Vitamin A deficiency, collections of keratin in the conjunctiva, known as Bitot's spots, and ulceration and necrosis of cornea keratomalacia can be seen. Nyctalopia is the earliest ocular sign of VAD. Conjunctival epithelial defects occur around lateral aspect of the limbus in the subclinical stage of VAD. These conjunctival epithelial defects are not visible on a biomicroscope, but they take up black stain and become readily visible after instillation of kajal ; this is called "Imtiaz's sign".
VAD can also lead to impaired immune function, cancer, and birth defects. Vitamin A deficiency is one of several hypovitaminoses implicated in follicular hyperkeratosis.
Night blindness
Night blindness is the difficulty for the eyes to adjust to dim light. Affected individuals are unable to distinguish images in low levels of illumination. People with night blindness have poor vision in the darkness but see normally when adequate light is present.VAD affects vision by inhibiting the production of rhodopsin, the eye pigment responsible for sensing low-light situations. Rhodopsin is found in the retina and is composed of retinal and opsin. Because the body cannot create retinal in sufficient amounts, a diet low in vitamin A leads to a decreased amount of rhodopsin in the eye, as the retinal is inadequate to bind with opsin. Night blindness results.
Night blindness caused by VAD has been associated with the loss of goblet cells in the conjunctiva, a membrane covering the outer surface of the eye. Goblet cells are responsible for secretion of mucus, and their absence results in xerophthalmia, a condition where the eyes fail to produce tears. Dead epithelial and microbial cells accumulate on the conjunctiva and form debris that can lead to infection and possibly blindness.
Decreasing night blindness requires the improvement of vitamin A status in at-risk populations. Supplements and fortification of food have been shown to be effective interventions. Supplement treatment for night blindness includes massive doses of vitamin A in the form of retinyl palmitate to be taken by mouth, which is administered two to four times a year. Intramuscular injections are poorly absorbed and are ineffective in delivering sufficient bioavailable vitamin A. Fortification of food with vitamin A is costly, but can be done in wheat, sugar, and milk. Households may circumvent expensive fortified food by altering dietary habits. Consumption of yellow-orange fruits and vegetables rich in carotenoids, specifically beta-carotene, provides provitamin A precursors that can prevent VAD-related night blindness. However, the conversion of carotene to retinol varies from person to person and bioavailability of carotene in food varies.
Infection
Along with poor diet, infection and disease are common in many developing communities. Infection depletes vitamin A reserves which in turn make the affected individual more susceptible to further infection. Increased incidence of xerophthalmia has been observed after an outbreak of measles, with mortality correlated with severity of eye disease. In longitudinal studies of preschool children, susceptibility to disease increased substantially when severe VAD was present.The reason for the increased infection rate in vitamin A deficient individuals is that T-killer cells require the retinol metabolite retinoic acid to proliferate correctly. Retinoic acid is a ligand for nuclear retinoic acid receptors that bind the promoter regions of specific genes, thus activating transcription and stimulating T cell replication. Vitamin A deficiency will often entail deficient retinol intake, resulting in a reduced number of T-cells and lymphocytes, leading to an inadequate immune response and consequently a greater susceptibility to infections. In the presence of dietary deficiency of vitamin A, VAD and infections reciprocally aggravate each other.
Causes
In addition to dietary problems, other causes of VAD are known. Iron deficiency can affect vitamin A uptake; other causes include fibrosis, pancreatic insufficiency, inflammatory bowel disease, and small-bowel bypass surgery. Protein energy malnutrition is often seen in VAD; suppressed synthesis of retinol binding protein due to protein deficiency leads to reduced retinol uptake. Excess alcohol consumption can deplete vitamin A, and a stressed liver may be more susceptible to vitamin A toxicity. People who consume large amounts of alcohol should seek medical advice before taking vitamin A supplements. In general, people should also seek medical advice before taking vitamin A supplements if they have any condition associated with fat malabsorption such as pancreatitis, cystic fibrosis, tropical sprue, and biliary obstruction. Other causes of vitamin A deficiency are inadequate intake, fat malabsorption, or liver disorders. Deficiency impairs immunity and hematopoiesis and causes rashes and typical ocular effects.Diagnosis
Initial assessment may be made based on clinical signs of VAD. Conjunctival impression cytology can be used to assess the presence of xerophthalmia which is strongly correlated with VAD status. Several methods of assessing bodily vitamin A levels are available, with HPLC the most reliable. Measurement of plasma retinol levels is a common laboratory assay used to diagnose VAD. Other biochemical assessments include measuring plasma retinyl ester levels, plasma and urinary retonioic acid levels, and vitamin A in breast milk.Treatment
Treatment of VAD can be undertaken with both oral vitamin A and injectable forms, generally as vitamin A palmitate.- As an oral form, the supplementation of vitamin A is effective for lowering the risk of morbidity, especially from severe diarrhea, and reducing mortality from measles and all-cause mortality. Vitamin A supplementation of children under five who are at risk of VAD can reduce all‐cause mortality by 23%. Some countries where VAD is a public-health problem address its elimination by including vitamin A supplements available in capsule form with national immunization days for polio eradication or measles. Additionally, the delivery of vitamin A supplements, during integrated child health events such as child health days, have helped ensure high coverage of vitamin A supplementation in a large number of least developed countries. Child health events enable many countries in West and Central Africa to achieve over 80% coverage of vitamin A supplementation. According to UNICEF data, in 2013 worldwide, 65% of children between the ages of 6 and 59 months were fully protected with two high-dose vitamin A supplements. Vitamin A capsules cost about US$0.02. The capsules are easy to handle; they do not need to be stored in a refrigerator or vaccine carrier. When the correct dosage is given, vitamin A is safe and has no negative effect on seroconversion rates for oral polio or measles vaccines. However, because the benefit of vitamin A supplements is transient, children need them regularly every four to six months. Since NIDs provide only one dose per year, NIDs-linked vitamin A distribution must be complemented by other programs to maintain vitamin A in children Maternal high supplementation benefits both mother and breast-fed infant: high-dose vitamin A supplementation of the lactating mother in the first month postpartum can provide the breast-fed infant with an appropriate amount of vitamin A through breast milk. However, high-dose supplementation of pregnant women should be avoided because it can cause miscarriage and birth defects.
- Food fortification is also useful for improving VAD. A variety of oily and dry forms of the retinol esters, retinyl acetates, and retinyl palmitate are available for food fortification of vitamin A. Margarine and oil are the ideal food vehicles for vitamin A fortification. They protect vitamin A from oxidation during storage and prompt absorption of vitamin A. Beta-carotene and retinyl acetate or retinyl palmitate are used as a form of vitamin A for vitamin A fortification of fat-based foods. Fortification of sugar with retinyl palmitate as a form of vitamin A has been used extensively throughout Central America. Cereal flours, milk powder, and liquid milk are also used as food vehicles for vitamin A fortification. Genetic engineering is another method that could be used to fortify food, and golden rice is a genetic engineering project designed to fortify rice with beta-carotene and thereby prevent and/or treat VAD. Although opposition to genetically modified foods resulted in the destruction of a field trial of golden rice prototypes , development of golden rice has proceeded and developers are currently to publicly release golden rice in the Philippines.
- Dietary diversification can also control VAD. Nonanimal sources of vitamin A like fruits and vegetables contain preformed vitamin A and account for greater than 80% of intake for most individuals in the developing world. The increase in consumption of vitamin A-rich foods of animal origin has beneficial effects on VAD.
Researchers at the U. S. Agricultural Research Service have been able to identify genetic sequences in corn that are associated with higher levels of beta-carotene, the precursor to vitamin A. They found that breeders can cross certain variations of corn to produce a crop with an 18-fold increase in beta-carotene. Such advancements in nutritional plant breeding could one day aid in the illnesses related to VAD in developing countries.