Vitamin A refers to a group of unsaturated nutritional organic compounds that includes retinol, retinal, retinoic acid, and several provitamin A carotenoids.
Normally, our liver stores 80 to 90% of the body’s vitamin A. Upton indication of body’s requirement, the liver releases the vitamin into the circulation bound to pre-albumin (transthyretin) and retinol-binding protein. There are two primary forms of Vitamin A depending on the source:
Vitamin A functions in the body
Retinoids are found in animal foods. The retinoid forms are important with respect to pregnancy and childbirth, infancy, childhood growth, night vision, red blood cell production, and resistance to infectious disease. They are present in animal sources such as fish, chicken, eggs etc.
Carotenoids are obtained from plant sources. Most carotenoid forms of the vitamin function as antioxidant and anti-inflammatory nutrients. Carotenoids are available in plant sources such as kale, spinach, broccoli etc. They are better absorbed when cooked with oil.
- Vitamin A is important for growth and development, for the maintenance of the immune system and good vision.
- It is needed by the retina of the eye in the form of retinal, which is necessary for both low-light and color vision.
- It also functions as retinoic acid which is an important hormone for epithelial and other cells.
Health Benefits of Vitamin A
Supports good vision
- The human retina contains four kinds of photo-pigments that store vitamin A compounds. The pigment rhodopsin, located in the rod cells of the retina, allows the rod cells to detect small amounts of light, and helps adapting the eye to low-light conditions and night vision.
- Retinal, the aldehyde form of the vitamin, participates in the synthesis of rhodopsin, and in the series of chemical reactions that causes visual excitation, which is triggered by light striking the rod cells.
- The remaining three pigments, collectively known as iodopsins, are found in the cone cells of the retina and are responsible for day vision.
Supports Immune System
Our digestive tract gets exposed on a daily basis to potentially unwanted substances (like pesticide residues in food), as well as unwanted micro-organisms (like certain kinds of bacteria). Our immune and inflammatory systems are designed to help prevent us from being harmed by these events.
It plays a key role in support of immune and inflammatory functions.
Cell Growth Support
- It is required for normal cell growth and development.
- The retinoic acid is necessary for the synthesis of many glycoproteins, which control cellular adhesion i.e. the ability of cells to attach to one another, cell growth, and cell differentiation.
- For example, the production of red blood cells in our bone marrow is a process that is known to require retinoid acid, which is a compound of the Vitamin.
Recommended Dose of Vitamin A
- For infants and children, Adequate Intake (AI) levels are:
- Birth to 6 months – 400 mcg/day (1300 units)
- 7 to 12 months – 500 mcg/day (1700 units)
- Eating 5 servings of fruits and vegetables per day provides about 50% to 65% of the adult RDA.
Foods Sources to fulfil Vitamin Needs
Carrots – When most people think of Vitamin A and eye health, they think of carrots! It is true that eating plenty of carrots improves your vision. One medium carrot accounts for over 200% of the average person’s needs for the day. They’re also a great source of Vitamin C, Vitamin K, and B Vitamin, plus magnesium and fiber.
Cod Liver Oil – Cod liver oil supplements, are a strong source of vitamins and minerals. Cod liver oil, which comes in both liquid and capsule form, contains Vitamin D, A, and omega 3 fatty acids. A tablespoon of cod liver oil will allow you to meet and exceed the daily recommended intake on daily basis.
Whole Milk –Whole milk over skim milk is preferred for its many nutrients and rich taste. A cup of whole milk is high in Vitamins D and A, and it’s a good source of calcium, protein, and magnesium. It’s also high in fat, though, so enjoy it in moderation or switch to skim milk if fat and calories become a concern.
Kale – Kale is so much more than a common garnish! It is a delicious and nutrient-rich vegetable that deserves a solid place in your diet. Kale can do wonders for your health, including helping you meet and exceed the amount of your RDA. A one-cup serving contains about 200% of what the average person needs.
Vitamin A Deficiency – VAD
- In pregnant women deficiency VAD
,especially in the last trimester may cause night blindness and may increase the risk of maternal mortality.
- Also, deficiency VAD is the leading cause of preventable blindness in children and increases the risk of disease and death from severe infections.
- Deficiency VAD is estimated to affect approximately one-third of children under the age of five around the world.
Causes of Vitamin A Deficiency
Deficiency VAD can occur as either a primary or a secondary deficiency.
A primary deficiency occurs among children and adults who do not consume an adequate intake of provitamin A carotenoids from fruits and vegetables or preformed from animal and dairy products. Early weaning from breast milk can also increase the risk of deficiency. Persistent deficiency gives rise to xerophthalmia.
Secondary deficiency VAD may be due to decreased bioavailability of provitamin A carotenoids or interference with absorption, storage, or transport of the vitamin.
- Impaired dark adaptation of the eyes, which can lead to night blindness, is an early symptom of the deficiency.
- Also, xerophathalmia occurs which involves drying and thickening of the conjunctivae and corneas.
- In advanced deficiency, the cornea becomes hazy and can develop erosions, which can lead to its destruction (keratomalacia).
- Other changes include impaired immunity (increased risk of ear infections, urinary tract infections, Meningococcal disease), hyperkeratosis (white lumps at hair follicles), keratosis pilaris and squamous metaplasia of the epithelium lining the upper respiratory passages and urinary bladder to a keratinized epithelium.
- Since breast milk is a natural source of vitamin A, promoting breastfeeding is the best way to protect babies from deficiency VAD.
- For deficient children, high-dose of vitamin produces remarkable results, reducing mortality by 23% overall and by up to 50% for acute measles sufferers.
- However, breastfeeding is time-limited and the effect of supplementation capsules lasts only 4-6 months, they are only initial steps towards ensuring better overall nutrition and not long-term solutions.
Vitamin A Toxicity
Acute vitamin A toxicity in children may result from taking large doses (> 300,000 IU), usually accidentally. In adults, acute toxicity has occurred when arctic explorers ingested polar bear or seal livers, which contain several million units of vitamin A.
Chronic vitamin A toxicity Adults who consume >4500 IU per day of vitamin A may develop osteoporosis. Infants who are given excessive doses (18,000 to 60,000 IU per day) of water-miscible vitamin A may develop toxicity within a few weeks.
Birth defects occur in children of women receiving isotretinoin (which is related to vitamin A) for acne treatment during pregnancy.
Although carotene is converted to vitamin A in the body, excessive ingestion of carotene causes carotenemia, not vitamin A toxicity. Carotenemia is usually asymptomatic but may lead to carotenosis, in which the skin becomes yellow.
When taken as a supplement, beta-carotene has been associated with increased cancer risk; risk does not seem to increase when carotenoids are consumed in fruits and vegetables.
Symptoms and Signs of Toxicity
- Although symptoms of vitamin A toxicity may vary, headache and rash usually develop during acute or chronic toxicity.
- Acute toxicity causes increased intracranial pressure. Drowsiness, irritability, abdominal pain, nausea, and vomiting are common. Sometimes the skin subsequently peels.
- Early symptoms of chronic toxicity are sparsely distributed, coarse hair; alopecia of the eyebrows; dry, rough skin; dry eyes; and cracked lips.
- Later, severe headache, pseudotumor cerebri, and generalized weakness develop.
- Fractures may occur easily, especially in the elderly. In children, toxicity can cause pruritus, anorexia, and failure to thrive. Hepatomegaly and splenomegaly may occur.
Complete recovery usually occurs if vitamin A ingestion stops. Symptoms and signs of chronic toxicity usually disappear within 1 to 4 wk. However, birth defects in the fetus of a mother who has taken megadoses of vitamin A are not reversible.