Vitamin A plays a crucial role in maintaining the overall functionality of the human body. It is essential for vision, cell development, and metabolism. Vitamin A is vital in keeping your immune and reproductive systems healthy. Your body can’t make vitamin A by itself, so it has to get it through your diet.
Table of Contents
What is vitamin A?
Vitamin A is an essential fat-soluble vitamin that occurs only in foods of animal origin. Vitamin A activity is also possessed by carotenoids found in plants. Hence, carotenoids are called provitamin A. Your vision depends on Vitamin A. Eyes need to make specific pigments for your retinas to see appropriately. A lack of vitamin A hampers your eyes’ ability to make these pigments, which can lead to night blindness. In other words, you need vitamin A to see at night or in dim lights. Your eyes also need vitamin A to produce moisture and adequately lubricate your corneas. If your corneas get too dry, they can become damaged, leading to blindness. Vitamin A also keeps your skin and the lining of your lungs, intestines, and urinary tract tip-top shape. Additionally, it keeps your immune system healthy and protects you against infections.
Chemistry and properties of vitamin A
Vitamin A, also known as retinol, contains a B-ionone ring. The side chain contains one alcoholic group and 4 double bonds. Due to the alcoholic group, esters are formed with acids, e.g., palmitate, acetate, etc. Because of double bonds in the side chain, oxidation quickly destroys vitamin A. Vitamin A is stable to heat (1000 C) for short periods in the presence of oxygen. Vitamin A is slowly destroyed when its solutions are exposed to light.
Different forms of Vitamin A
Vitamin A is considered a singular nutrient but has a group of compounds, including retinol, retinal, and retinyl esters.
Vitamin A2
Vitamin A2 occurs in the livers of freshwater fish. It differs from vitamin A in having one more double bond in the B-ionone ring. Many cold-blooded vertebrates use A2 for their visual system to obtain a red-shifted sensitive spectrum. Human skin naturally contains vitamin A2. Protein CYP27C1 converts ordinary A1 (all-trans retinoids) to A2 in humans.
Vitamin A aldehyde (Retinal, Retinene)
Vitamin A aldehyde occurs in the rods and cones of the retina of the eyes. Many vertebrates ingest retinal directly from meat or produce retinal from carotenoids such as α-carotene or β-carotene, both of which are carotenes.
Vitamin A acid (Retinoic acid)
It is one of the nutrients the body needs in lesser amounts to properly function in day-to-day life and keep you healthy. Vitamin A acid is made in your body from vitamin A and supports cells in growing and developing, especially in the embryo. In one study, it promoted the growth of rats but did not support reproduction in female rats. Due to vitamin A deficiency, it cannot be converted into retinal and does not prevent or cure eye lesions.
Carotenoids (Provitamin A)
A large number of carotenoids occur in plants. Of these, the following are of nutritional importance: α, β, γ carotenes, and cryptoxanthin.
Absorption and conversion into Vitamin A
The carotenoids present in the vegetables are absorbed 25-50 percent depending on the quantity of fat in the diet and the extent to which the vegetables have been homogenized. The conversion of vitamin A from carotene occurs in the intestinal walls. The relative biological potency of the essential carotenoids is as follows:
β-carotene 100
α-carotene 53
γ-carotene 45
cryptoxanthin 57
According to the Nutrition Expert Group ICMR, four µg carotene is equivalent to 1 µg vitamin A.
Absorption and storage
Vitamin A alcohol and its esters are almost entirely absorbed in normal humans and pass along with fat through the lymphatic system into the bloodstream. In adults, the liver can store large amounts (about 100,000 µg) of vitamin A when fed diets rich in vitamin A.
Functions of Vitamin A
Vitamin A and vision:
Vitamin A is essential in vision in dim light. Vitamin A deficient person cannot see objects in dim light. The characteristics pigments of the rods and cones in the retina of the eyes are rhodopsin and iodopsin. They contain vitamin A aldehyde (retinene) and a specific protein. In bright light, these pigments are bleached and reformed again in dim light. A part of the retinene is destroyed in the process.
Vitamin A and epithelial tissue:
In vitamin A deficiency, the epithelial tissue is keratinized. The tissues affected are salivary glands, the respiratory tract, the eyes, the skin, and the sex organs.
Vitamin A and bone:
Vitamin A is essential for normal bone formation. Excess of vitamin A is toxic and causes brittleness of bones and, hence, causes bone fractures.
Effects of vitamin A deficiency
Critical deficiency states due to a lack of vitamin A in the diet are Night blindness, xerosis conjunctivae, Xerosis cornea, Bitot’s spots, Keratomalacia, and follicular hyperkeratosis.
Night blindness:
In advanced deficiency, the person cannot see objects in dim light and has difficulty reading in dim light.
Xerosis conjunctivae:
The conjunctiva is dry, wrinkled, thickened, and pigmented. This is because of the keratinization of the epithelial cells. The pigmentation gives the conjunctiva a smoky appearance.
Xerosis cornea:
When dryness spreads to the cornea, it becomes dull, hazy, and lusterless.
Bitot’s spots:
Bitot described this condition in 1863. They are greyish or glistening, and plaques formed of desquamated thickened conjunctival epithelium, usually triangular and firmly adhering to the conjunctiva, are frequently found in children with other signs of vitamin A deficiency.
Keratomalacia:
When xerosis of the conjunctivae and cornea is not treated, it may develop into the known condition ‘keratomalacia.’ The corneal epithelium becomes opaque, and ulceration and bacterial invasion destroy it, resulting in blindness.
Follicular hyperkeratosis (phrynoderma):
In this condition, the epithelium lining the hair follicle is hyper-keratinized. The skin becomes rough and dry, and papules of varying sizes are observed. Earlier workers showed that this condition responds to treatment with vitamin A. More recent studies, however, indicate that the above condition can be cured by administering essential fatty acids and pyridoxine.
Infertility:
Vitamin A is vital in keeping your reproductive system healthy. Deficiency of Vitamin A can lead to infertility and cause trouble in conceiving.
Growth issues in children:
Vitamin A deficiency can lead to delayed growth and development.
Respiratory tract infections:
Vitamin A deficiency leads to immune system problems, which can lead to infections in the throat and chest, resulting in respiratory tract infections.
What could be the reasons for vitamin A deficiency?
There are a few crucial reasons for vitamin A deficiency; some can be avoided if noticed at the right time.
Inadequate Dietary Intake:
Vitamin A deficiency can occur when you don’t get enough vitamin A in your daily meals. While common in India, vitamin A deficiency also appears frequently in developing countries worldwide. People in underprivileged nations don’t get enough vitamin A in their food.
Liver Disorders:
Liver disorders are also one cause of vitamin A deficiency. Your liver stores vitamin A; hence, liver disorders could affect vitamin storage, leading to vitamin A deficiency.
Increased Demand:
Pregnancy, lactation, or childhood growth increase vitamin A requirements, which can lead to deficiency if unmet.
Other causes:
Many diseases and conditions, such as malabsorption disorders that affect your intestinal lining to absorb nutrients and fat, could also lead to vitamin A deficiency. These conditions reduce your intestine’s absorption of nutrients and vitamins, including vitamin A. These conditions are as follows:
- Bile duct blockage.
- Celiac disease.
- Crohn’s disease
- Chronic diarrhea.
- Cystic fibrosis.
- Certain pancreatic disorders.
- Small bowel bypass or bariatric surgery.
- Intestine or pancreas surgery.
- Whipple’s Disease
- Zinc or iron deficiency.
Dietary sources of vitamin A
The table below shows the Vitamin A and carotene contents of some foods. Fish liver oils are the richest source of vitamin A, butter and eggs are good sources, and milk is a fair source. Green leafy vegetables and red palm oil are rich sources of carotene.
| Source | Vitamin A content (µg/100g) | |
| Rich Source | Fish liver oils Cod liver oil Cod liver oil(B.P) Shark liver oil Shark liver oil(I.P) Liver (sheep, goat, ox) | 6,660 – 1000,000 10,000 – 100,000 18,000 9,000 – 16,000 6,660 6,000 – 10,000 |
| Good Source | Butter Ghee (clarified butter fat) Egg(hen), Whole Egg, yolk Milk powder, Full cream | 720 – 1200 600 – 700 300 – 400 600 – 800 400 – 450 |
| Fair Sources | Milk cow’s or buffalo (whole) Fatty fish | 50 – 60 30 – 40 |
| Source | Carotene content (µg/100g) | Vitamin A equivalent* (µg) |
| Red palm oil Green leafy vegetables Carrot Pumpkin, Yellow Mango, ripe | 25,000 – 33,000 1,300 – 8,000 1.300 – 2,600 600 – 720 1,500 – 3,000 | 6,250 – 8,250 325 – 2,000 325 – 650 150 – 180 375 – 750 |
The harmful effect of excess intake of vitamin A
Children receiving daily large doses of fat-soluble vitamin A (30,000 to 150,000 µg) can cause toxic effects caused by Anorexia, headache, dry itching skin, and swelling of the bones.
Treatment and prevention of vitamin A deficiency
Mild to moderate cases of vitamin A deficiency can be treated with a daily oral dose of 10,000 µg of fat-soluble vitamin A for ten days. In severe cases, large doses of 50,000 µg of fat-soluble vitamin A must be administered daily for a few weeks. Studies carried out at the National Institute of Nutrition, Hyderabad, have shown that a single massive dose of 100,000 µg of fat-soluble vitamin A will help to prevent vitamin A deficiency in children for about six months.
Conclusion
Vitamin A deficiency could lead to severe complications, and cases of vitamin A deficiency are concerning in India and developing countries, especially in children. Awareness about the importance of vitamin A and its deficiency should be made public by us and the government, and underlying conditions related to vitamin A should be monitored, especially in rural areas. The primary reason is insufficient intake of vitamin A in dietary food, which could lead to vitamin A deficiency. It’s essential to take rich sources of vitamin A, like meat, dairy, and liver oils, and pro-vitamin A food through green leafy veggies, carrots, and yellow fruits and veggies. You could also consider taking vitamin A supplements.
Sources & references: Handbook of Food and Nutrition by Dr. M. Swaminathan, Vitamin A Deficiency from Cleveland Clinic.
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