Vitamin A, also known as retinol, is one of the water-soluble vitamins along with vitamins D, E, and K. It is an essential micronutrient for our body and plays a key role in many essential biological processes.
Its main function in the visual process has been recognized since ages. Besides, it is important for the growth of the embryo and the maintenance of a strong immune system.
The fat-soluble vitamin A is present only in animal foods and its provitamin carotenoids are found in plant foods.
Types
There are two main types of vitamin A:
- Preformed fat-soluble vitamin A (retinoid) is found in animal foods such as liver, eggs, fish, and dairy. This form your body can easily digest and absorb. Retinol, a type of retinoid, is the main active form of vitamin A in blood.
- Provitamin A (carotenoid) is found in plant foods such as carrots, tomatoes, sweet potatoes, and leafy greens. Your body converts it into usable forms of vitamin A that it can digest and absorb. Carotenoids are naturally occurring pigments (yellow, orange, and red) synthesized by plants. Beta- carotene also called β-Carotene, a precursor of vitamin A, is a potent antioxidant.
The liver stores such a high amount of vitamin A that a healthy man can go on a vitamin A-deficient diet without its blood levels falling. Signs of its deficiency show only when the liver stores get depleted.
Deficiency can occur at any age but is most commonly seen in pre-school-age children and pregnant women, due to fetal needs.
Various countries have set norms for recommended dietary intake and fixed the upper limits for safe intake.
Functions
Vitamin A, a water-soluble vitamin is an essential micronutrient with varying vital functions.
Vision
Vitamin A is crucial for normal vision. Its deficiency can lead to visual defects. In the eyes, a derivative of vitamin A called retinaldehyde interacts with a protein called opsin to form rhodopsin.
Rhodopsin is the light receptor found in the rod photoreceptor cells of the retina and is essential for color vision and to see at night.
Immune system
Vitamin A is essential for maintaining a healthy immune system and its deficiency can lead to the body’s impaired response to infection.
Retinoic acid (RA) is the biologically active form of this vitamin, which plays a crucial role in the development and regulation of the immune system.
It plays a vital role in helping the immune cells communicate with each other and fight infections.
Vitamin A deficiency (VAD) has been shown to impair immunity against pathogens leading to many autoimmune disorders and infectious diseases.
Cell growth
Retinoic acid is a key growth-stimulating factor for epithelial cells and other cells in the body. It promotes cellular communication.
Besides promoting cellular growth, Vitamin A plays a critical role in the normal development of the heart, lungs, eyes, and other organs in addition to proper maintenance of their functions.
Gene transcription and protein formation
Retinoic acid, the active form of Vitamin A is essential for gene transcription, the process of making an RNA copy of a gene’s DNA sequence, and facilitating the flow of genetic information from DNA to RNA.
Retinol is taken up by the cell and oxidized to retinaldehyde and then retinoic acid. This process is irreversible and tightly regulated because retinoic acid binds to activate nuclear receptors to regulate gene transcription.
Skin health
Retinoic acid facilitates immature skin cells to develop into mature epidermal cells and maintain healthy skin by activating genes.
For example, isotretinoin, a retinoid (Vitamin A) is the most commonly prescribed drug in the treatment of acne. This drug decreases the size of sebaceous glands and reduces the secretion of sebum. This inhibits the bacteria present in the sebaceous ducts and the skin’s surface due to the reduced sebum, which bacteria rely on for nutrients.
Health Benefits
Here are the six vital benefits vitamin A imparts to our body.
Potent antioxidant
Provitamin A carotenoids, beta-carotene, alpha-carotene, and beta-cryptoxanthin have strong antioxidant properties.
They protect your body against free radicals, which are highly reactive and unstable molecules that generate oxidative stress.
Oxidative stress causes damage to organs and tissues and results in various chronic diseases such as diabetes, cancer, heart disease, and cognitive decline. Having a good amount of vitamin A reduces your risk of these chronic conditions.
Essential for eye health
Vitamin A is essential for good vision and eye health. Provitamin A carotenoids, beta-carotene, alpha-carotene, and beta-cryptoxanthin protect your eyes against eye diseases including age-related macular degeneration (AMD). This protection against AMD is due to its property of lowering oxidative stress by its antioxidant properties.
Vitamin A deficiency can lead to night blindness, which if not treated can lead to permanent loss of vision.
According to one study, vitamin A sufficiency can reduce your risk of developing cataracts and can even restore some vision loss, caused by bilateral visual field loss, xerophthalmia (dry eyes due to its deficiency), and night blindness.
Boosts your immune system
Vitamin A promotes the body’s immunity and gives it a boost by stimulating responses that neutralize the free radicals and protect your body from infections and illnesses.
It is involved in the production of B cells and T cells. These are powerful components of the immune system. B-cells produce antibodies to fight infection, while T-cells protect you from being infected by destroying the invading pathogens such as the cancerous and infected cells.
For example, according to the World Health Organization, giving vitamin A supplements to children with measles has been seen to reduce the severity of the disease.
Reduces cancer risk
The powerful antioxidant properties of carotenoids offer protection against cancer and reduce your risk of many cancers including cancers of the lung, bladder, breast, and ovarian cancer.
The oxidative stress caused by free radicals during cellular metabolism increases your risk of many diseases including cancer. Vitamin A with its potent antioxidative stress neutralizes such free radicals and prevents you from this dreaded disease.
The role of vitamin A against cancer risk can be judged from this study.
This study was conducted in more than 10,000 adults. It found that smokers with peak blood levels of alpha-carotene and beta-cryptoxanthin had a 46% and 61% lower risk of dying from lung cancer, than nonsmokers with the lowest intake of these nutrients.
Vital for reproductive health
Vitamin A is essential for both male and female reproduction because it is important in the development of sperm and egg.
Besides, during pregnancy, it is crucial for the proper health of the placenta, and the development and maintenance of good fetal health.
Therefore, it is vital for the good health of a pregnant woman and her developing baby. Its good levels in the body also facilitate easy pregnancy.
Vitamin A deficiency can lead to infertility and can hamper growth in children.
Vitamin A foods
Vitamin A is found naturally in many foods and some foods come fortified with this vitamin including juices, milk, and cereal.
Food sources of vitamin A include:
- Some fish, such as herring and salmon
- Organ meats such as beef lever
- Leafy green vegetables such as kale, spinach, broccoli
- Orange and yellow vegetables such as carrots, sweet potatoes, pumpkin, tomatoes, peppers, and winter squash
- Yellow fruits such as cantaloupe, mangos, papaya, and apricots
- Dairy products, such as milk and cheese
- Fortified breakfast cereals
- Eggs
Recommended vitamin A dose per day
The recommended dietary allowance (RDA) per day differs according to age and gender. Here is a chart showing the recommended dietary allowance per day of vitamin A
Age | Recommended daily dose in mcg |
6 to 11 months | 500 mcg |
6 to 11 months | 300 mcg |
2 to 18 years (female) | 300 to 700 mcg |
Over 18 years (female) | 1600 to 1800 mcg |
2 to 18 years (male) | 1400 to 2200 mcg |
Over 18 years (male) | 2000 to 2400 mcg |
Over 14 years and pregnant | 750 to 770 mcg |
Over 14 years & lactating | 1200 to 1300 mcg |
Tolerable upper intake levels
The tolerable upper intake levels of a supplement are the highest amount that you can safely take daily. Higher doses are mostly prescribed to treat vitamin A deficiencies.
Age | Tolerable Upper Intake Levels |
0-3 years | 600 mcg/day |
4-8 years | 900 mcg/day |
9-13 years | 1,700 mcg/day |
14-18 years | 2,800 mcg/day |
19 years and up | 3,000 mcg/day |
Vitamin A deficiency
Vitamin A deficiency is not having enough of it in the body resulting from an inadequate dietary intake that cannot meet the body’s needs. It can also occur when the gut cannot absorb this nutrient due to health complications, which can include Celiac disease, cirrhosis of the liver, pancreatic insufficiency, and duodenal bypass.
Its deficiency can increase the risk of serious, sometimes even fatal infections due to lowered immunity. Symptoms include night blindness, Bitot spots, dry skin, and frequent infections. Further complications can include the potentially blinding stages of corneal xerosis, ulceration, and necrosis.
Vitamin A deficiency is most commonly seen in developing countries of Africa and Southeast Asia. The most severe complications of this deficiency are seen in young children and pregnant women in these countries. This deficiency is rare in developed countries.
Testing of blood concentration levels of retinol helps diagnose this condition. A serum retinol concentration <0.70 μmol/L indicates subclinical vitamin A deficiency in children and adults, and a concentration of <0.35 µmol/L indicates severe deficiency.
Treatments typically include supplements and dietary changes. Vitamin A deficiency is treated with high doses of its supplement for several days. Later, your doctor will lower the dose of supplement until your vision and skin issues start to resolve. Correcting the deficiency with vitamin A supplements can cure night blindness and lubricate your dry eyes. However, vision loss due to scarring from corneal ulcers can’t be cured.
Risks of too much vitamin A
Too much vitamin A can be harmful. Taking more than 3,000 mcg a day of its oral supplements over the long term can cause hypervitaminosis A. Symptoms include:
- Nausea
- Vomiting and diarrhea
- Dizziness
- Blurry vision
- Thinning of bones
- Liver damage
- Changes in skin
- Pain in the joints and bone
- Birth defects in pregnant women with hypervitaminosis A usually due to too much supplements
Hypervitaminosis A can be corrected by stopping its supplements. Complications such as liver damage are treated separately though sometimes these complications may not be reversible.
Vitamin A supplements – Do’s and Don’ts.
Vitamin A supplements are available as either retinol or retinyl palmitate. Most multivitamins contain the recommended dietary allowance (RDA) for vitamin A. You should take dietary supplements only under the advice of a healthcare provider.
Taking too much of it when you are pregnant or even otherwise can cause serious problems.
In pregnancy, it can cause birth defects in the baby. Since prenatal vitamins contain the required amount of vitamin A, there may not be any need to take its separate supplement unless you are severely deficient.
Women who are pregnant or want to become pregnant shouldn’t take synthetic vitamin A because it can cause birth defects in the baby.
The tolerable upper limit, or safe upper limit, is 10,000 IU. and vitamin A tablets or capsules are available in a variety of doses. Always take its supplements at the dose recommended by your doctor.