- What is Vitamin A?
- Functions of Vitamin A in Our Bodies
- The Health Benefits of Vitamin A
- Vitamin A Deficiency
- Vitamin A Food Sources
- Vitamin A Toxicity & Dosage Recommendations
- Vitamin A Interactions
- Bottom Line
Vitamin A is one of the most crucial elements in our bodies. It is a fat-soluble nutrient that exists in many foods and can be taken via supplements as well.
In this article, we will discuss everything you need to know about vitamin A; benefits, deficiency, toxicity, food sources, and recommended dosage.
What is Vitamin A?
Vitamin A includes many fat-soluble compounds including retinol, retinal, and retinyl esters. (1)
Foods include two main forms of vitamin A.
The first is preformed vitamin A (retinol and retinyl esters compounds) exists in animal sources only such as fish, liver, and dairy. On the other hand, provitamin A carotenoids are found in plant sources such as oils, fruits, and vegetables. (2)
Our bodies must convert all the previous forms of vitamin A to retinoic acid and retinal to be able to utilize them.
Retinoic acid and retinal are the active forms of vitamin A.
Also, vitamin A is stored in many body tissues because it is fat-soluble. The liver stores most of it in retinyl ester form. (3)
Our bodies can utilize the stored vitamin A after breaking down the esters and converting them into all-trans-retinol, which binds to RBP (Retinol-Binding Proteins) so that they can enter the bloodstream and be utilized. (4)
Functions of Vitamin A in Our Bodies
Vitamin A is essential for our health. It plays an essential role in cell growth, supporting our immune system, the development of the fetuses, and protecting our vision.
It is well-known for its functions and benefits for eye health. When retinal, an active form of vitamin A, is combined with opsin proteins, it makes them light-sensitive and converts them into rhodopsin. Rhodopsin is one of the essential molecules for low-light and color vision. (5)
Besides, it participates in protecting and maintaining the cornea and the conjunctiva. The cornea is the clear outermost later located at the front of our eyes that helps the eye to focus light to enable us to see clearly. The conjunctiva is a loose connective tissue covering the surface of the eye and the inner aspects of the eyelids. It also adheres to the sclera and meets the cornea at the limbus.
In a nutshell, vitamin A is essential for the function and structure of the eyes. (6)
Furthermore, it is important for the maintenance of other surface tissues such as the lungs, urinary bladder, inner ear, intestinal walls, and skin.
Also, it supports the immune system by participating in the growth and distribution of T-cells, a type of White Blood Cells (WBCs). T-cells are part of the immune system that protect our bodies from infections and some studies suggest that they play a role in fighting cancerous cells. (7)
Other functions include the support of healthy skin and male and female reproduction systems. (8)
The Health Benefits of Vitamin A
Vitamin A is one of the most important nutrients for our health due to the following benefits.
Beta-carotene, alpha-carotene, and beta-cryptoxanthin belong to provitamin A carotenoids, which are precursors of vitamin A and have powerful antioxidant properties.
Free radicals are reactive molecules that can cause severe damages to our bodies via a process called oxidative stress. Carotenoids fight and prevent them from causing these damages. (9)
Studies show that oxidative stress is one of the main causes of many chronic diseases including diabetes, cancers, heart problems, and cognition problems. (10)
Important for Eye Health
Vitamin A is extremely important for vision and eye health. The adequate intake is essential for protecting the eye against many eye problems, especially age-related macular degeneration.
According to studies, increasing your blood levels of carotenoids lowers the risk of age-related macular degeneration significantly due to the ability of carotenoids to fight oxidative stress. (14)
Protects the Body Against Certain Types of Cancers
Fruits and vegetables that are rich in carotenoids protect the body against many types of cancer due to their antioxidant properties.
A study was done on more than 10,000 participants found that smokers who consume carotenoids, especially alpha-carotene and beta-cryptoxanthin, had a 46% lower risk of dying from lung cancer than non-smokers. The same study found that non-smokers had a 61% lower risk of dying from lung cancer than people who do not consume carotenoids. (15)
Furthermore, laboratory studies suggest that retinoids can prevent and inhibit the risk of the growth of bladder, breast, and ovarian cancer cells. (16)
Essential for Fertility and the Proper Development of Fetuses
Vitamin A is one of the most important nutrients for both female and male reproduction. For males, it is essential for the health of the sperms, and for females, it plays a crucial role in the development of the eggs.
In addition, it is important for the health of the placenta, the proper development, growth, and maintenance of fetal tissues. (17)
In a nutshell, people trying to conceive should increase their vitamin A intake.
Crucial for the Immune System
Vitamin A has a direct impact on our immunity since it is essential for stimulating certain immune responses and reactions that protect our bodies from infections and diseases.
it is essential for the production of many immune cells such as B-Cells and T-Cells, which are vital for the immune responses that protect our bodies against diseases.
Vitamin A deficiency leads to increasing the risk of inflammations and inhibits our immunity’s responses and reactions. (18)
Vitamin A Deficiency
Vitamin A deficiency is rare in developed Vitamin A deficiency is not a problem in developed countries because they get enough vitamin A via food sources. However, it is one of the biggest problems in developing countries since the foods there do not contain enough provitamin A carotenoids especially preformed and vitamin A.
Vitamin A deficiency is associated with many severe health problems.
According to the World Health Organization (WHO), it is one of the most common causes of preventable blindness in children in the world, especially in developing countries.
Furthermore, it increases the risk of anemia and death during pregnancy, and also affects the fetuses by inhibiting their growth and development. (21)
The risk of deficiency increases in certain groups such as premature infants, cystic fibrosis patients, pregnant and breastfeeding women. The risk is higher in developing countries. (24)
Vitamin A Food Sources
Preformed vitamin A is found in foods from animal sources, including dairy products, fish, and meat (especially liver) and beta-carotene is the most important provitamin A carotenoid. Other provitamin A carotenoids include alpha-carotene and beta-cryptoxanthin.
Preformed vitamin A is absorbed and utilized in the body faster than provitamin A carotenoids, especially from plant-based sources.
The ability of the body to convert carotenoids into active forms of the vitamin depends on many factors including your overall health status, diet, genetics, medications, and more. (25)
That is why vegans and other people who follow plant-based diets should try to increase their intake of carotenoid-rich foods.
- Egg Yolk.
- Beef Liver.
- Cod Liver Oil.
- Chicken Liver.
- Cheddar Cheese.
- King Mackerel.
Foods that contain high amounts of provitamin A carotenoids such as beta-carotene include:
- Dandelion Greens.
- Swiss Chard.
- Red Peppers.
- Butternut Squash.
- Collard Greens.
Vitamin A Toxicity & Dosage Recommendations
High does of vitamin A can be as dangerous as its deficiency.
The Recommended Daily Allowance (RDA) is 900 mcg for men and 700 mcg for women. This can be obtained easily by following a balanced, whole-food diet without the need for any supplements or multivitamins. (28)
To avoid reaching the toxic levels, you need to watch your intake and not exceed the maximum dosage, which is 3,000 mcg for adults. (29)
It is rare to suffer from vitamin A toxicity due to consuming animal-based foods such as the liver. However, it is more common in people who take multivitamins and supplements in addition to certain medications such as Isotretinoin (A form of vitamin A that is used to treat severe nodular acne). (30)
Vitamin A is fat-soluble, which means that when it is stored in our bodies, it could reach toxic levels over time if it exceeded the maximum levels.
Vitamin A toxicity has many side effects and sometimes, it can be life-threatening, especially large doses taken in a short period.
Acute toxicity can occur if one extremely high dosage of vitamin A is taken over a short period. On the other hand, chronic toxicity occurs if you consumed doses that are higher than the Recommended Daily Allowance 10 times over a longer period.
Hypervitaminosis A (Chronic Vitamin A Toxicity) has many symptoms and side effects including:
- Visual Disturbance.
- Joint and Bone Pain.
- Loss of Appetite.
- Sensitivity to the Sunlight.
- Moderate to Severe Headaches.
- Hair Loss.
- Dry Skin.
- Liver Problems such as Jaundice.
- Delayed Development and Growth.
- Itchy and/or Dry Skin.
Acute vitamin A toxicity is less common than hypervitaminosis A. However, it is usually associated with more severe side effects including liver failure, high cranial pressure, coma, and even death. (31)
Furthermore, vitamin A toxicity can have severe negative effects on fetal and maternal health, which may lead to severe birth defects. (32)
Therefore, to avoid vitamin A toxicity, you should stick to the recommended daily doses of vitamin A supplements and steer clear of high-dose supplements.
The maximum dose of vitamin A must be applied to both vitamin A supplements and animal-based food sources of vitamin A.
On the other hand, toxicity is not linked with a high intake of dietary carotenoids. However, studies show that beta-carotene supplements may increase the risk of lung cancers and heart problems, especially in smokers. (33)
In conclusion, since excessive vitamin A intake can lead to severe problems, you should talk to your doctor before starting taking multivitamins or vitamin A supplements.
Vitamin A Interactions
Vitamin A has many interactions. Therefore, you should consult your doctor if you take any medications before taking any supplements.
Possible interactions include:
- Anticoagulants: Taking vitamin A supplements orally while taking anticoagulant medications to prevent the formation of blood clots may increase the risk of bleeding.
- Bexarotene (Targretin): This is a topical cancer medication. Vitamin A supplements increase the risk of the side effects of this medication, especially itchy and dry skin.
- Hepatotoxic Medications: High doses of vitamin A supplements can lead to severe liver damages. Taking high doses of vitamin A along with medications that are toxic to the liver can increase the risk of liver damage significantly.
- Orlistat (Alli, Xenical): Orlistat is a weight-loss supplement that may decrease the absorption of vitamin a from food sources. It is highly recommended to take a multivitamin supplement with vitamin A and beta-carotene supplements while taking orlistat.
- Retinoids: Try to Avoid taking of vitamin A supplements Along with oral retinoids. This way you will avoid reaching high level of vitamin A in your blood.
Vitamin A is a fat-soluble nutrient that is crucial for our immune system, eye health, reproduction systems, and fetal development.
Both deficiency and toxicity can lead to severe side effects and damages. Therefore, That is why you should stick to the Recommended Daily Allowance of 700 to 900 mcg for adults per day without exceeding the maximum daily dose of 3,000 mcg per day.
Also, you should consult your doctor if you have any medical conditions or take any medications.
Vitamin A supplements are great. However, a healthy, well-balanced diet can provide you with your needs completely.