• David Steuer

Vitamin A


Source: Healthline

Author: Atli Arnarson


Vitamins can be classified based on their solubility. Most are water-soluble, meaning they dissolve in water. In contrast, the fat-soluble vitamins are similar to oil and do not dissolve in water. Fat-soluble vitamins are most abundant in high-fat foods and are much better absorbed into your bloodstream when you eat them with fat. This article provides a comprehensive overview of one of these, vitamin A, its health benefits, functions and main dietary sources.


Vitamin A

Vitamin A plays a key role in maintaining your vision. Without it, you would go blind.

Types

Vitamin A is not a single compound. Rather, it is a group of fat-soluble compounds collectively known as retinoids.


The most common dietary form of vitamin A is retinol. Other forms — retinal and retinoic acid — are found in the body, but absent or rare in foods.


Vitamin A2 (3,4-dehydroretinal) is an alternative, less active form found in freshwater fish (1).


Summary: The main dietary form of vitamin A is known as retinol.

Role and Function of Vitamin A

Vitamin A supports many critical aspects of body function, including:

  • Vision maintenance: Vitamin A is essential for maintaining the light-sensing cells in the eyes and for the formation of tear fluid (2).

  • Immune function: Vitamin A deficiency impairs immune function, increasing susceptibility to infections (3, 4).

  • Body growth: Vitamin A is necessary for cell growth. Deficiency may slow or prevent growth in children (5).

  • Hair growth: It is also vital for hair growth. Deficiency leads to alopecia, or hair loss (6).

  • Reproductive function: Vitamin A maintains fertility and is vital for fetal development (7).

Summary: Vitamin A is best known for its vital role in maintaining vision. It’s also essential for body growth, immune function and reproductive health.

Dietary Sources

Vitamin A is only found in animal-sourced foods. The main natural food sources are liver, fish liver oil and butter.


Vitamin A can also be derived from certain carotenoid antioxidants found in plants. They are collectively known as provitamin A.


The most efficient of these is beta-carotene, which is abundant in many vegetables, such as carrots, kale and spinach (9, 10).

Summary: The best dietary sources of vitamin A include liver and fish oil. Sufficient amounts can also be derived from provitamin A carotenoids, like beta-carotene, which are found in vegetables.

Recommended Intake


The table below shows the recommended daily allowance (RDA) for vitamin A. The RDA is the estimated amount of vitamin A that the vast majority (about 97.5%) of people need to meet their daily requirements.


This table also shows the tolerable upper intake limit (UL), which is the highest level of daily intake considered safe for 97.5% of healthy people (11).


RDA (IU / mcg)UL (IU / mcg)


Infants

0–6 months1,333 / 4002,000 / 600

7–12 months1,667 / 5002,000 / 600


Children

1–3 years1,000 / 3002,000 / 600

4–8 years1,333 / 4003,000 / 900

9–13 years2,000 / 6005,667 / 1700


Women

14–18 years2,333 / 7009,333 / 2800

19–70 years2,333 / 70010,000 / 3000


Men

14–18 years3,000 / 9009,333 / 2800

19–70 years3,000 / 90010,000 / 3000


Summary: The RDA for vitamin A is 3,000 IU (900 mcg) for adult men and 2,333 (700 mcg) for women. For children, it ranges from 1,000 IU (300 mcg) to 2,000 IU (600 mcg).

Vitamin A Deficiency

Vitamin A deficiency is rare in developed countries.


However, vegans may be at risk, since pre-formed vitamin A is only found in animal-sourced foods.


Although provitamin A is abundant in many fruits and vegetables, it is not always efficiently converted into retinol, the active form of vitamin A. The efficiency of this conversion depends on people’s genetics (12, 13).


Deficiency is also widespread in some developing countries where food variety is limited. It is common in populations whose diet is dominated by refined rice, white potatoes or cassava and lacking in meat, fat and vegetables.


A common symptom of early deficiency includes night blindness. As it progresses, it may lead to more serious conditions, such as:

  • Dry eyes: Severe deficiency may cause xerophthalmia, a condition characterized by dry eyes caused by reduced tear fluid formation (2).

  • Blindness: Serious vitamin A deficiency may lead to total blindness. In fact, it is among the most common preventable causes of blindness in the world (14).

  • Hair loss: If you are vitamin A deficient, you may start to lose your hair (15).

  • Skin problems: Deficiency leads to a skin condition known as hyperkeratosis or goose flesh (16).

  • Poor immune function: Poor vitamin A status or deficiency makes people prone to infections (3).

Summary: Severe vitamin A deficiency may lead to blindness. Other symptoms may include hair loss, skin problems and an increased risk of infections.


Vitamin A Toxicity

Overdosing on vitamin A leads to an adverse condition known as hypervitaminosis A. It’s rare, but may have serious health effects.


Its main causes are excessive doses of vitamin A from supplements, liver or fish liver oil. In contrast, high intake of provitamin A does not cause hypervitaminosis.


The main symptoms and consequences of toxicity include fatigue, headache, irritability, stomach pain, joint pain, lack of appetite, vomiting, blurred vision, skin problems and inflammation in the mouth and eyes.


It may also lead to liver damage, bone loss and hair loss. At extremely high doses, vitamin A can be fatal (17).


People are advised to avoid exceeding the upper limit for intake, which is 10,000 IU (900 mcg) per day for adults.


Higher amounts, or 300,000 IU (900 mg), may cause acute hypervitaminosis A in adults. Children can experience harmful effects at much lower amounts (18).


Individual tolerance varies considerably. Children and people with liver diseases like cirrhosis and hepatitis are at an increased risk and need to take extra care.


Pregnant women should also be especially careful, since high doses of vitamin A may harm the fetus. Doses as low as 25,000 IU per day have been linked with birth defects (19).


Summary: High doses of vitamin A may lead to hypervitaminosis A, which is associated with various symptoms. Pregnant women should avoid eating high amounts of vitamin A because of the risk of birth defects.


Benefits of Vitamin A Supplements

While supplements are beneficial for those who suffer from deficiency, most people get enough vitamin A from their diet and do not need to take supplements.


Yet, controlled studies suggest that vitamin A supplements may benefit certain people even if their diet meets the basic requirements.


For instance, vitamin A supplements may help treat measles in children (20, 21).


They protect against measles-associated pneumonia and reduce the risk of death by 50–80%. Studies suggest that vitamin A acts by suppressing the measles virus (22).


Summary: Supplements mainly benefit those who are low or deficient in vitamin A. One exception is children with measles, as studies show that supplements may help treat the disease.


Summary of Vitamin A

Vitamin A, also known as retinol, is a fat-soluble vitamin traditionally associated with vision and eye health.


The most abundant dietary sources of vitamin A are liver, fish liver oil and butter.


It can also be derived from provitamin A carotenoids found in red, yellow and orange vegetables, as well as some leafy, dark-green vegetables.


Deficiency is rare in developed countries, but is most common among people who follow diets lacking in diversity, especially those dominated by rice, white potatoes and cassava.


Early symptoms of vitamin A deficiency include night blindness, and severe deficiency may eventually lead to total blindness.


Nevertheless, while getting enough vitamin A is vital, too much may cause harm.


Pregnant women should be extra careful not to eat excessive amounts of vitamin A because of the risk of birth defects.


The Bottom Line


Vitamin A is essential for health and plays many important roles in the body. Because it is a fat soluble vitamin, you can enhance its absorption by adding fat or oil to an otherwise low-fat meal.


For optimal health, make sure you get all of the fat-soluble vitamins in adequate amounts.



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