Vitamin A

Introduction

Vitamin A belongs to the fat-soluble vitamins. There is vitamin A and provitamin A. Vitamin A is found in animal-based food and provitamin A in plants-based food. The body converts provitamin A to vitamin A. Vitamin A has the forms of retinol and retinyl ester. Beta-carotene is an important form of provitamin A, but alpha-carotene and beta-cryptoxanthin are also provitamin A. 1

This article contains information about vitamin A deficiency (not enough), overdose (too much), recommended daily intake, assessment of the personal status, natural sources of vitamin A, types of supplements, and other topics.

Not Enough

A vitamin A deficiency can lead to dry eyes, night blindness, corneal scarring, weakened immune system, higher risk of deadly infections and other negative effects2.

Too Much

Too much vitamin A is poisonous1. It is differentiated between a one-time very large intake and longer term too high intake.

One-Time Too High Intake (Acute Hypervitaminosis A)
A sudden high vitamin A intake can lead to a acute symptoms. These have been described as drowsiness, sluggishness, irritability, headaches, as well as peeling of skin by polar researches who ate large amounts of polar bear liver3.

Long-Term Too High Intake (Chronic Hypervitaminosis A)
A long-term too high intake of vitamin A leads to increased intracranial pressure (head), dizziness, nausea, headaches, skin irritation, pain in joints and bones, coma, and can also result in death1. If pregnant, then too much vitamin A can cause birth defects1.

A too high vitamin A intake from food is possible, but usually the cause is too much supplementation with vitamin A (Retinol, Retinyl-Ester)1. Supplements with beta-carotene (a form of provitamin A) are harmless as discussed below. After stopping the excess consumption of vitamin A, it takes a long time until the vitamin A content in tissue has normalized again1.

There are no known negative effects of a long term too high intake of beta-carotene (a form of provitamin a)4. A high beta-carotene intake may lead carotenemia, which shows up as a yellowish coloring of the skin4. Carotenemia is considered harmless, and disappears if the excess intake of beta-carotene is stopped4.
An exception could be smokers and asbestos workers, for whom a increased beta-carotene intake might increase the risk of lung cancer4.

Conclusion
A vitamin A overdose from normal food rarely occurs, but one should not eat uncontrolled often larger amounts of liver. If taking supplements containing vitamin A then caution is advised. If taking supplements containing provitamin A then the risk of negative consequences is very little. But for safety concerns, one should also not eat unnecessary large amounts of provitamin A supplements.

Recommended Daily Intake (Total)

The Institute of Medicine (US) Panel on Micronutrients recommends the below listed daily total intake. This includes intake from food and supplements together.
A RAE (retinol activity equivalent) can be 1 μg vitamin A (retinol) or, for example, 12 μg beta-carotene in food. The conversion table can be found in the "Types of Supplements" section on this page.

More detailed tables are available here.

Assessment of Status

There are blood tests for retinol1, retinyl-ester5 and cartenoids1. In the following, tests are discussed for vitamin A deficiency and chronic hypervitaminosis (long-term too much). Acute hypervitaminosis (once too much) is not discussed.

Retinol Blood Test Vitamin A Deficiency
There is a retinol blood plasma test. The results can be interpreted as in the table below.

Natural Sources

Vitamin A is contained in animal-based food, like milk products, fish, meat, and especially liver. There are two forms of vitamin A: retinol and retinyl ester. 1

Provitamin A is contained in some plant-based food1. There are different forms of provitamin A. The most important one is beta-carotene, and the other forms are apha-carotene and beta-cryptoxanthine1. The absorption of provitamin A is improved if eaten together with fat4.

There are other forms of carotenoids in food, but they cannot be converted to vitamin A. The following carotenoids are not sources of vitamin A: lycopene, lutein, zeaxanthine.1

A sufficient intake of vitamin A and provitamin A with normal food is possible. This is also true for vegans if enough fruits and vegetables with high carotenoid content are eaten2.

The table below shows selected food and its vitamin or provitamin A content:

As visible in the table above, muscle meat only contains small amounts of vitamin A. But liver contains much. Egg yolks contains some vitamin A as well as butter and cheese. For the plant-based sources, carrots contain very much provitamin A and other strongly orange or dark green colored vegetables also contain much. Bread does not contain provitamin A.

Types of Supplements

Vitamin A is available by food or supplements as either vitamin A or provitamin A2. The body must convert provitamin A to vitamin A and this does not happen one to one. To produce one microgram retinol (vitamin A) more than one microgram beta-carotene or another form of provitamin A is required2.

The US Institute of Medicine has published the following conversion table (2001)2. The values are in μg (microgram; a million μg are one gram).

That means that a person can either use 1 μg retinol as supplement, or 2 μg beta-carotene as supplement, or 12 μg as beta-carotene in food, to achieve the same effect.

Sometimes the vitamin A or provitamin A content on supplements is expressed as IU. This can be converted to RAE using the table below.

Maximal Dosage

The maximum safe dosage of vitamin A (as retinol) per day is according to the Institute of Medicine (US) Panel on Micronutrients2: for people older than 14 years the value is between 2'800-3'000 μg per day2. For children younger than 14 years, lower values apply2. More detailed tables by age and gender are available.

This does not mean that it is appropriate to consume that much. It only means that one should not consume more than this amount. This includes vitamin A from food as well as supplements.

There is no limit set for provitamin A (e.g. beta-carotene) as there are no dangerous effects known even at high doses2.

Biological Details

The body converts provitamin A to vitamin A. There are two forms of vitamin A: retinol and retinyl-ester. Inside the cells the vitamin A is then converted to retinal and retinoic acid and used by the cells.1

Conclusion

With a balanced diet it should be possible to get enough vitamin A from normal food. Muscle meat does not contain much vitamin A. If no milk or milk products are eaten, then one should consume regularly carrots or other fruits and vegetables high in provitamin A together with some fat.
A deficiency should be avoided, because it weakens the body and if the deficiency becomes pronounces also harms it.
An excess intake of vitamin A is poisonous. But it is almost impossible to consume too much vitamin A with food, unless one regularly eats meaningful amounts of liver. If taking supplements then one should pay attention not to consume too much vitamin A or only use supplements in the form of provitamin A, which the body can convert to vitamin A if needed.