Pure vitamin A (retinol) is a lipid-soluble vitamin. It is primarily found in animal sources that contain fat. Vegetable sources contain a lipid-soluble precursor of vitamin A called beta-carotene. Pure vitamin A is primarily stored in the liver and in fatty tissue, for which reason we do not need a daily supply of the nutrient. Vitamin A and zinc work together. A deficiency in one nutrient will therefore affect the other. On a global scale, vitamin A deficiencies are widespread. In Western countries vitamin A deficiency is normally seen in connection with chronic disease. Vitamin A is destroyed when exposed to oxygen or heat, for instance during cooking.
Functions and importance
- Eyes and light perception: Needed for the synthesis of rhodopsin which we humans need in order to see during twilight and in the dark.
- Immune defence and protection against infections
- Mucous membranes
- The condition and moisture level of skin
- Hormonal system, reproduction, and formation of new sperm cells
- Genetic coding, fetal growth and development
- Antioxidant function (especially beta-carotene)
- Protection against harmful impact from free radicals (from sunlight, tobacco smoke, inflammation, etc.)
Deficiencies and poor utilisation may be caused by:
- Impaired fat absorption caused by gastro-intestinal disease or medical drugs
- Diabetes and subsequent lack of ability to convert beta-carotene
- Long-term use of antacids
- Unbalanced diets, alcoholism, and old age
- Pregnancy and lactation
- Night blindness, dry eyes, and conjunctivitis
- Dry, flaky skin and other skin conditions
- Dry and sensitive mucous membranes
- Impaired immunity
- Fragile hair and nails that crack or split
- Reduced ability to hear, smell and taste
- Abnormal fetal development
A vitamin A deficiency is detectable by means of a blood test that measures the blood content of retinol.
Animal and plant sources
The two most important sources are:
Pure vitamin A from animal sources
Precursor from plant sources
|Found in animal sources such as cod liver oil, liver, liver pate, kidneys, butter, cheese, egg yolk, and oily fish||Found in green/red/yellow vegetable sources such as carrots, rosehips, parsley, kale, spinach, tomatoes, bell pepper, and melon.|
Stored in the liver and in fatty tissue for future need.
Requires zinc to be released from the liver.
Stays a relatively long time in the body.
Gets converted to retinol in the intestinal mucous membrane as needed.
Does not get stored.
A particularly powerful antioxidant.
|The most potent type of vitamin A.
12 times stronger effect than seen with beta-carotene.
Possible to overdose.
|The weakest type of vitamin A.
The need is correspondingly greater. Practically impossible to overdose, which makes it safer for supplementation.
Measuring units for retinol and beta-carotene
The content of vitamin A is specified in RE (retinol equivalents) that corresponds with the total vitamin A effect.
Recent studies have shown that the effect of beta-carotene is not as great as previously assumed. For that reason, the vitamin A effect of beta-carotene has been halved in the new food listings.
1 RE= 1 microgram of retinol = 12 micrograms of beta-carotene
The former declaration known as IU (International Units) is still seen.
1 IU = 0.3 micrograms of retinol
Vitamin A content in selected foods (per 100 grams).
The content of retinol and beta-carotene is listed both in micrograms and in RE:
Cod liver oil (retinol)
|30,000 micrograms/ RE|
|Pork liver (retinol)||14,125 micrograms /RE|
|Carrots (beta-carotene)||9,790 micrograms = 816 RE|
|Spinach (beta-carotene)||4,186 micrograms = 349 RE|
Recomended daily allowance
Adults: 11 years of age and older: 800 micrograms/RE
Children: 1-10 years of age: 400 micrograms/RE.
Supplements are available both with retinol (pure vitamin A) and beta-carotene.
Always remember to take supplements together with a meal, as the fat content increases nutrient absorption.
- In connection with the mentioned deficiency symptoms
- Pregnancy and lactation
- Gastro-intestinal diseases with impaired fat absorption
- Diabetics who in contrast to healthy people are unable to convert beta-carotene
- Unbalanced diets, extended periods of dieting, and pure vegetarianism (vegans)
- Old age
- Large alcohol consumption
- Women who are pregnant or lactating should not ingest over 1,000 micrograms/ RE daily
- Women who are pregnant or lactating should avoid ingesting large quantities of vitamin A, as this may cause fetal damage or harm the infant via the breast milk. Also, pregnant and lactating women must refrain from eating pure liver or taking supplements, unless the preparations are designed to be used during pregnancy and/or lactation
- In case of seriously compromised renal function, special caution must be exercised with large dosages
- It is safer to take beta-carotene, as this nutrient cannot be overdosed
- Contraceptive pills increase blood levels of vitamin A and therefore the need for this nutrient is reduced
- Large amounts of beta-carotene (from e.g. carrots) may cause the skin to turn yellowish, but this is completely harmless
- People who use prescription medicine such as Roacutane (chemical vitamin A in pill form) and retinoic acid cream should avoid taking vitamin A supplements. There is a risk of side effects from these treatments.
Beta-carotene and smokers
Nutritional supplements with beta-carotene must carry a warning stating that these products are not suited for smokers. This warning is a consequence of two American studies that were conducted with synthetically manufactured beta-carotene. When ingested in excessive amounts, this may slightly increase the risk of lung cancer in heavy smokers. In contrast, Chinese research has shown that beta-carotene combined with selenium and vitamin E reduces the risk of lung cancer.
Overdosing and side effects
- Overdosing is rarely seen and only occurs when the depots in the liver are saturated and the intake of large amounts of vitamin A (retinol) continues
- If ingested on an empty stomach, transient side effects such as nausea may occur
- Acute poisoning may occur if more than 100 milligrams of vitamin A are ingested. This is more than 100 times the RDA level of 800 micrograms (RE)
- The symptoms include nausea, vomiting, elevated brain pressure and headache. Also, cramps, itching, and dry and flaky skin may occur in the case of very large single doses.
- Chronic poisoning only occurs after several months of ingesting daily dosages of several thousand micrograms (RE)
- Symptoms include nausea, headache, aching joints, dry and flaky skin, and constipation
- Increased risk of fetal damage may be seen in pregnant women who ingest over 3,000 micrograms (RE) daily
- Pregnant women should not ingest over 1,000 micrograms (RE) daily