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Vitamin A

Also known as: retinol, retinal, retinoic acid, tretinoin, beta-carotene, Retinyl ester, all-trans retinoic acid, 13-cis-retinoic acid

Overview

Vitamin A describes a group of fat-soluble metabolites that include preformed vitamin A molecules — retinols, retinals, retinyl esters, retinoic acids — and provitamin A molecules — carotenoids like beta-carotene — which are precursors that the body can convert into the other forms of vitamin A. Vitamin A metabolites are involved in cell and tissue growth and in several bodily functions, including vision, bone metabolism, and immune function. Because they are fat soluble, vitamin A metabolites can be stored in the body (mainly in the liver) as a retinyl ester. However, dietary intake is necessary because humans cannot synthesize vitamin A from scratch. Humans obtain preformed vitamin A — retinols, retinals, retinyl esters, and retinoic acids — from animal products (e.g., fish, eggs, dairy, liver), and obtain provitamin A carotenoids (like beta-carotene) from plants (e.g., sweet potatoes, carrots, leafy greens); the latter can be converted to preformed vitamin A by the body. In some countries, some foods — milk, margarine, cereals, etc. — are also fortified with vitamin A. Vitamin A is also taken as a dietary supplement. Such supplements typically contain preformed vitamin A (retinyl acetate or retinyl palmitate), provitamin A (beta-carotene), or a combination of both. Vitamin A is also found in high levels in some fish oil supplements, such as cod liver oil, and in many multivitamins.

Benefits

When ingested, beta-carotene, a provitamin A carotenoid, is converted in the gastrointestinal system into a retinal form of vitamin A, which can be subsequently converted into retinol, retinoic acid, or retinyl ester forms of vitamin A. Retinyl esters are the storage form of vitamin A, and are primarily stored in the liver. The metabolism of vitamin A is complex: there are many interconvertible forms, and each has a different role. For example, different types of retinols, retinals, and retinyl esters are precursor molecules to the synthesis of rhodopsin, a pigment involved in vision; retinols can act as cofactors in several enzymatic processes; and retinoic acids directly regulate gene expression while also regulating vitamin A metabolism by, for example, modifying the intestinal absorption of beta-carotene.

How it works

Sufficient amounts of vitamin A can be obtained through a balanced diet, but vitamin A deficiency can occur due to insufficient intake. Supplementation with vitamin A is used to treat vitamin A deficiency, which is most common in malnourished infants/children living in developing countries. The evidence shows that supplementation with vitamin A can improve growth, vision, and survival in malnourished infants/children and in premature babies. However, the precise recommendations for treating vitamin A deficiency are unclear because results are inconsistent among studies. Observational studies show that higher dietary intake of vitamin A, which includes total vitamin A intake from all sources (foods, drinks, and supplements), is associated with a reduced risk of depression. However, this association is derived from cross-sectional and cohort study designs in which vitamin A intake was estimated from historical diet recall using food frequency questionnaires. This makes it difficult to prove a causal link between vitamin A intake and depression. Besides dietary vitamin A, all-trans retinoic acid (Tretinoin) and 13-cis-retinoic acid (Isotretinoin) are types of vitamin A used in prescription drugs that are effective in treating acne.

Side effects

Recommended intake is similar between males and females, except during pregnancy and lactation when vitamin A requirements are elevated. The recommended dietary allowances (RDA) are: Birth to 6 months: 400 micrograms (mcg, or µg); Infants 7–12 months: 500 mcg; Children 1–3 years: 300 mcg; Children 4–8 years: 400 mcg; Children 9–13 years: 600 mcg; Teen males 14–18 years: 900 mcg; Teen females 14–18 years: 700 mcg; Teen females, pregnant: 750 mcg; Teen females, breastfeeding: 1,200 mcg; Adult males: 900 mcg; Adult females: 700 mcg; Adult females, pregnant: 770 mcg; Adult females, breastfeeding: 1,300 mcg. Due to potential toxicity, the upper limit of total daily intake of preformed vitamin A (from food, beverages, and supplements combined) is as follows: Birth to 12 months: 600 mcg; Children 1–3 years: 600 mcg; Children 4–8 years: 900 mcg; Children 9–13 years: 1,700 mcg; Teens 14–18 years: 2,800 mcg; Adults 19 years and older: 3,000 mcg.

Dosage

High intakes of preformed vitamin A can cause serious side effects — symptoms include severe headache, blurred vision, nausea, dizziness, muscle aches, and problems with coordination. Excessive intakes of preformed vitamin A have been associated with an increased risk of bone problems — e.g., osteoporosis and hip fracture — and some types of cancer, including ovarian cancer, pancreatic cancer, lung cancer, and gastric cancer. However, this evidence is derived from observational studies using cross-sectional and cohort study designs in which vitamin A intake data was estimated from historical diet recall using food frequency questionnaires. This makes it difficult to prove the causality between vitamin A intake and disease risk. Excessive intake of preformed vitamin A during pregnancy can also cause birth defects, and excessive intake when breastfeeding can impair a child’s growth. Therefore, high-dose supplementation with preformed vitamin A is not recommended when pregnant or lactating — consult your doctor if you are unsure. In severe cases, excessive intake of preformed vitamin A can cause coma and death, and supplementation with high doses of vitamin A has also been associated with increased mortality in well-nourished populations. There are also several drug interactions with Vitamin A, including, but not limited to, several antibiotics, some weight loss drugs (e.g., orlistat), and vitamin-A-containing drugs (e.g., isotretinoin). Always consult your doctor if you are taking over-the-counter or prescription medicines and planning to use a vitamin A supplement.

FAQs

What is vitamin A?

What are the signs and symptoms of vitamin A deficiency?

Inadequate dietary intake of vitamin A — either preformed vitamin A molecules or provitamin A molecules, like beta-carotene — is the primary cause of vitamin A deficiency. Common signs and symptoms of Vitamin A deficiency include damage to the skin, vision problems under dim light conditions (night blindness), and poor immune system function. Vitamin A deficiency during pregnancy can also cause birth defects in babies and growth impairments in children.

How common is vitamin A deficiency?

Due to the many approaches by which vitamin A status has been assessed, there is large variability in the prevalence estimates for vitamin A deficiency. However, vitamin A deficiency is most common in infants/children living in low- and middle-income countries, in whom prevalence of vitamin A deficiency is approximately 30%. Some conditions also increase a person’s risk of developing vitamin A deficiency. These include chronic alcohol intake, liver cirrhosis, intestinal absorption disorders like Crohn’s disease, and conditions that affect the exocrine pancreas (e.g., chronic pancreatitis).

What is retinol binding protein 4 (RBP4)?

Retinol binding protein 4 (RBP4) is a carrier protein that transports retinol (preformed vitamin A) in the blood from the liver to other tissues. Single-nucleotide polymorphisms (variants) in the gene that codes for the RBP4 protein appear to be associated with retinol concentrations in the blood and the risk of some conditions, including childhood obesity, coronary artery disease, insulin resistance, gestational diabetes, and type 2 diabetes. The concentration of RBP4 protein in the blood has also been shown to be associated with the risk of gestational diabetes and type 2 diabetes.