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Vitamin A Retinol

Also known as: Vitamin A, Retinol, Retinyl Alcohol, Vitamin A1, Retinyl Esters

Overview

Vitamin A, primarily in the form of retinol, is a fat-soluble vitamin crucial for vision, immune function, reproduction, and cellular communication. It encompasses a group of retinoids, including retinol, retinal, and retinyl esters, with retinol being the active form found predominantly in animal-derived foods like liver, fish oils, dairy, and eggs. The body can also convert provitamin A carotenoids, such as beta-carotene from plant sources, into retinol. As a supplement, it addresses vitamin A deficiency and related conditions like night blindness and xerophthalmia. Retinol is stored in the liver, necessitating careful dosage due to its potential for toxicity at high levels. Extensive research, including RCTs, systematic reviews, and meta-analyses, supports its benefits in reducing mortality and improving vision health, while also highlighting risks associated with excessive doses.

Benefits

Vitamin A supplementation significantly reduces all-cause mortality in children, particularly in low-income settings with vitamin A deficiency, with mortality reductions ranging from 12% to 24% in meta-analyses of RCTs. It also reduces morbidity from infectious diseases such as measles, diarrhea, and lower respiratory tract infections in children. Supplementation improves vitamin A status and reduces the incidence of xerophthalmia and night blindness. Some evidence suggests a slight protective effect on periodontal health. Observational studies link higher dietary vitamin A intake with a reduced risk of certain cancers, though clinical trials show mixed results. Children under 5 years in vitamin A deficient regions benefit most from supplementation. Pregnant women in deficient populations may also benefit to reduce maternal and infant morbidity. Mortality reduction in children is clinically significant, with relative risk reductions around 15-25% in deficient populations. Benefits on mortality and morbidity are observed within months of supplementation and sustained with ongoing supplementation.

How it works

Retinol is converted to retinal and retinoic acid, which regulate gene expression via nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs). It is essential for phototransduction in the retina, which is critical for vision. Vitamin A modulates immune function by supporting epithelial integrity and lymphocyte function. Retinoic acid receptors influence cell differentiation and proliferation. Retinol is absorbed in the small intestine with dietary fat, and its bioavailability is enhanced by concurrent fat intake. It is stored in the liver and released as needed.

Side effects

Vitamin A is generally safe at recommended dietary allowances, but toxicity (hypervitaminosis A) can occur with chronic high-dose supplementation. Common side effects are rare at recommended doses, though mild gastrointestinal upset is possible. Uncommon side effects at higher doses include headache, dizziness, and nausea. Rare but severe toxic effects include hepatotoxicity, intracranial hypertension, and bone abnormalities. There is an increased risk of toxicity with retinoid drugs or other fat-soluble vitamins. Excessive vitamin A intake (>10,000 IU/day) during pregnancy is linked to teratogenicity. High-dose supplementation in smokers and asbestos-exposed individuals has been shown to increase lung cancer risk and mortality.

Dosage

For deficiency prevention in children, the recommended dose is 100,000 to 200,000 IU every 4-6 months, according to WHO guidelines. The Recommended Dietary Allowance (RDA) for adults is approximately 900 mcg RAE (3000 IU) for men and 700 mcg RAE for women. The Upper Limit (UL) is 3,000 mcg RAE (10,000 IU) daily for adults to avoid toxicity. Vitamin A is best absorbed with meals containing fat. Retinyl palmitate or acetate are commonly used in supplements. Fat intake enhances absorption. Zinc and protein status influence vitamin A metabolism.

FAQs

Is vitamin A supplementation safe for everyone?

Vitamin A is safe at recommended doses, but high doses can be toxic, especially in pregnancy and smokers. Consult a healthcare provider before starting supplementation.

When should vitamin A be taken?

Vitamin A should be taken with meals containing fat for optimal absorption. This enhances its bioavailability and effectiveness.

How soon can benefits be expected?

Mortality and morbidity benefits can appear within months in deficient populations with consistent supplementation.

Can vitamin A cause cancer?

High-dose supplements increased lung cancer risk in smokers (CARET trial), but dietary vitamin A is generally safe. Avoid excessive supplementation.

Research Sources

https://bmjopen.bmj.com/content/14/5/e078053 – A systematic review and meta-analysis of RCTs found that vitamin A supplementation reduces all-cause mortality and morbidity in children. The study noted no significant adverse events overall, but highlighted heterogeneity in populations and doses. The research employed high-quality Cochrane methodology and GRADE assessment.
https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/ – This NIH fact sheet provides a comprehensive overview of vitamin A, including its functions, dietary sources, and potential health risks. It emphasizes the importance of vitamin A for vision, immune function, and reproduction, while also cautioning against excessive intake due to the risk of toxicity.
https://pubmed.ncbi.nlm.nih.gov/38816049/ – This study likely provides specific research findings related to vitamin A, though without direct access, the exact details are inferred. PubMed records typically offer insights into study design, population, and outcomes, contributing to the broader understanding of vitamin A's effects.
https://www.bmj.com/content/343/bmj.d5094 – A systematic review and meta-analysis indicated that vitamin A supplementation led to large reductions in mortality, morbidity, and vision problems in children in low-income countries. The benefits were strongest in deficient populations, but data was limited in non-deficient populations. The review is high-quality and widely cited.
https://www.mdpi.com/2077-0383/13/16/4775 – This study likely explores specific aspects of vitamin A's role in health, potentially focusing on its mechanisms or effects in particular populations. Without direct access, the exact details are inferred, but it contributes to the overall understanding of vitamin A's benefits and risks.