Vitamin A

Vitamin A (Retinol and Carotenoids)

1. Identity: Chemical Names, Natural Sources, and Common Forms

Vitamin A is the collective name for a group of fat-soluble compounds that share biological activity with retinol. Vitamin A is a group of vital micronutrients widely present in the human diet. Animal-based products are a rich source of the retinyl ester form of the vitamin, while vegetables and fruits contain carotenoids, most of which are provitamin A. The human organism can metabolize natural forms of vitamin A and provitamin A into biologically active forms — retinol, retinal, and retinoic acid — which interact with multiple molecular targets, including nuclear receptors and opsin in the retina.

The two main forms of vitamin A in the human diet are preformed vitamin A (retinol and retinyl esters), and provitamin A carotenoids such as alpha-carotene and beta-carotene that are converted to retinol. Preformed vitamin A comes from animal products, fortified foods, and vitamin supplements. Carotenoids are found naturally in plant foods. There are other types of carotenoids found in food that are not converted to vitamin A but have health-promoting properties; these include lycopene, lutein, and zeaxanthin.

Chemical Forms

• Retinol — the primary alcohol form; the reference standard for vitamin A activity.
• Retinal (retinaldehyde) — the aldehyde form, essential for visual phototransduction.
• Retinoic acid (all-trans-retinoic acid) — the acid form; functions as a ligand for nuclear receptors regulating gene transcription.
• Retinyl esters (e.g., retinyl palmitate, retinyl acetate) — storage forms of the vitamin found in animal tissues and used in supplements.
• Provitamin A carotenoids — principally beta-carotene, alpha-carotene, and beta-cryptoxanthin, which are cleaved in the intestinal mucosa and liver to yield retinal and subsequently retinol.

Vitamin A is a crucial component of various fat-soluble compounds, including retinol, retinyl palmitate, and β-carotene. These compounds are essential for vision, cellular differentiation, epithelial integrity, immune function, and gene regulation.

Natural Food Sources

Among the best animal sources of vitamin A are eggs, liver, butter, milk, and such fish as tuna, sardines, and herring. Natural sources of vitamin A include dark leafy greens, orange-colored vegetables, milk products, liver, and fish.

Supplement and Pharmaceutical Preparations

Vitamin A is available in stand-alone supplements and most multivitamins, often in the form of retinol or its ester forms, such as retinyl palmitate. However, many dietary supplements (such as multivitamins) do not provide all of their vitamin A as retinol or its ester forms. For example, the vitamin A in some supplements consists partly or entirely of beta-carotene or other carotenoids.

Vitamin A (retinol) was first discovered in 1916; in 1931, it was first isolated and the structure was determined. The first chemical synthesis was achieved in 1947 and, shortly after, in 1948, the first commercial batches of synthetic vitamin A were produced. In 1948, the first kilograms of synthetic vitamin A (acetate) were produced by F. Hoffmann-La Roche, eliminating the need to extract this vital compound from natural sources.

Correct formulation is essential for stabilizing vitamin A derivatives, which are sensitive to light and oxidation.

Units of Measurement

The Daily Value for vitamin A is 900 mcg RAE for adults and children age 4 years and older, where 1 mcg RAE = 1 mcg retinol, 2 mcg beta-carotene from supplements, 12 mcg beta-carotene from foods, 24 mcg alpha-carotene, or 24 mcg beta-cryptoxanthin. The Institute of Medicine lists the Recommended Dietary Allowances (RDA) of vitamin A in micrograms (mcg) of retinol activity equivalents (RAE) to account for different absorption rates of preformed vitamin A and provitamin A carotenoids.

2. Traditional and Historical Use

Ancient Civilizations

The Ebers Papyrus, a medical document from approximately 1500 B.C., describes night blindness and suggests a treatment involving the consumption of roasted animal liver. The Ebers Papyrus describes night blindness in ancient Egypt. Physicians treated the condition by squeezing the "juices" of a grilled lamb's liver into the eyes of afflicted patients.

The old Egyptians, the Babylonians, the Greeks, and the Arabs used animal liver for treatment and successfully cured the disease. Hippocrates (460–327 BC) prescribed liver soaked in honey for blindness in malnourished children. Assyrian texts dating from 700 BC and Chinese medical writings from the 7th century AD both call for the use of liver in the treatment of night blindness.

Treating blindness with animal liver must have originated in empirical observations and was a widespread medical practice in the ancient world. This suggests that it must have been beneficial — it would be difficult to explain otherwise why this motif survived for an impressive two millennia. The cause of night blindness is a deficiency of vitamin A, and during the long history of medicine, the eating or application of liver (often the liver of a goat) or its "blood" has been recommended as the primary remedy.

Early Modern Period

Night blindness was recognized by the ancient Egyptians and Greeks, and many authorities from Galen onward advocated liver as a curative. Outbreaks of night blindness were linked to nutritional causes in the 18th and 19th centuries by von Bergen, Schwarz, and others.

Night blindness was a recurring problem among sailors on long voyages. The first to systematically test the old folk remedy was Eduard Schwarz (1831–1862), a ship's doctor on an Austrian frigate sent around the world on a scientific exploration. Before his departure from Vienna, several physicians had asked Schwarz to test the old folk remedy of boiled ox liver against night blindness. Schwarz fed sailors ox or pork liver and found that the night vision in all of the afflicted was restored. The cure was described as "a true miracle," and he concluded emphatically that night blindness was a nutritional disease.

Scientific Discovery Era (19th–Early 20th Century)

The physiologist François Magendie conducted nutritional deprivation experiments with dogs in 1816 that resulted in corneal ulcers and high mortality — a finding similar to the common clinical situation in poorly fed, abandoned infants in Paris. In the 1880s, Nicolai Lunin showed that there was an unknown substance in milk that was essential for nutrition. Carl Socin suggested that an unknown substance for growth in egg yolk was fat soluble.

In 1912, an English biochemist called Frederick Gowland Hopkins found unknown factors present in milk that were not fats, proteins, or carbohydrates, but were required to aid growth in rats. Hopkins was later awarded the Nobel Prize (in 1929) for this discovery. In 1917, Elmer McCollum from the University of Wisconsin–Madison, along with Lafayette Mendel and Thomas Burr Osborne from Yale University, discovered one of these substances while researching the role of dietary fats. In 1918, these "accessory factors" were described as fat-soluble, and in 1920 they were referred to as vitamin A.

A pivotal breakthrough in understanding the fundamental nature of Vitamin A occurred in 1932 when the Swiss chemist Paul Karrer successfully described its precise chemical structure, providing a crucial foundation for subsequent research into its function and enabling attempts at its artificial synthesis. In 1925, Fridericia and Holm directly linked vitamin A to night blindness in animal experiments using rats, and in 1929, Holm demonstrated the presence of vitamin A in retinal tissue. In the 1930s, Moore, Karrer, Wald, and others established the provitamin role of β-carotene.

During World War I, Bloch conducted a controlled clinical trial of different diets among malnourished Danish children with night blindness and keratomalacia and concluded that whole milk, butter, and cod-liver oil contain a fat-soluble substance that protects against xerophthalmia.

3. Key Constituents and Active Compounds: Mechanisms of Action

Absorption and Storage

Vitamin A is absorbed in the duodenum after hydrolyzation by pancreatic and intestinal enzymes and emulsified with dietary fats and bile acids. The majority is then stored in the hepatic stellate cells. Significant amounts are also stored in adipose tissue and the pancreas.

Visual Cycle

The most significant role of vitamin A in vision is to regenerate the visual chromophore of rhodopsin for receiving light. Light perception in vertebrates is initiated by activation of rhodopsin, which leads to a cascade reaction called phototransduction in the photoreceptor outer segments. To make rhodopsin light sensitive, covalent linkage between a vitamin A derivative 11-cis-retinal and opsin is essential. Photoisomerization of 11-cis-retinal to all-trans-retinal causes conformational changes in the opsin molecule that enable it to stimulate transducin. Continuous vision depends on recycling of the photoproduct all-trans-retinal back to visual chromophore 11-cis-retinal. This process is enabled by the visual (retinoid) cycle, a series of biochemical reactions in photoreceptor, adjacent RPE, and Müller cells.

Nuclear Receptor–Mediated Gene Regulation

Mammals use vitamin A not only for the generation of the visual chromophore, but also for the synthesis of the important signaling molecule retinoic acid, which binds to nuclear receptors — ligand-regulated transcription factors that directly control gene expression upon activation. In the eye, retinoic acid is generated by the oxidation of retinal by Retinaldehyde Dehydrogenase (RALDH). Retinoic acid binds to the transporter Cellular Retinoic Acid Binding Protein (CRABP) that facilitates its transport into the nucleus.

RA signaling is mediated by two families of nuclear receptors: RARs and RXRs, including three members α, β, and γ for each family. Nuclear retinoid receptors are frequently composed of RXR and RAR heterodimers, although RXRs can form homodimers or heterodimers with other nuclear receptors such as the vitamin D receptor or PPARs (peroxisome proliferator-activated receptors). In the nucleus, the RAR/RXR complex is bound to a specific sequence of DNA (RARE: retinoic acid response element), usually performing repressor roles in the absence of ligands and activating transcription of target genes when bound to them.

Biological Functions of Active Metabolites

The various functions of vitamin A are carried out by several metabolically active derivatives including 11-cis-retinal and all-trans-retinoic acid, which are required for vision and transcriptional gene regulation, respectively. Nuclear retinoic acid receptors regulate the transcription of a large number of genes. In addition to its essential roles in embryonic development, retinoic acid is also important in the function of many adult organs such as the nervous system, the immune system, the male and female reproductive systems, the respiratory system, and the skin. Retinoids have also been used successfully as therapeutic agents in treating human diseases including leukemia and acne.

Through its various metabolites, vitamin A sustains fetal development, immunity, vision, and the maintenance, regulation, and repair of adult tissues. Abnormal tissue levels of the vitamin A metabolite retinoic acid can result in detrimental effects, including congenital defects, immune deficiencies, proliferative defects, and toxicity. Intricate feedback mechanisms have evolved to allow tissues to generate appropriate levels of active retinoid metabolites despite variations in the level and format or in the absorption and conversion efficiency of dietary vitamin A precursors.

Hematopoiesis and Development

RA signaling appears to be essential for expression of genes involved in developmental hematopoiesis, regulating the endothelial/blood cell balance in the yolk sac, promoting the hemogenic program in the aorta-gonad-mesonephros area, and stimulating erythropoiesis in fetal liver by activating the expression of erythropoietin. In adults, RA signaling regulates differentiation of granulocytes and enhances erythropoiesis.

4. Scientific Evidence by Area of Use

4.1 Vision and Xerophthalmia

It is estimated that 3 to 10 million children, mostly in developing countries, become xerophthalmic annually, and 250,000 to 500,000 go blind annually. The WHO classified various stages of xerophthalmia to include night blindness, conjunctival xerosis, Bitot's spots, corneal xerosis, corneal ulceration, and scarring, all related to vitamin A deficiency. Night blindness is the first ocular symptom observed with vitamin A deficiency, and it responds rapidly to treatment with vitamin A. High-dose (60 mg) vitamin A supplementation reduced the incidence of night blindness by 63 percent in Nepalese children.

Xerophthalmia is the leading cause of preventable blindness and also childhood blindness. Treatment in the early stages can restore vision, but in patients with corneal ulcers, surgery is required; however, this still does not guarantee full restoration of vision.

The evidence for vitamin A in preventing and reversing vitamin A–deficiency eye disease is considered definitive and forms the basis for global public health programs. Normal levels of vitamin A are essential for good vision, while either too much or too little can be harmful.

4.2 Child Mortality, Morbidity, and Infectious Disease

The pediatric supplementation evidence base is among the strongest in nutritional science. A systematic review and meta-analysis including 43 trials with about 215,633 children found that 17 trials including 194,483 participants reported a 24% reduction in all-cause mortality (rate ratio = 0.76, 95% CI 0.69 to 0.83). Seven trials reported a 28% reduction in mortality associated with diarrhoea. Vitamin A supplementation was associated with a reduced incidence of diarrhoea and measles (50% reduction, rate ratio = 0.50, 95% CI 0.37 to 0.67) and a reduced prevalence of vision problems, including night blindness and xerophthalmia.

The 2022 Cochrane Review, the most current comprehensive evaluation, confirmed and updated these findings. It identified 47 studies involving approximately 1,223,856 children. Vitamin A supplementation (VAS) reduced the incidence of diarrhoea (RR 0.85, 95% CI 0.82 to 0.87; 15 studies, 77,946 children; low-certainty evidence), measles (RR 0.45, 95% CI 0.30 to 0.69; 2 studies, 1,982 children; low-certainty evidence), Bitot's spots (RR 0.42, 95% CI 0.33 to 0.53; 5 studies, 1,063,278 children; moderate-certainty evidence), night blindness (RR 0.32, 95% CI 0.21 to 0.50; 2 studies, 22,972 children; moderate-certainty evidence), and VAD (RR 0.71, 95% CI 0.65 to 0.78; 4 studies, 2,262 children; moderate-certainty evidence).

However, there was no evidence of a difference in incidence of respiratory disease (RR 0.99, 95% CI 0.92 to 1.06; 11 studies, 27,540 children; low-certainty evidence) or hospitalisations due to diarrhoea or pneumonia. Nine trials reported mortality due to diarrhoea and showed a 12% overall reduction for VAS (RR 0.88, 95% CI 0.79 to 0.98; 1,098,538 children; high-certainty evidence).

Evidence strength: The evidence for VAS reducing all-cause mortality and specific infections in deficient child populations in low- and middle-income countries is rated high to moderate certainty by GRADE methodology. The evidence does not extend clearly to well-nourished children in high-income settings.

4.3 Global Burden and Deficiency

Vitamin A deficiency (VAD) persists as a significant global health concern, disproportionately affecting populations in low- and middle-income countries (LMICs), where malnutrition is a persistent issue. Between 1990 and 2021, the global number of deaths attributable to VAD decreased from 188,458 to 17,374; the age-standardized rate declined from 3.04 to 0.27 per 100,000 population. Globally, VAD-attributable DALYs decreased from 18.79 million to 2.63 million. Since 1990, global VAD-attributable mortality and DALYs have declined significantly, driven by socioeconomic development and public health interventions. However, substantial inequalities persist, with low-SDI regions and males bearing a disproportionate residual burden.

4.4 Lung Cancer — Beta-Carotene and Retinol Supplementation (CARET Trial)

One of the most significant and cautionary findings in vitamin A supplementation research concerns high-dose supplementation in smokers. The Beta-Carotene and Retinol Efficacy Trial (CARET) was a multicenter, randomized, double-blind, placebo-controlled primary prevention trial involving a total of 18,314 smokers, former smokers, and workers exposed to asbestos. The effects of a combination of 30 mg of beta-carotene per day and 25,000 IU of retinol (vitamin A) in the form of retinyl palmitate per day were compared with placebo.

CARET was stopped ahead of schedule in January 1996 because participants who were randomly assigned to receive the active intervention were found to have a 28% increase in incidence of lung cancer, a 17% increase in incidence of death, and a higher rate of cardiovascular disease mortality compared with participants in the placebo group.

The CARET intervention was stopped 21 months early because of clear evidence of no benefit and substantial evidence of possible harm; there were 28% more lung cancers and 17% more deaths in the active intervention group. According to CARET's pre-specified analysis, there was an RR of 1.36 (95% CI = 1.07–1.73; P = .01) for weighted lung cancer incidence for the active intervention group compared with the placebo group, and RR = 1.59 (95% CI = 1.13–2.23; P = .01) for weighted lung cancer mortality.

It is important to note that the harm observed in CARET was from the combination of high-dose supplemental beta-carotene and preformed vitamin A in a high-risk population (smokers, asbestos-exposed workers). A subsequent meta-analysis of 19 observational studies found a pooled relative risk of 0.855 for higher category of dietary vitamin A intake and lung cancer risk. Evidence from 18 studies suggested that higher category of dietary beta-carotene intake could reduce lung cancer risk (RR 0.768 [95% CI 0.675–0.874]). The investigators concluded that higher category of dietary beta-carotene and vitamin A intakes could reduce the risk of lung cancer. This illustrates the important distinction between dietary intake and high-dose supplementation.

There is a lack of evidence to support the use of naturally occurring retinoids for the treatment and prevention of lung cancers.

4.5 Bone Health

The current evidence has yielded inconsistent outcomes showing positive, negative, and negligible effects of vitamin A on bone health. Animal studies showed that the negative effects of retinol on the skeleton were observed at higher concentrations, especially on the cortical bone. In humans, the direct relationship between vitamin A and poor bone health was more pronounced in individuals with obesity or vitamin D deficiency. Mechanistically, vitamin A differentially influenced the stages of osteogenesis by enhancing early osteoblastic differentiation and inhibiting bone mineralisation via retinoic acid receptor (RAR) signalling. However, adequate vitamin A intake through food or supplements was shown to maintain healthy bones.

Outcomes-based systematic reviews and meta-analyses have shown positive associations for vitamin A intake and serum retinol with risk of hip fracture. The tolerable upper daily intake level of vitamin A is approximately 3,000 μg RAE; higher levels increase the risk of vitamin A-induced chronic liver damage and fetal teratogenicity. Importantly, chronic daily ingestion of preformed vitamin A above the recommended intake (700 to 900 μg RAE) but below the tolerable upper intake (3,000 μg RAE) may still be harmful, especially to the musculoskeletal system.

Evidence strength: The bone health relationship with vitamin A is mixed and incomplete. Available data suggest that high intakes of preformed retinol may be associated with increased fracture risk, but evidence is not fully consistent across studies.

4.6 Reproductive Health and Embryonic Development

Retinoic acid, acting through nuclear retinoic acid receptors (RARs), is a potent regulator of patterning during embryonic development, as well as being necessary for adult tissue homeostasis. Vitamin A deficiency during pregnancy increases risk of maternal night blindness and anemia and may be a cause of congenital malformations. Childhood vitamin A deficiency can cause xerophthalmia, lower resistance to infection, and increased risk of mortality.

Through its various metabolites, vitamin A sustains fetal development, immunity, vision, and the maintenance, regulation, and repair of adult tissues. Conversely, excessive intake of preformed vitamin A during pregnancy is teratogenic — this is one of the primary reasons a Tolerable Upper Intake Level has been established specifically for preformed vitamin A.

5. Body Systems and Health Areas

Vitamin A has documented or established roles in the following body systems:

• Visual system: Metabolites of vitamin A are essential for vision, cellular differentiation, epithelial integrity, and immune function. Specifically, 11-cis-retinal is the chromophore of rod and cone photopigments.
• Immune system: Vitamin A deficiency is widespread in developing countries. Besides prominent ocular consequences, anemia and immune deficiency highlight other critical roles of vitamin A/RA signaling.
• Epithelial and skin integrity: Retinoic acid regulates epithelial cell differentiation throughout the body, including skin, respiratory tract, and gastrointestinal lining. Vitamin A is important for the integrity and regeneration of respiratory and gastrointestinal epithelia and is involved in regulating human immune function.
• Reproductive system: Retinoic acid is important in the function of both the male and female reproductive systems.
• Hematopoietic system: RA signaling appears to be essential for expression of genes involved in developmental hematopoiesis and stimulating erythropoiesis. In adults, RA signaling regulates differentiation of granulocytes and enhances erythropoiesis.
• Skeletal system: Vitamin A differentially influenced the stages of osteogenesis by enhancing early osteoblastic differentiation and inhibiting bone mineralisation via RAR signalling and modulation of osteocyte/osteoblast-related bone peptides.
• Nervous system: Retinoic acid is important in the function of the nervous system.
• Gene regulation (systemic): Nuclear retinoic acid receptors regulate the transcription of a large number of genes.

6. Dosage Forms and Dosages Reported in Studies

Recommended Dietary Allowances (RDA)

The Recommended Dietary Allowance (RDA) for men and women is 900 and 700 μg retinol activity equivalents (RAE)/day, respectively. The Tolerable Upper Intake Level (UL) for adults is set at 3,000 μg/day of preformed vitamin A.

Clinical Trial Dosages

• Child mortality/morbidity (Cochrane/meta-analysis): Randomised trials of synthetic oral vitamin A supplements in children aged 6 months to 5 years were included in the major systematic reviews. In most trials, single large oral doses (100,000–200,000 IU) were administered every 4–6 months.
• Night blindness in children (Nepal study): High-dose (60 mg) vitamin A supplementation reduced the incidence of night blindness by 63 percent in Nepalese children.
• CARET trial: The CARET trial tested oral administration of beta-carotene (30 mg/day) plus retinyl palmitate (25,000 IU/day) to decrease the incidence of lung cancer in high-risk populations.
• Burn/wound healing (case series): Patients received vitamin A at 25,000 IU thrice weekly.

Deficiency Assessment Thresholds

A serum or plasma retinol concentration of 20 mcg/dL (0.70 micromoles/L) or less frequently reflects moderate vitamin A deficiency, and a level of 10 mcg/dL (0.35 micromoles/L) or less is considered an indicator of severe vitamin A deficiency.

European Safety Reference Values

Teratogenicity was selected as the critical effect on which to base the UL for preformed vitamin A. The European Food Safety Authority (EFSA) Panel proposes to retain the UL for preformed vitamin A of 3,000 μg RE/day for adults. This UL applies to men and women, including women of child-bearing age, pregnant and lactating women, and post-menopausal women. This value was scaled down to other population groups using allometric scaling, leading to ULs between 600 μg RE/day (infants 4–11 months) and 2,600 μg RE/day (adolescents 15–17 years).

7. Safety Considerations and Drug/Nutrient Interactions

Preformed Vitamin A vs. Provitamin A Carotenoids

Only preformed vitamin A has a tolerable upper limit (UL) because high amounts can cause health problems, such as birth defects during pregnancy and liver damage. Beta-carotene has no UL because high amounts don't cause these problems. The absorption of provitamin A is variable and subject to feedback regulation, making it unlikely to lead to toxicity with excessive intake.

Acute Toxicity (Hypervitaminosis A)

Acute toxicity is caused by a single or a few repeated very high doses (generally greater than 100 times the RDA), arising within days to weeks with a typical symptom complex of severe headache, nausea, vertigo, blurred vision, muscle aches and lack of coordination, followed by skin desquamation and alopecia. Severe overdose can cause increased cerebrospinal fluid pressure, progressive drowsiness, and coma.

Acute ingestion (a single dose) of more than 200,000 μg RAE of preformed vitamin A is required to cause acute hypervitaminosis A syndrome in adults. This condition is uncommon and was seen historically in Atlantic explorers who inadvertently ate large amounts of animal livers containing preformed vitamin A.

Chronic Toxicity

Chronic hypervitaminosis A usually arises 3 months to many years after starting moderately high levels of vitamin A (generally 10 times the RDA) and is marked by dry skin, cheilosis, gingivitis, muscle and joint pains, fatigue, mental dullness, depression, and liver test abnormalities. Serum bilirubin is typically only mildly elevated. Serum aminotransferase and alkaline phosphatase levels are variably increased, but usually only 1 to 4 times the upper limit of normal.

The liver may be hypoechogenic on ultrasound examination and suggest the diagnosis of nonalcoholic fatty liver disease, but the lipid-laden cells found on liver biopsy are not hepatocytes, but rather stellate cells (formerly known as Ito cells) which contain excess vitamin A. Chronic, moderately high doses of vitamin A (generally over 1 to 8 years) can lead to portal hypertension with ascites and esophageal varices, even before frank cirrhosis can be shown to be present.

Teratogenicity

The condition characterized by elevated levels of vitamin A in the body is referred to as hypervitaminosis A. Vitamin A, also known as a teratogen, is capable of causing severe malformations. Certain dermatological medications, such as isotretinoin, contain analogs of vitamin A. Prolonged or excessive usage of these medications can lead to an accumulation of vitamin A stores, resulting in hypervitaminosis A, toxicity, and teratogenic effects.

Hypercalcemia

Excessive intake of preformed vitamin A, such as that found in supplements and animal sources (animal liver, fish liver oil, dairy, and eggs), is associated with multisystem effects that can include bone resorption and hypercalcemia. Hence, vitamin A toxicity should be explored in unexplained cases of parathyroid hormone-independent hypercalcemia. Clinical case series have found that approximately a third of patients who received high-dose vitamin A as per a micronutrient supplementation protocol developed hypercalcemia.

Retinoid Medications (Additive Toxicity Risk)

Certain dermatological medications, such as isotretinoin, contain analogs of vitamin A. Prolonged or excessive usage of these medications can lead to an accumulation of vitamin A stores, resulting in hypervitaminosis A, toxicity, and teratogenic effects. Concurrent use of supplemental vitamin A with prescription retinoids (isotretinoin, tretinoin, acitretin) substantially increases total retinoid burden.

Carotenoid Supplementation Risk in Smokers

The Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study (ATBC) and Carotene and Vitamin A Efficacy Trial (CARET), the most representative intervention studies of beta-carotene supplements, found strong positive associations between beta-carotene supplements and lung cancer risk among current smokers and asbestos-exposed workers. The implications for supplemental beta-carotene use in smokers represent one of the most well-established adverse findings in nutritional supplementation research.

Drug Interactions

Contraindications to high-dose vitamin A supplementation include pregnancy, breastfeeding, and hepatic or renal disease. While high doses of vitamin A are usually achieved by vitamin A supplements, hypervitaminosis A can also occur with excessive dietary intake of liver, particularly that of carnivores (bears, seals, dogs) or salt-water fish (cod liver oil).

The interaction profile of vitamin A with pharmaceutical agents includes several clinically relevant interactions. High-dose retinol can enhance the blood-thinning effects of warfarin, increasing bleeding risk. Interactions between retinoids/carotenoids and acetylsalicylic acid (ASA) have been barely explored in the published literature.

Regarding orlistat (a lipase inhibitor used for obesity): because vitamin A is fat-soluble, medications that reduce fat absorption — including orlistat — may reduce absorption of all fat-soluble vitamins, including vitamin A. This is a clinically recognized drug-nutrient interaction reported in prescribing information for orlistat, though it is not further elaborated in the primary sources retrieved here.

Vomiting Associated with Supplementation

In the pediatric supplementation literature, there was an increased risk of vomiting within the first 48 hours of VAS (RR 1.97, 95% CI 1.44 to 2.69; 4 studies, 10,541 children; moderate-certainty evidence). This is generally transient and dose-related, and has been observed consistently with high-dose oral supplementation protocols in children.

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