Table of Contents
The retinoids, a group of molecules related to the dietary retinol (vitamin A) , are essential for vision and reproduction, growth and the maintenance of epithelial tissue. They also play an essential part in the immune system. Retinoic acid, which is derived by oxidation of retinol plays a major role in the action of the retinoids. This is except for the function of vision which relies on the retinal, an aldehyde derivative of the retinol.
What is Vitamin A?
- Vitamin A, a crucial fat-soluble vitamin, plays a pivotal role in numerous physiological functions. It is essential for normal vision, gene expression, embryonic development, growth, and the proper functioning of the immune system. Derived from various sources, vitamin A can be found in foods of animal origin as preformed vitamin A, while plants provide provitamin A carotenoids, such as β-carotene.
- Therefore, vitamin A is not just a singular compound. It encompasses a group of related compounds known as retinoids. These include retinal, retinol, retinoic acid, and several provitamin A carotenoids like beta-carotene. Each of these compounds has specific roles in the body. For instance, retinol is vital for vision as it combines with the protein opsin to form rhodopsin, a molecule necessary for both color and low-light vision.
- Besides animal sources, vitamin A can also be derived from plant sources in the form of provitamin A carotenoids. These carotenoids, when consumed, are converted into retinal and then retinol in the body. It’s important to note that β-carotene from ripe, colored fruits and cooked yellow tubers is more efficiently converted to vitamin A than from dark green leafy vegetables.
- Then, there’s the aspect of vitamin A storage and deficiency. The liver stores vitamin A in lipid droplets, ensuring that well-nourished individuals can sustain months on a diet deficient in vitamin A and β-carotene without showing signs of deficiency. However, when these liver stores are nearly depleted, deficiency symptoms, such as night blindness, begin to manifest.
- Furthermore, vitamin A and its metabolites have diverse roles in physiology. They are involved in everything from the formation of vision pigments to the regulation of numerous genes. Maintaining vitamin A levels within a specific range is crucial for health, as both its deficiency and excess can lead to severe diseases.
- In terms of dietary recommendations, the Recommended Dietary Allowance (RDA) for vitamin A is set at 900 μg retinol activity equivalents (RAE) per day for men and 700 μg RAE/day for women. However, it’s essential to strike a balance. While vitamin A is vital for health, excessive intake can lead to toxicity, known as hypervitaminosis A, which can have adverse effects on the body.
- In conclusion, vitamin A is a multifaceted nutrient with a myriad of functions in the body. It is vital for vision, growth, reproduction, and the maintenance of epithelial cells. Whether sourced from animals or plants, it is imperative to ensure adequate intake to reap its health benefits while avoiding the pitfalls of deficiency or excess.
Definition of Vitamin A
Vitamin A is a fat-soluble vitamin essential for normal vision, gene expression, reproduction, embryonic development, growth, and immune function. It is found in animal-derived foods as preformed vitamin A and in plant-based foods as provitamin A carotenoids. The vitamin plays a crucial role in various physiological processes, including vision, growth, and maintenance of epithelial cells. Consuming adequate amounts of vitamin A is vital for health, as both deficiency and excess can lead to serious health issues.
Recommended dietary allowance (RDA)
The Recommended Dietary Allowance (RDA) is a guideline that indicates the daily nutritional intake considered sufficient to meet the requirements of nearly all healthy individuals in a specific life stage and gender group. When discussing vitamin A, it’s essential to understand its measurement units and how they relate to one another.
Vitamin A’s daily requirement is typically expressed in terms of retinol equivalents (RE) rather than International Units (IU). To break this down:
- 1 retinol equivalent (RE) is equivalent to 1 µg of retinol.
- This same amount, 1 RE, can also be equated to 6 µg of β-carotene or 12 µg of other carotenoids.
- In terms of International Units, 1 RE corresponds to 3.33 IU of vitamin A activity when derived from retinol and 10 IU of vitamin A activity when sourced from β-carotene.
Furthermore, the RDA for vitamin A varies based on age, gender, and specific conditions such as pregnancy. For adults:
- Men require approximately 1,000 retinol equivalents, which translates to 3,500 IU.
- Women, on the other hand, need around 800 retinol equivalents, equivalent to 2,500 IU.
It’s also noteworthy that 1 International Unit (IU) of vitamin A is equivalent to 0.3 mg of retinol. Additionally, specific groups, such as pregnant women and lactating mothers, have increased vitamin A requirements to support their unique physiological needs.
Therefore, understanding these measurements and conversions is crucial for ensuring adequate vitamin A intake, which plays a vital role in various biochemical functions in the body.
|Life stage group||US RDAs or AIs
|US Upper limits
|Infants||0–6 months||400 (AI)||600|
|7–12 months||500 (AI)||600|
Structure of vitamin A
Vitamin A, often referred to as retinoids, encompasses a group of structurally related and biologically active molecules. These molecules play a pivotal role in various biological processes and are essential for maintaining optimal health. Therefore, understanding the structure of vitamin A is crucial for comprehending its functions and significance in the human body.
To begin with, retinoids include both natural and synthetic forms of vitamin A. Among these, retinol, retinal, and retinoic acid are considered the primary vitamers of vitamin A. Delving deeper into their structures:
- Retinol (vitamin A alcohol): This molecule is characterized by a primary alcohol containing a β-ionone ring. Besides, its side chain is composed of two isoprenoid units, four double bonds, and a hydroxyl group. In animal tissues, retinol predominantly exists as retinyl ester, which forms a bond with long-chain fatty acids.
- Retinal (vitamin A aldehyde): Originating from the oxidation of retinol, retinal is its aldehyde counterpart. These two forms, retinal and retinol, can interconvert, ensuring the body maintains an equilibrium between them. In earlier scientific literature, retinal was often referred to as retinine.
- Retinoic acid (vitamin A acid): This molecule emerges from the oxidation of retinal. However, it’s essential to note that retinoic acid cannot revert to form retinal or retinol, making its structure and function distinct.
- β-Carotene (provitamin A): Found abundantly in plant-based foods, β-carotene undergoes cleavage in the intestine, resulting in the production of two moles of retinal. Then, it’s worth noting that in humans, the conversion of β-carotene to retinal is not highly efficient. Therefore, β-carotene offers only about one-sixth of the vitamin A activity when compared to retinol.
Properties of Vitamin A
Vitamin A, a vital nutrient for the human body, possesses a range of properties that contribute to its biological significance. Delving into a detailed and sequential explanation of these properties:
- Physical Appearance: Vitamin A is an oily substance that is colorless in nature. When it interacts with saturated “antimony trichloride” (AnCl3), it momentarily exhibits a blue color. This particular characteristic is pivotal in the “Cataract” method, which is employed to determine the presence of vitamin A.
- Absorption Properties: One of the defining features of vitamin A is its ability to absorb UV light. Specifically, it demonstrates maximum absorption at a wavelength of 325 nm.
- Stability: Vitamin A remains stable when shielded from air and light. Besides, it has the capability to form esters with higher fatty acids, further emphasizing its chemical versatility.
- Solubility: Vitamin A is soluble in fats and fat solvents. However, it is insoluble in water, which is a crucial aspect to consider when studying its distribution and storage in the body.
- Retinol: Retinol, a form of vitamin A, presents itself as a viscid, colorless oil. It plays a pivotal role as a steroid hormone, aiding in cell growth and differentiation. Moreover, retinol can be isolated through careful fractionation, resulting in pale yellowish needles. It exhibits a characteristic absorption band in the ultraviolet spectrum at 328 nm. Furthermore, retinol is stored as retinyl ester in animal tissues, ensuring its availability for biological processes.
- Stability and Storage: While vitamin A remains relatively stable during processes like cooking, canning, and freezing, it is susceptible to destruction by oxidizing agents. Exposure to UV light also leads to its degradation. Therefore, understanding these properties is essential for its preservation and optimal utilization.
- Solubility of Retinol: Retinol, being fat-soluble, integrates well with fats. However, its insolubility in water is a defining characteristic that influences its distribution in the body.
Sources of vitamin A
- Animal Tissues: Vitamin A is naturally present in numerous animal tissues. When consumed, it is efficiently absorbed in the terminal small intestine. Among all dietary sources, the liver stands out as the most abundant reservoir of vitamin A.
- Plant Sources: Interestingly, plants do not inherently contain vitamin A. However, many dark-green or dark-yellow plants, such as the renowned carrot, are rich in carotenoids like beta-carotene. These carotenoids act as provitamins. During absorption in the intestinal mucosa, they undergo a transformation to retinol, making them vital for those who rely on plant-based diets.
- Storage and Distribution: The liver plays a pivotal role in the storage of vitamin A. Within the liver, stellate cells predominantly store vitamin A as retinyl esters. When the body requires vitamin A, it is released from the liver, transported in the blood by the retinol binding protein, and subsequently delivered to other tissues.
- Animal Sources: Animal-derived foods are rich in preformed vitamin A. Prime sources include liver, kidney, egg yolk, milk, cheese, and butter. Additionally, fish liver oils, especially from cod or shark, are exceptionally rich in vitamin A, making them a sought-after source for many.
- Vegetable Sources: While vegetables do not contain vitamin A directly, they are abundant in provitamin A-carotenes. Yellow and dark green vegetables and fruits are exemplary sources of these carotenes. Examples include carrots, spinach, pumpkins, mango, and papaya, among others.
Absorption, transport and mobilization of vitamin A
- Absorption of Vitamin A: Dietary retinyl esters undergo hydrolysis in the intestine, facilitated by pancreatic or intestinal brush border hydrolases. This process releases retinol and free fatty acids. Carotenes, on the other hand, are acted upon by β-carotene 15-15′ dioxygenase in intestinal cells, producing two moles of retinal. This retinal is then reduced to retinol. Within the intestinal mucosal cells, retinol undergoes reesterification with long-chain fatty acids. Subsequently, it gets incorporated into chylomicrons and is transferred to the lymphatic system.
- Transport to the Liver: The chylomicrons containing retinol esters are absorbed by the liver for storage. When the body requires vitamin A, the liver releases it in the form of free retinol. Zinc is believed to play a crucial role in this mobilization process. In the bloodstream, retinol binds to the plasma retinol binding protein (RBP) and associates with pre-albumin. This retinol-RBP complex then binds to specific receptors on peripheral tissue cells, facilitating its entry.
- Cellular Uptake and Function: Many target tissues contain a cellular retinol-binding protein that transports retinol to the cell nucleus, where it binds to chromatin (DNA). In this location, retinol functions similarly to a steroid hormone, influencing gene expression.
- Additional Insights: Retinyl esters from animal sources or dietary supplements undergo hydrolysis in the small intestine, releasing free retinol. This retinol passively diffuses into intestinal absorptive cells. Once inside, it binds to retinol binding protein 2 (RBP2) and is reesterified by lecithin retinol acyltransferase before being incorporated into chylomicrons. β-carotene, in contrast, is absorbed by enterocytes through the scavenger receptor B1 (SCARB1) protein. Once inside the cell, β-carotene can either be incorporated directly into chylomicrons or first converted to retinal and then retinol.
- Storage and Mobilization: The liver’s ability to store retinol ensures that well-nourished individuals can survive for extended periods without vitamin A intake. Two primary liver cell types, hepatocytes and hepatic stellate cells (HSCs), are responsible for this storage and release. Mobilization involves the release of free retinol from HSCs, which is then transferred to hepatocytes, bound to RBP4, and released into the bloodstream.
- Dietary Considerations for Carnivores and Herbivores: Carnivores have a unique ability to tolerate high retinol intakes due to their capacity to excrete excess retinol and retinyl esters in urine. Herbivores, on the other hand, rely on ionone-containing carotenoids, converting them to retinal.
- Activation and Excretion: In the liver and other tissues, retinol is converted to retinal by alcohol dehydrogenases. Retinal is then irreversibly oxidized to retinoic acid (RA) by aldehyde dehydrogenases. RA plays a role in gene activation or deactivation. Its oxidative degradation is initiated by its presence, ensuring a short-lived gene transcription signal. This degradation involves cytochrome P450 enzymes, which convert RA into various oxidized forms. These forms are then conjugated with glucuronic acid, making them water-soluble and ready for excretion in urine and feces.
Mechanism of action of vitamin A
- Vision: Vitamin A (all-trans Retinol) is converted by the retina to the 11-cis isomer of retinaldehyde (11-cis-retinal). 11-cis-retinal is a component of the retina by converting of light into neural signals that are necessary to see. 11-cis retina, when linked to opsin within rhodopsin is isomerized to all-trans-retinal via light. This is the process that causes the neural signal to the brain that allows the brain to perceive light. Trans-retinal then gets released from opsin , and then decreased to all-trans-retinol. All-trans-retinol can be isomerized into 11-cis when it is dark and later transformed into 11-cis-retinol. 11-cis retina recombines with opsin in order to form the rhodopsin. Vision loss or night blindness in low light is caused by an inability to synthesize the 11-cis retinal quickly.
- Epithelial differentiation: The function in the role of Vitamin A during epithelial differentiation, as in different physiological procedures, is it binding Vitamin A in two nuclear retinal receptors (retinoic acid receptors also known as RARs and retinoid X receptors, RXRs). These receptors act as factors activated by ligands that regulate the transcription of genes. If there isn’t sufficient Vitamin A to connect to these receptors, the natural cell growth and differentiation are disrupted.
Toxicity of vitamin A
Vitamin A, a crucial nutrient for various bodily functions, can become toxic when consumed in excessive amounts. This phenomenon is termed hypervitaminosis A. The body’s inability to easily eliminate excess fat-soluble vitamins, such as vitamin A, makes it susceptible to toxicity. Therefore, understanding the implications of vitamin A toxicity is essential for maintaining optimal health.
Understanding the Thresholds: The upper limit (UL) for vitamin A intake is set at 3,000 mg/day. Consuming amounts exceeding 7.5 mg/day of retinol is discouraged. For context, acute toxicity can manifest with doses as high as 25,000 IU/kg of body weight, while chronic toxicity can occur with daily intakes of 4,000 IU/kg body weight for durations ranging from 6 to 15 weeks. However, it’s worth noting that liver-related toxicities can manifest with daily dosages as low as 15,000 IU and as high as 1.4 million IU. In individuals with renal failure, even a dose of 4,000 IU can be detrimental.
Symptoms and Manifestations: Early signs of chronic hypervitaminosis A include dry, pruritic skin due to decreased keratin synthesis, an enlarged liver that can progress to cirrhosis, and symptoms mimicking a brain tumor due to increased intracranial pressure. Other symptoms encompass nausea, jaundice, anorexia, vomiting, blurred vision, headaches, abdominal pains, fatigue, drowsiness, and diminished mental health. In severe cases, individuals may experience hair loss, mucous membrane dryness, insomnia, fever, bone fractures, weight loss, diarrhea, and anemia.
Moreover, an elevated intake of vitamin A over extended periods can lead to a condition termed “pseudotumor cerebri.” This condition, characterized by increased intracranial pressure, presents with symptoms like headaches, blurred vision, and confusion.
Special Considerations: Pregnant women should exercise caution with vitamin A intake due to its teratogenic potential, which can cause congenital malformations in the fetus. Additionally, excessive alcohol consumption can exacerbate vitamin A toxicity. Children, with their lower body weight, can reach toxic levels with doses as high as 1,500 IU/kg.
Bone Health and Vitamin A: A noteworthy concern is the association between high vitamin A intake and decreased bone mineral density, leading to an increased risk of fractures. This is particularly alarming considering that approximately 75% of the population in developed countries consumes more than the recommended daily allowance (RDA) of vitamin A. Chronic intake of vitamin A, especially double the RDA, has been linked to conditions like osteoporosis and hip fractures.
Vitamin A and Fetal Development: Vitamin A’s toxic effects can profoundly impact fetal development. For instance, treatment doses used for acne have been shown to disrupt neural activity in the fetal brain. The period of organogenesis is particularly sensitive to vitamin A toxicity.
Carotenoids and Vitamin A: While the toxicities are primarily associated with preformed (retinoid) vitamin A, carotenoid forms, like beta-carotene, are generally considered safe. However, excessive beta-carotene consumption can lead to carotenodermia, resulting in an orange-yellow skin discoloration.
Recent Developments: Researchers have explored water-soluble forms of vitamin A to mitigate toxicity risks. However, some studies suggest that water-soluble vitamin A might be more toxic than its fat-soluble counterpart.
Biochemical Functions of Vitamin A
Vitamin A, a vital nutrient, plays a pivotal role in a myriad of biochemical processes essential for maintaining overall health. This article delves into the intricate functions of Vitamin A, elucidating its significance in vision, cellular growth, and other vital processes.
- Vitamin A and Vision: Vitamin A is indispensable for vision. The biochemical mechanism underlying vision involves a complex process known as the Rhodopsin or Wald’s visual cycle. The retina, a crucial component of the eye, comprises two types of cells: rods and cones. While rods facilitate vision in dim light, cones are instrumental in bright light and color perception. The primary event in the visual cycle is the isomerization of 11-cis-retinal to all-trans retinal, a process that generates nerve impulses. This cycle ensures the continuous regeneration of rhodopsin, a conjugated protein present in rods, essential for vision.
- Dark Adaptation Time: Transitioning from a brightly lit environment to a dimly lit one depletes rhodopsin stores, momentarily impairing vision. However, with the resynthesis of rhodopsin, vision improves—a process termed as dark adaptation. Vitamin A deficiency prolongs this adaptation time.
- Color Vision: Cones, specialized for bright and color vision, operate through a visual cycle akin to rods. The perception of different colors arises from the bleaching of color-sensitive pigments like porphyropsin (red), iodopsin (green), and cyanopsin (blue). The dissociation of these pigments in varying proportions allows the brain to perceive a spectrum of colors.
- Maintenance of Healthy Epithelial Surfaces: Vitamin A ensures the health of epithelial tissues. Retinol and retinoic acid inhibit keratin synthesis, preventing the formation of keratinized surfaces. Additionally, retinyl phosphate, derived from retinol, aids in the synthesis of glycoproteins and mucopolysaccharides, essential for growth and mucus secretion.
- Cell Growth and Differentiation: Functioning analogously to steroid hormones, retinol and retinoic acid regulate protein synthesis, playing a pivotal role in cell growth and differentiation.
- Lipid Metabolism: In the realm of lipid metabolism, Vitamin A is crucial. Mevalonate, an intermediate in cholesterol biosynthesis, is redirected towards coenzyme Q synthesis in the absence of Vitamin A.
- Immune System: Furthermore, Vitamin A fortifies the immune system, bolstering the body’s defense against infections.
- Antioxidant Properties: Carotenoids, especially β-carotene, act as potent antioxidants. They mitigate the risk of cancers initiated by free radicals and potent oxidants. Additionally, β-carotene has been found to be beneficial in averting heart attacks, attributed to its antioxidant properties.
Vitamin A And Immune System
- Vitamin A (carotenoids) is a vitamin that has an anti-inflammatory effect and plays an important role in improving immunity. It has been proven to have therapeutic effects in treating a variety of infectious diseases.
- It was discovered that nutrients that are rich in vitamin A lower the risk of developing cancer however, its absence has been linked with the risk increase of contracting infections.
- Vitamin A is essential in the growth and function of B and T lymphocytes.
- Thus, a decline in the level of vitamin A naturally reduces immune cell reactions and also reduces specific antibodies after vaccination.
- It has been proven that vitamin A may also inhibit normal apoptosis of bone-marrow cells, resulting in an increased quantity of marrow cells found in the bone marrow as well as in spleen , and peripheral blood. This indicates that vitamin A plays a role in controlling the homeostasis of bone marrow.
- Vitamin A supplements also work as an adjuvant to boost immune responses when it is given during immunization. It was observed that vitamin A is essential for measles recovery in children.
- So, supplements with large doses of vitamin A that are administered within the first few days after hospitalization, greatly reduce the risk of dying and morbidity among children.
- The improved results were due to an increase in measles antibody tests in children receiving vitamin A supplements.
- Vitamin A supplements for children are identified to decrease the risk of dying and morbidity caused by certain types of measles, diarrhea, HIV disease and malaria.
- Additionally, recent studies have revealed a link between vitamin A deficiencies and infectious diseases like tuberculosis, AIDS and other infectious diseases that propagated through the digestive and respiratory systems of children.
- The exposure to UV radiation directly increases the burden of free radicals that is imposed on the body. It also reduces immune reactions, specifically cell-mediated reactions. It also raises the likelihood of developing skin cancer.
- However, b-Carotene as well as other carotenoids are able to block singlet oxygen production triggered from ultraviolet radiation. In fact, this singlet oxygen may trigger the creation of immunosuppression.
Vitamin A Deficiency
Vitamin A deficiency is a significant health concern that primarily arises from malnutrition. However, it can also be attributed to abnormalities in the intestinal absorption of retinol or carotenoids. This deficiency is prevalent in certain underdeveloped countries, especially among children. In herbivores, such as cattle, the deficiency typically stems from a lack of green feed, often seen in animals transitioning from dry summer pastures or those consuming low-quality hay. It’s crucial to note that the liver stores substantial amounts of retinol. Therefore, the manifestations of vitamin A deficiency usually take several months to surface.
Manifestations of Vitamin A Deficiency:
- Visual Impairments: One of the most severe consequences of vitamin A deficiency is blindness, which results from the inability to synthesize adequate quantities of rhodopsin. A moderate deficiency can lead to night blindness, where individuals experience difficulties seeing in low light conditions. In contrast, a severe deficiency can cause xerophthalmia, characterized by extreme dryness and opacity of the cornea.
- Increased Susceptibility to Infections: There’s a heightened risk of mortality from infectious diseases, especially in malnourished children. Supplementation with vitamin A has proven to significantly reduce mortality rates from diseases such as measles and gastrointestinal infections.
- Epithelial Cell Dysfunction: A deficiency in vitamin A can lead to the abnormal function of various epithelial cells. This manifests as dry, scaly skin, inadequate secretion from mucosal surfaces, infertility, decreased synthesis of thyroid hormones, and elevated cerebrospinal fluid pressure due to inadequate absorption in the meninges.
- Bone Growth Abnormalities: Animals deficient in vitamin A can experience abnormal bone growth, leading to malformations. When the skull is affected, it can result in central nervous system disorders and optic nerve issues.
Besides the above, vitamin A deficiency can stem from factors such as inadequate dietary intake, impaired intestinal absorption, reduced liver storage, and chronic alcoholism. The symptoms of this deficiency are not immediate since the liver can sustain the body’s requirements for a few months. However, prolonged deficiency can lead to severe health issues.
Additional Manifestations Include:
- Growth Impact: Vitamin A deficiency can hinder growth due to skeletal formation impairment.
- Reproductive Issues: The deficiency adversely affects the reproductive system, leading to germinal epithelium degeneration and male sterility.
- Skin and Epithelial Cell Effects: The skin becomes rough and dry, with keratinization of the epithelial cells in the gastrointestinal, urinary, and respiratory tracts. This increases susceptibility to bacterial infections. Furthermore, vitamin A deficiency can lead to urinary stone formation.
Vitamin A excess/Symptoms of hypervitaminosis A
Hypervitaminosis A refers to the toxic effects that arise from excessive consumption of vitamin A. While the body requires vitamin A for various essential functions, an overabundance can lead to a range of adverse symptoms. It’s crucial to understand the manifestations and causes of this condition to ensure optimal health.
Symptoms of Hypervitaminosis A:
- Dermatological Effects: One of the primary symptoms of vitamin A toxicity is dermatitis, a condition characterized by inflamed and itchy skin.
- Neurological Concerns: Elevated intracranial tension is another manifestation of excessive vitamin A intake. This condition can lead to headaches and other neurological issues.
- Liver Enlargement: The liver plays a pivotal role in storing vitamin A. Therefore, an excessive intake can lead to liver enlargement, impacting its function and overall health.
- Skeletal Issues: Hypervitaminosis A can lead to skeletal decalcification, making bones more fragile. Additionally, individuals may experience tenderness in long bones.
- General Health Concerns: Other symptoms include weight loss, irritability, hair loss, and joint pains. It’s worth noting that elderly individuals are more susceptible to vitamin A toxicity, emphasizing the importance of monitoring vitamin A intake in this demographic.
The total serum vitamin A level, which typically ranges between 20–50 µg/dl, is elevated in cases of hypervitaminosis A. It’s essential to understand that free retinol or retinol bound to plasma lipoproteins can be harmful to the body. The symptoms of vitamin A toxicosis become evident when the retinol binding capacity of the retinol binding protein is exceeded. Furthermore, higher concentrations of retinol stimulate the synthesis of lysosomal hydrolases. The symptoms of hypervitaminosis A are believed to be a result of the destructive action of these hydrolases, especially on cell membranes.
Contrast with Carotenoids:
While excessive vitamin A intake can lead to toxicity, it’s important to differentiate between vitamin A and carotenoids. Excessive intake of carotenoids, such as those found in carrots and green vegetables, has not been reported to cause disease. Therefore, individuals need not avoid consuming these nutrient-rich foods out of fear of potential vitamin A toxicity.