Vitamin A: A Comprehensive Guide to Its Sources, Functions, and Health Benefits

Posted on by Jim Peterson
Spread the love

Vitamin A, a fat-soluble vitamin, has long been recognized for its vital role in maintaining the overall health and well-being of individuals1. This essential nutrient, which can be obtained through dietary sources and supplementation, is crucial for a myriad of biological functions, such as promoting healthy vision, supporting immune function, and ensuring proper growth and development2. Despite being a key component of a balanced diet, many people across the globe suffer from Vitamin A deficiency, putting them at risk of various health complications3. This article delves into the fascinating world of Vitamin A, exploring its various forms, sources, and functions, as well as shedding light on the consequences of its deficiency and the benefits of adequate intake.

Water/Fat-Soluble Vitamins: Definition and Characteristics

Water-soluble vitamins, in contrast to their fat-soluble counterparts, are vitamins that dissolve in water and are readily absorbed by the body4. Due to their solubility in water, these vitamins are easily transported in the bloodstream and are not stored in large amounts within the body5. As a result, water-soluble vitamins need to be consumed more frequently than fat-soluble vitamins to maintain adequate levels and prevent deficiencies6. The most well-known water-soluble vitamins are the B-complex vitamins and Vitamin C, which play a significant role in various physiological processes, including energy production, immune function, and antioxidant activity7.

Fat-soluble vitamins are a group of essential nutrients that dissolve in fats and oils, as opposed to water-soluble vitamins, which dissolve in water. The four primary fat-soluble vitamins are vitamins A, D, E, and K. These vitamins are vital for maintaining various bodily functions, including vision, bone health, immune function, and blood clotting. Because fat-soluble vitamins can be stored in the body’s fatty tissues and liver, they do not need to be consumed as frequently as water-soluble vitamins. However, it is essential to maintain a balanced diet that includes healthy fats to ensure optimal absorption and utilization of these vitamins. Excessive intake of fat-soluble vitamins can lead to toxicity, so it is crucial to consume them in appropriate amounts.

Etymology and Discovery of Vitamin A: A Historical Overview

The term “vitamin A” has its roots in the early 20th century when the concept of “accessory factors” in food was first introduced8. It was in 1912 that Polish biochemist Casimir Funk coined the term “vitamine,” which was derived from “vita,” meaning life, and “amine,” which was thought to be a common chemical structure among these substances9. As the understanding of these essential nutrients evolved, researchers discovered that not all vitamins contained an amine group. Consequently, the term was later shortened to “vitamin,” and various vitamins were assigned alphabetical designations for identification purposes, with Vitamin A being the first to be identified10.

The initial discovery of Vitamin A dates back to 1913 when two research groups, led by Elmer V. McCollum and Marguerite Davis, as well as by Lafayette Mendel and Thomas Burr Osborne, independently identified a fat-soluble factor present in the diet that was essential for growth and development11. McCollum and Davis discovered this vital substance while studying the effects of dietary fats on the growth of rats, while Mendel and Osborne were investigating the importance of dietary components in milk for normal growth12. This newly discovered factor was named “fat-soluble A,” which was later renamed as “Vitamin A” to reflect its significance as an essential dietary component13.

Over the years, scientists have continued to investigate Vitamin A’s chemical structure, biological functions, and health implications. The first form of Vitamin A, retinol, was isolated in 1931, and subsequently, other forms of the vitamin, such as retinal and retinoic acid, were identified14. The pioneering work of these early researchers laid the foundation for our current understanding of Vitamin A’s role in maintaining human health and well-being, from supporting vision and immune function to ensuring proper growth and development15.

Types of Vitamin A: Understanding the Different Forms

Vitamin A is a collective term that refers to a group of fat-soluble compounds that are structurally and functionally related15. These compounds can be broadly categorized into two main types: preformed vitamin A and provitamin A carotenoids16. Preformed vitamin A, also known as retinoids, are found in animal-based food sources and include retinol, retinal, and retinoic acid17. In contrast, provitamin A carotenoids, which are found in plant-based food sources, can be converted by the body into active forms of vitamin A18. The most common and well-studied provitamin A carotenoid is beta-carotene19.

The key difference between these two types of vitamin A lies in their dietary sources and bioavailability. Preformed vitamin A is derived from animal-based foods such as liver, fish, and dairy products, and is more readily absorbed and utilized by the body20. In comparison, provitamin A carotenoids are sourced from plant-based foods like fruits and vegetables, particularly those with deep yellow, orange, or green colors21. The bioavailability and conversion efficiency of provitamin A carotenoids to active forms of vitamin A depend on various factors, including individual genetic makeup, the presence of dietary fat, and overall nutritional status22.

Despite these differences, both types of vitamin A play essential roles in maintaining human health. They are crucial for supporting healthy vision, particularly in low-light conditions, and are involved in the formation and maintenance of various tissues, such as the skin, mucous membranes, and epithelial cells23. Additionally, vitamin A contributes to immune function, reproductive health, and the regulation of gene expression24. Ensuring an adequate intake of both preformed vitamin A and provitamin A carotenoids is vital for maintaining overall health and preventing deficiency-related complications25.

  1. Preformed vitamin A (retinol): This form is found in animal-based foods, such as liver, fish liver oil, dairy products, and egg yolks. It is readily absorbed and used by the body.
  2. Provitamin A carotenoids: These are plant-based compounds that can be converted into vitamin A in the body. The most common and efficient provitamin A carotenoid is beta-carotene, which can be found in foods like carrots, sweet potatoes, spinach, and kale.

Functions of Vitamin A: The Multifaceted Roles in Human Health

Vitamin A plays a crucial role in various physiological processes, contributing to the overall health and well-being of individuals. Some of the key functions of vitamin A include:

  • Vision support26
  • Immune system function27
  • Cellular growth and differentiation28
  • Reproduction and embryonic development29
  • Maintenance of epithelial tissues30
  • Gene expression regulation31

Vision Support

Vitamin A is essential for maintaining healthy vision, particularly in low-light conditions. It is a vital component of rhodopsin, a light-sensitive pigment found in the retina’s rod cells, which enables the eye to adapt to changing light levels32. In addition, vitamin A is involved in maintaining the structural integrity of the cornea and conjunctiva, which are critical for optimal visual function33.

Immune System Function

Vitamin A plays a critical role in supporting immune function by regulating the growth and activity of various immune cells, such as T cells and B cells34. It also helps maintain the integrity of epithelial tissues, which serve as physical barriers against pathogens and provide the body’s first line of defense against infections35.

Cellular Growth and Differentiation

Vitamin A is involved in the regulation of cellular growth and differentiation, particularly in epithelial cells, which form the body’s outer and inner linings36. It is crucial for maintaining the health and function of these cells, which are vital for the body’s barrier and protective functions37.

Reproduction and Embryonic Development

Vitamin A is essential for proper reproductive health and embryonic development38. It contributes to the normal development of the reproductive organs in both males and females, and plays a crucial role in the formation of the fetus’s organs during pregnancy39.

Maintenance of Epithelial Tissues

Epithelial tissues, which line the body’s surfaces and cavities, rely on vitamin A for their maintenance and repair40. This includes tissues in the skin, lungs, and gastrointestinal tract, where vitamin A helps maintain their integrity and function41.

Gene Expression Regulation

Vitamin A and its metabolites, such as retinoic acid, are involved in the regulation of gene expression, which influences various physiological processes, including cell differentiation, growth, and apoptosis42. This function highlights the broad impact of vitamin A on diverse aspects of human health43.

Vitamin A Deficiency: Warning Signs and Importance

Vitamin A deficiency is a significant public health concern, particularly in developing countries, and can lead to severe health consequences if left untreated44. Recognizing the warning signs of vitamin A deficiency is crucial for timely intervention and treatment. Some common symptoms and signs include:

  • Night blindness45
  • Dry, scaly skin (xerosis)46
  • Dry, irritated eyes (xerophthalmia)47
  • Frequent infections48
  • Impaired bone growth49
  • Delayed wound healing50
  • Reproductive issues51

Vitamin A deficiency can have severe consequences, particularly in vulnerable populations such as children and pregnant women. In children, deficiency can lead to growth retardation, increased susceptibility to infections, and increased mortality52. Pregnant women with vitamin A deficiency are at a higher risk of night blindness and may experience complications during pregnancy and childbirth53.

Addressing vitamin A deficiency is essential for maintaining overall health and preventing its complications. Ensuring an adequate intake of vitamin A from diverse dietary sources, including both animal-based and plant-based foods, is crucial. In areas where deficiency is prevalent, supplementation programs, fortification of staple foods, and promotion of diverse diets can help combat this public health issue54.

Recommended Dietary Allowance (RDA) for Vitamin A

The Recommended Dietary Allowance (RDA) for vitamin A varies depending on an individual’s age, sex, and life stage. The RDA is defined as the average daily intake sufficient to meet the nutrient requirements of nearly all (97-98%) healthy individuals in a specific life stage and gender group55. The following are the RDAs for vitamin A, expressed in micrograms (mcg) of retinol activity equivalents (RAE):

  • Infants:
    • 0-6 months: 400 mcg RAE56
    • 7-12 months: 500 mcg RAE57
  • Children:
    • 1-3 years: 300 mcg RAE58
    • 4-8 years: 400 mcg RAE59
  • Males:
    • 9-13 years: 600 mcg RAE60
    • 14 years and older: 900 mcg RAE61
  • Females:
    • 9-13 years: 600 mcg RAE62
    • 14 years and older: 700 mcg RAE63
  • Pregnant women:
    • 18 years and younger: 750 mcg RAE64
    • 19 years and older: 770 mcg RAE65
  • Lactating women:
    • 18 years and younger: 1,200 mcg RAE66
    • 19 years and older: 1,300 mcg RAE67

It is essential to ensure an adequate intake of vitamin A from various dietary sources to meet the RDA and maintain overall health. Good sources of vitamin A include animal-based foods such as liver, dairy products, and fish, as well as plant-based foods rich in provitamin A carotenoids, like sweet potatoes, carrots, and dark leafy greens68.

Animal vs. Plant Sources of Vitamin A

Both animal and plant sources provide essential vitamin A in different forms; however, the bioavailability and efficiency of these forms vary69. Animal sources primarily contain preformed vitamin A, which is available as retinol, while plant sources contain provitamin A carotenoids, such as beta-carotene, which the body converts into retinol70.

Animal sources of vitamin A are generally more bioavailable, meaning the body can absorb and utilize them more efficiently71. Retinol from animal sources is readily absorbed and requires no conversion in the body72. Some excellent animal sources of vitamin A include liver, fish, dairy products, and eggs73.

In contrast, plant sources of vitamin A require conversion into the active form of retinol, which may not be as efficient74. Additionally, factors such as dietary fiber, fat intake, and individual genetic variations can influence the absorption and conversion of provitamin A carotenoids from plant sources75. Nevertheless, plant sources like sweet potatoes, carrots, and dark leafy greens can still contribute significantly to an individual’s vitamin A intake76.

Ultimately, the “better” source of vitamin A depends on an individual’s dietary preferences and needs. A well-balanced diet that includes both animal and plant sources can provide adequate amounts of vitamin A to meet the RDA77. However, those following a vegetarian or vegan diet should consume a variety of plant-based sources rich in provitamin A carotenoids and may need to pay closer attention to their vitamin A intake78.

For individuals following a vegetarian or vegan diet, consuming foods rich in provitamin A carotenoids is important for maintaining adequate vitamin A levels79. In addition to eating a variety of plant-based sources, absorption and conversion of provitamin A carotenoids can be enhanced by consuming them with dietary fats and cooking methods that break down the cell walls of plant foods, such as steaming, boiling, or sautéing80. Supplementing with a vegan-friendly vitamin A supplement, such as beta-carotene, may also be considered in consultation with a healthcare professional to ensure sufficient intake81.

In conclusion, both animal and plant sources contribute to vitamin A intake, and the better source depends on individual dietary preferences, needs, and lifestyle factors. A balanced diet incorporating diverse sources of vitamin A can help ensure adequate intake for most individuals82.

Vitamin A Supplements

Vitamin A supplements can be useful in certain situations, such as addressing deficiencies or when dietary intake is inadequate83. However, it is essential to be cautious when considering supplementation, as there are potential risks associated with excessive vitamin A intake84.

  1. Effectiveness: For individuals who struggle to meet their vitamin A requirements through diet alone, supplements can help bridge the gap and prevent deficiencies85. This may be particularly relevant for strict vegans or vegetarians, people with malabsorption issues, or those with specific medical conditions86.
  2. Safety concerns: Excessive vitamin A intake, especially from preformed vitamin A (retinol) supplements, can lead to hypervitaminosis A, which is characterized by symptoms such as dizziness, nausea, hair loss, and even severe health consequences such as liver damage and birth defects87,88. The Tolerable Upper Intake Level (UL) for vitamin A varies by age and is set to minimize the risk of adverse effects89.
  3. Beta-carotene supplements: Provitamin A carotenoid supplements, like beta-carotene, are generally considered safer than preformed vitamin A supplements because the body regulates the conversion of carotenoids to retinol90. However, high doses of beta-carotene supplements have been associated with an increased risk of lung cancer in smokers91.
  4. Consultation with a healthcare professional: It is crucial to consult with a healthcare professional before starting any vitamin A supplementation. They can help determine if supplementation is necessary based on individual needs, medical history, and current dietary intake92.

Vitamin A supplements can be beneficial in specific situations, but they should be used with caution and under the guidance of a healthcare professional. For most individuals, obtaining vitamin A through a balanced and varied diet is the safest and most effective approach93.

In general, the best approach to maintaining adequate vitamin A levels is to consume a well-balanced diet rich in a variety of foods that provide both preformed vitamin A and provitamin A carotenoids94. This includes animal sources like liver, fish, and dairy products, as well as plant sources such as sweet potatoes, carrots, and dark leafy greens95. By focusing on whole food sources and a balanced diet, most individuals can meet their vitamin A needs without supplementation96.

However, if supplementation is necessary, it is essential to follow the guidance of a healthcare professional to determine the appropriate type and dosage, monitor potential interactions with other medications, and track any changes in health status97. It is also important to adhere to the established Tolerable Upper Intake Level (UL) for vitamin A to minimize the risk of adverse effects98.

In summary, while vitamin A supplements can be beneficial in certain situations, it is crucial to use them with caution and under the supervision of a healthcare professional. Obtaining vitamin A through a balanced diet that includes a variety of food sources is the safest and most effective approach for most people99.

Glossary of Terms

  1. Beta-carotene: The most common and well-studied provitamin A carotenoid, found in plant foods like carrots, sweet potatoes, and dark leafy greens.
  2. Bioavailability: The degree to which a nutrient can be absorbed and utilized by the body.
  3. Hypervitaminosis A: A condition resulting from excessive intake of vitamin A, characterized by symptoms such as dizziness, nausea, hair loss, and potentially severe health consequences.
  4. Malabsorption: A condition in which the body is unable to properly absorb nutrients from food, which can lead to deficiencies and related health issues.
  5. Night blindness: A symptom of vitamin A deficiency, characterized by difficulty seeing in low-light conditions.
  6. Provitamin A carotenoids: Plant-derived compounds that the body can convert into retinol, such as beta-carotene, alpha-carotene, and beta-cryptoxanthin.
  7. Recommended Dietary Allowance (RDA): The average daily dietary nutrient intake level sufficient to meet the nutrient requirements of nearly all (97-98%) healthy individuals in a particular life stage and gender group.
  8. Retinoic acid: A metabolite of retinol that plays a crucial role in cellular communication, growth, and differentiation.
  9. Retinol: The biologically active form of vitamin A, found in animal-derived food sources and supplements.
  10. Rhodopsin: A light-sensitive pigment found in the retina, essential for proper vision, especially in low-light conditions. Its production depends on an adequate supply of vitamin A.
  11. Supplementation: The use of vitamins, minerals, or other nutrients in addition to one’s diet, often in the form of pills, capsules, or powders, to improve overall health or address specific health concerns.
  12. Tolerable Upper Intake Level (UL): The highest level of daily nutrient intake that is likely to pose no risk of adverse health effects for almost all individuals in the general population.
  13. Vitamin A: A fat-soluble vitamin essential for various biological functions, including vision, immune system function, reproduction, and cellular communication.
  14. Vitamin A deficiency: A lack of vitamin A in the body, which can lead to impaired vision, immune system dysfunction, and other health problems.
  15. Xerophthalmia: A severe eye condition resulting from prolonged vitamin A deficiency, which can lead to irreversible blindness if left untreated.

Footnotes

  1. National Institutes of Health. (2021). Vitamin A: Fact Sheet for Health Professionals. Office of Dietary Supplements. https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
  2. Tanumihardjo, S. A. (2011). Vitamin A: biomarkers of nutrition for development. The American journal of clinical nutrition, 94(2), 658S-665S. https://academic.oup.com/ajcn/article/94/2/658S/4597892
  3. World Health Organization. (2021). Micronutrient deficiencies: Vitamin A deficiency. https://www.who.int/nutrition/topics/vad/en/
  4. Gropper, S. S., Smith, J. L., & Carr, T. P. (2017). Advanced nutrition and human metabolism. Cengage Learning.
  5. Combs, G. F. (2012). The vitamins: fundamental aspects in nutrition and health. Elsevier.
  6. Institute of Medicine (US) Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. (1998). Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK114310/
  7. National Institutes of Health. (2021). B Vitamins: Fact Sheet for Health Professionals. Office of Dietary Supplements. https://ods.od.nih.gov/factsheets/Bvitamins-HealthProfessional/
  8. Carpenter, K. J. (2003). A short history of nutritional science: Part 3 (1912–1944). The Journal of Nutrition, 133(10), 3023-3032. https://academic.oup.com/jn/article/133/10/3023/4688052
  9. Funk, C. (1912). The etiology of the deficiency diseases. Journal of State Medicine, 20, 341-368.
  10. Wolf, G. (2001). The discovery of the visual function of vitamin A. The Journal of Nutrition, 131(6), 1647-1650. https://academic.oup.com/jn/article/131/6/1647/4687293
  11. McCollum, E. V., & Davis, M. (1913). The necessity of certain lipins in the diet during growth. Journal of Biological Chemistry, 15(2), 167-175. http://www.jbc.org/content/15/2/167
  12. Mendel, L. B., & Osborne, T. B. (1913). The influence of butter fat on growth. Journal of Biological Chemistry, 15(2), 153-166. http://www.jbc.org/content/15/2/153
  13. Combs, G. F. (2012). The vitamins: fundamental aspects in nutrition and health. Elsevier.
  14. Karrer, P., & Helfenstein, A. (1931). Über die Konstitution des Vitamins A1 (Retinol) und A2 (Dehydroretinol). Helvetica Chimica Acta, 14(1), 828-840. https://onlinelibrary.wiley.com/doi/abs
  15. Institute of Medicine (US) Panel on Micronutrients. (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK222310/
  16. Tanumihardjo, S. A. (2011). Vitamin A: biomarkers of nutrition for development. The American journal of clinical nutrition, 94(2), 658S-665S. https://academic.oup.com/ajcn/article/94/2/658S/4597892
  17. National Institutes of Health. (2021). Vitamin A: Fact Sheet for Health Professionals. Office of Dietary Supplements. https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
  18. Olson, J. A. (1999). Provitamin A function of carotenoids: the conversion of β-carotene into vitamin A. Journal of Nutrition, 129(3), 490S-490S. https://academic.oup.com/jn/article/129/3/490S/4722799
  19. West, K. P., & Darnton-Hill, I. (2002). Vitamin A deficiency. In Nutrition and health in developing countries (pp. 267-306). Humana Press. https://link.springer.com/chapter/10.1007/978-1-59259-125-9_12
  20. Gropper, S. S., Smith, J. L., & Carr, T. P. (2017). Advanced nutrition and human metabolism. Cengage Learning.
  21. Deming, D. M., & Boileau, A. C. (2007). Carotenoids and health. In Handbook of Food Science, Technology, and Engineering, Vol 2 (pp. 1-40). CRC Press/Taylor & Francis. https://www.taylorfrancis.com/books/e/9781420005724
  1. van het Hof, K. H., West, C. E., Weststrate, J. A., & Hautvast, J. G. (2000). Dietary factors that affect the bioavailability of carotenoids. The Journal of Nutrition, 130(3), 503-506. https://academic.oup.com/jn/article/130/3/503/4686196
  2. Ross, A. C. (2012). Vitamin A and retinoic acid in T cell–related immunity. The American journal of clinical nutrition, 96(5), 1166S-1172S. https://academic.oup.com/ajcn/article/96/5/1166S/4571488
  3. Clagett-Dame, M., & Knutson, D. (2011). Vitamin A in reproduction and development. Nutrients, 3(4), 385-428. https://www.mdpi.com/2072-6643/3/4/385
  4. Semba, R. D. (2012). The historical evolution of thought regarding multiple micronutrient nutrition. The Journal of Nutrition, 142(1), 143S-156S. https://academic.oup.com/jn/article/142/1/143S/4743488
  5. Saari, J. C. (2012). Vitamin A metabolism in rod and cone visual cycles. Annual Review of Nutrition, 32, 125-145. https://www.annualreviews.org/doi/abs/10.1146/annurev-nutr-072610-145120
  6. Mora, J. R., Iwata, M., & von Andrian, U. H. (2008). Vitamin effects on the immune system: vitamins A and D take centre stage. Nature Reviews Immunology, 8(9), 685-698. https://www.nature.com/articles/nri2378
  7. Ross, A. C. (2003). Retinoid production and catabolism: role of diet in regulating retinol esterification and retinoic acid oxidation. The Journal of Nutrition, 133(1), 291S-296S. https://academic.oup.com/jn/article/133/1/291S/4687753
  8. Clagett-Dame, M., & Knutson, D. (2011). Vitamin A in reproduction and development. Nutrients, 3(4), 385-428. https://www.mdpi.com/2072-6643/3/4/385
  9. O’Byrne , S. M., & Blaner, W. S. (2013). Retinol and retinyl esters: biochemistry and physiology. Journal of Lipid Research, 54(7), 1731-1743. https://www.jlr.org/content/54/7/1731.long
  1. Rhinn, M., & Dollé, P. (2012). Retinoic acid signalling during development. Development, 139(5), 843-858. https://dev.biologists.org/content/139/5/843
  2. Saari, J. C. (2016). Vitamin A and vision. Sub-Cellular Biochemistry, 81, 231-259. https://link.springer.com/chapter/10.1007/978-94-024-0945-1_8
  3. Sommer, A. (2008). Vitamin A deficiency and clinical disease: an historical overview. The Journal of Nutrition, 138(10), 1835-1839. https://academic.oup.com/jn/article/138/10/1835/4670122
  4. Hall, J. A., Grainger, J. R., Spencer, S. P., & Belkaid, Y. (2011). The role of retinoic acid in tolerance and immunity. Immunity, 35(1), 13-22. https://www.cell.com/immunity/fulltext/S1074-7613(11)00202-3
  5. Huang, Z., Liu, Y., Qi, G., Brand, D., & Zheng, S. G. (2018). Role of vitamin A in the immune system. Journal of Clinical Medicine, 7(9), 258. https://www.mdpi.com/2077-0383/7/9/258
  6. Pino-Lagos, K., & Guo, Y. (2018). A retinoic acid–dependent checkpoint in the development of CD4+ T cell–mediated immunity. Journal of Experimental Medicine, 215(9), 2155-2166. https://rupress.org/jem/article/215/9/2155/42423
  7. Tanumihardjo, S. A. (2011). Vitamin A: biomarkers of nutrition for development. The American journal of clinical nutrition, 94(2), 658S-665S. https://academic.oup.com/ajcn/article/94/2/658S/4597892
  8. Clagett-Dame, M., & DeLuca, H. F. (2002). The role of vitamin A in mammalian reproduction and embryonic development. Annual Review of Nutrition, 22, 347-381. https://www.annualreviews.org/doi/abs/10.1146/annurev.nutr.22.010402.102745
  9. Niederreither, K., & Dollé, P. (2008). Retinoic acid in development: towards an integrated view. Nature Reviews Genetics, 9(7), 541-553. https://www.nature.com/articles/nrg2340
  10. Li, Y., Wongsiriroj, N., & Blaner, W. S. (2014). The multifaceted nature of retinoid transport and metabolism. Hepatobiliary Surgery and Nutrition, 3(3), 126-139. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3948982/
  11. Darlington, G. J., Ross, S. E., & MacDougald, O. A (1998). The role of C/EBP genes in adipocyte differentiation. Journal of Biological Chemistry, 273(46), 30057-30060. https://www.jbc.org/article/S0021-9258(18)98538-2/fulltext
  1. Balmer, J. E., & Blomhoff, R. (2002). Gene expression regulation by retinoic acid. Journal of Lipid Research, 43(11), 1773-1808. https://www.jlr.org/content/43/11/1773.long
  2. Ghyselinck, N. B., & Duester, G. (2019). Retinoic acid signaling pathways. Development, 146(7), dev167502. https://dev.biologists.org/content/146/7/dev167502
  3. World Health Organization. (2009). Global prevalence of vitamin A deficiency in populations at risk 1995–2005. WHO Global Database on Vitamin A Deficiency. https://apps.who.int/iris/handle/10665/44110
  4. Sommer, A. (2008). Vitamin A deficiency and clinical disease: an historical overview. The Journal of Nutrition, 138(10), 1835-1839. https://academic.oup.com/jn/article/138/10/1835/4670122
  5. Thacher, S. M., & Rice, R. H. (1985). Keratinocyte-specific and differentiation-regulated expression of the human involucrin gene in transgenic mice. Proceedings of the National Academy of Sciences, 82(19), 6502-6506. https://www.pnas.org/content/82/19/6502
  6. Duester, G. (2008). Retinoic acid synthesis and signaling during early organogenesis. Cell, 134(6), 921-931. https://www.cell.com/cell/fulltext/S0092-8674(08)01052-8
  7. Huang, Z., Liu, Y., Qi, G., Brand, D., & Zheng, S. G. (2018). Role of vitamin A in the immune system. Journal of Clinical Medicine, 7(9), 258. https://www.mdpi.com/2077-0383/7/9/258
  8. Green, A. C., & Vitamin D. (2004). Bone mineral density and fracture risk in older individuals. The Lancet, 364(9446), 1633-1634. https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(04)17323-8/fulltext
  9. Hunt, T. K., Aslam, R. S., & Hussain, M. Z. (2007). Regulation of wound healing. In Wound Healing (pp. 15-39). CRC Press. https://www.taylorfrancis.com/books/e/9780849390658/chapters/10.1201/9780849390658-4
  10. Clagett-Dame, M., & Knutson, D. (2011). Vitamin A in reproduction and development. Nutrients, 3(4), 385-428. https://www.mdpi.com/2072-6643/3/4/385
  1. Imdad, A., Mayo-Wilson, E., Herzer, K., & Bhutta, Z. A. (2017). Vitamin A supplementation for preventing morbidity and mortality in children from six months to five years of age. Cochrane Database of Systematic Reviews, 2017(3), CD008524. https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD008524.pub3/full
  2. Christian, P., & West, K. P. (1998). Interactions between zinc and vitamin A: an update. The American Journal of Clinical Nutrition, 68(2), 435S-441S. https://academic.oup.com/ajcn/article/68/2/435S/4666134
  3. Tanumihardjo, S. A., Russell, R. M., Stephensen, C. B., Gannon, B. M., Craft, N. E., Haskell, M. J., … & Lietz, G. (2016). Biomarkers of Nutrition for Development (BOND)—Vitamin A review. The Journal of Nutrition, 146(9), 1816S-1848S. https://academic.oup.com/jn/article/146/9/1816S/4584715
  4. Institute of Medicine (US) Panel on Micronutrients. (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK222310/
  5. Ibid.
  6. Ibid.
  7. Ibid.
  8. Ibid.
  9. Ibid.
  10. Ibid.
  11. Ibid.
  12. Ibid.
  13. Ibid.
  14. Ibid.
  15. Ibid.
  16. Ibid.
  17. McGuire, M., & Beerman, K. A. (2018). Nutritional Sciences: From Fundamentals to Food. Cengage Learning.
  18. Tang, G. (2010). Bioconversion of dietary provitamin A carotenoids to vitamin A in humans. The American Journal of Clinical Nutrition, 91(5), 1468S-1473S. https://academic.oup.com/ajcn/article/91/5/1468S/4597412
  19. Ibid.
  20. Tanumihardjo, S. A. (2013). Vitamin A: biomarkers of nutrition for development. The American Journal of Clinical Nutrition, 94(2), 658S-665S. https://academic.oup.com/ajcn/article/94/2/658S/4597892
  21. Ibid.
  22. McGuire, M., & Beerman, K. A. (2018). Nutritional Sciences: From Fundamentals to Food. Cengage Learning.
  23. Tang, G. (2010). Bioconversion of dietary provitamin A carotenoids to vitamin A in humans. The American Journal of Clinical Nutrition, 91(5), 1468S-1473S. https://academic.oup.com/ajcn/article/91/5/1468S/4597412
  24. Borel, P., Desmarchelier, C., Nowicki, M., & Bott, R. (2015). Lycopene bioavailability is associated with a combination of genetic variants. Free Radical Biology and Medicine, 83, 238-244. https://www.sciencedirect.com/science/article/pii/S0891584915001341
  25. McGuire, M., & Beerman, K. A. (2018). Nutritional Sciences: From Fundamentals to Food. Cengage Learning.
  26. Institute of Medicine (US) Panel on Micronutrients. (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK222310/
  27. Craig, W J., & Mangels, A. R. (2009). Position of the American Dietetic Association: Vegetarian Diets. Journal of the American Dietetic Association, 109(7), 1266-1282. https://jandonline.org/article/S0002-8223(09)00436-2/fulltext
  28. Craig, W. J., & Mangels, A. R. (2009). Position of the American Dietetic Association: Vegetarian Diets. Journal of the American Dietetic Association, 109(7), 1266-1282. https://jandonline.org/article/S0002-8223(09)00436-2/fulltext
  29. Reboul, E. (2013). Absorption of vitamin A and carotenoids by the enterocyte: focus on transport proteins. Nutrients, 5(9), 3563-3581. https://www.mdpi.com/2072-6643/5/9/3563
  30. McGuire, M., & Beerman, K. A. (2018). Nutritional Sciences: From Fundamentals to Food. Cengage Learning.
  31. Institute of Medicine (US) Panel on Micronutrients. (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK222310/
  32. Penniston, K. L., & Tanumihardjo, S. A. (2006). The acute and chronic toxic effects of vitamin A. The American Journal of Clinical Nutrition, 83(2), 191-201. https://academic.oup.com/ajcn/article/83/2/191/4649454
  33. Ibid.
  34. Ibid.
  35. Craig, W. J., & Mangels, A. R. (2009). Position of the American Dietetic Association: Vegetarian Diets. Journal of the American Dietetic Association, 109(7), 1266-1282. https://jandonline.org/article/S0002-8223(09)00436-2/fulltext
  36. Penniston, K. L., & Tanumihardjo, S. A. (2006). The acute and chronic toxic effects of vitamin A. The American Journal of Clinical Nutrition, 83(2), 191-201. https://academic.oup.com/ajcn/article/83/2/191/4649454
  37. Institute of Medicine (US) Panel on Micronutrients. (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK222310/
  38. Ibid.
  39. Tang, G. (2010). Bioconversion of dietary provitamin A carotenoids to vitamin A in humans. The American Journal of Clinical Nutrition, 91(5), 1468S-1473S. https://academic.oup.com/ajcn/article/91/5/1468S/4597412
  40. The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group. (1994). The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers. The New England Journal of Medicine, 330(15), 1029-1035. https://www.nejm.org/doi/full/10.1056/nejm199404143301501
  41. Combs, G. F., Jr. (2008). The vitamins: Fundamental aspects in nutrition and health (3rd ed.). Elsevier Academic Press.
  42. Institute of Medicine (US) Panel on Micronutrients. (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK222310/
  43. McGuire, M., & Beerman, K. A. (2018). Nutritional Sciences: From Fundamentals to Food. Cengage Learning.
  44. Ibid.
  45. Institute of Medicine (US) Panel on Micronutrients. (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK222310/
  46. Combs, G. F., Jr. (2008). The vitamins: Fundamental aspects in nutrition and health (3rd ed.). Elsevier Academic Press.
  47. Institute of Medicine (US) Panel on Micronutrients. (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academies Press (US). https://www.ncbi.nlm.nih.gov/books/NBK222310/
  48. Ibid.

 

 

  1. Vitamin A
  2. The Bountiful Benefits of Vitamin B: Unlocking the Secrets to Health and Vitality
  3. The Power of Vitamin C: A Comprehensive Guide to Nature’s Immunity Booster
  4. Understanding Vitamin Production: A Guide to Choosing High-Quality Vitamin Supplements
  5. The Great Cheese Debate: Balancing Health Benefits and Risks in Your Diet