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8.3: Vitamin A and Vision

  • Page ID
    21150
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    Learning Objectives

    • Describe the functions of Vitamin A.
    • Give examples of good food sources of Vitamin A.

    Vitamin A

    Vitamin A is a generic term for a group of similar compounds called retinoids. Retinol is the form of vitamin A found in animal-derived foods, and it is converted in the body to the biologically active forms of vitamin A: retinal and retinoic acid. About 10% of plant-derived carotenoids, including beta-carotene, can be converted in the body to retinoids and are another source of functional vitamin A. Vitamin A is fat-soluble and is packaged into chylomicrons in small intestine mucosal cells, and then transported to the liver. The liver stores and exports vitamin A as needed; it is released into the blood bound to a retinol-binding protein, which transports it to cells. Although the liver stores about 90% of our vitamin A, some vitamin A is also stored in our adipose (fat) tissue, kidneys, and lungs.

    Functions of Vitamin A

    Vitamin A has several important functions in the body, including maintaining vision and a healthy immune system. Many of vitamin A’s functions in the body are similar to the functions of hormones (for example, vitamin A can interact with DNA, causing a change in protein function). Vitamin A assists in maintaining healthy skin and the linings and coverings of tissues; it also regulates growth and development.

    Vision

    Retinol that is circulating in the blood is taken up by cells in the retina, where it is converted to retinal and is used as part of the pigment rhodopsin, which is involved in the eye’s ability to see under low light conditions. A deficiency in vitamin A thus results in less rhodopsin and a decrease in the detection of low-level light, a condition referred to as night blindness.

    Insufficient intake of dietary vitamin A over time can also cause complete vision loss. In fact, vitamin A deficiency is the number one cause of preventable blindness worldwide. Vitamin A not only supports the vision function of eyes but also maintains the coverings and linings of the eyes. Vitamin A deficiency can lead to the dysfunction of the linings and coverings of the eye, causing dryness of the eyes, a condition called xerophthalmia. This condition can progress, causing ulceration of the cornea and eventually blindness.

    Immunity

    The common occurrence of advanced xerophthalmia in children who died from infectious diseases led scientists to hypothesize that supplementing vitamin A in the diet for children with xerophthalmia might reduce disease-related mortality. In Asia in the late 1980s, targeted populations of children were administered vitamin A supplements, and the death rates from measles and diarrhea declined by up to 50%. Vitamin A supplementation in these deficient populations did not reduce the number of children who contracted these diseases, but it did decrease the severity of the diseases so that they were no longer fatal. Soon after the results of these studies were communicated to the rest of the world, the World Health Organization (WHO) and the United Nations Children’s Fund (UNICEF) commenced worldwide campaigns against vitamin A deficiency. UNICEF estimates that the distribution of over half a billion vitamin A capsules prevents 350,000 childhood deaths annually1.

    In the twenty-first century, science has demonstrated that vitamin A greatly affects the immune system. What we are still lacking are clinical trials investigating the proper doses of vitamin A required to help ward off infectious disease and how large of an effect vitamin A supplementation has on populations that are not deficient in this vitamin. This brings up one of our common themes in this text—micronutrient deficiencies may contribute to the development, progression, and severity of a disease, but this does not mean that an increased intake of these micronutrients will solely prevent or cure disease. The effect, as usual, is cumulative and depends on the diet as a whole, among other things.

    Growth and Development

    Vitamin A acts similarly to some hormones in that it is able to change the amount of proteins in cells by interacting with DNA. This is the primary way that vitamin A affects growth and development. Vitamin A deficiency in children is linked to growth retardation; however, vitamin A deficiency is often accompanied by protein malnutrition and iron deficiency, thereby confounding the investigation of vitamin A’s specific effects on growth and development.

    Both males and females require vitamin A in the diet to effectively reproduce. In the fetal stages of life, vitamin A is important for limb, heart, eye, and ear development; too much vitamin A in the diet of pregnant women can lead to birth defects.

    Vitamin A Imbalances

    Vitamin A is highly toxic, especially from supplements. During pregnancy, vitamin A toxicity can lead to spontaneous abortion or the baby being born with birth defects (as noted above). Toxicity can also lead to permanent damage of the liver and/or eyes. On the other end of the spectrum, night blindness is the most common disease of vitamin A deficiency.

    Vitamin A Recommendations

    There is more than one source of vitamin A in the diet. There is preformed vitamin A, which is abundant in many animal-derived foods, and there are carotenoids, which are found in high concentrations in yellow and orange colored fruits and vegetables and some oils.

    Some carotenoids are converted to retinol in the body by intestinal cells and liver cells. However, only small amounts of certain carotenoids are converted to retinol, meaning fruits and vegetables are not necessarily good sources of vitamin A. Beta-carotene dissolved in oil is more readily converted to retinol; one-half of a microgram of beta-carotene is converted to retinol. Overall, the carotenoids do not have the same biological potency of preformed vitamin A, but as you will soon find out, they have other attributes that influence health, most notably their antioxidant activity.

    The Recommended Dietary Allowances (RDAs) and Tolerable Upper Intake Levels (ULs) for different age groups for vitamin A are provided in Table \(\PageIndex{1}\)2.The RDA for vitamin A includes all sources of vitamin A. The amount of vitamin A obtained from carotenoids—the retinol activity equivalent (RAE)—can be calculated. For example, 12 micrograms of fruit- or vegetable-based beta-carotene will yield 1 microgram of retinol. The RDA for vitamin A is considered sufficient to support growth and development, reproduction, vision, and immune system function while maintaining adequate stores (good for four months) in the liver.

    Table \(\PageIndex{1}\): Dietary Reference Intakes for Vitamin A
    Age Group RDA (mcg/day) UL (mcg/day)
    Infants (0–6 months) 400* 600
    Infants (7–12 months) 500* 600
    Children (1–3 years) 300 600
    Children (4–8 years) 400 900
    Children (9–13 years) 600 1,700
    Adolescents (14–18 years) 900 (males), 700 (females) 2,800
    Adults (> 19 years) 900 (males), 700 (females) 3,000
    *denotes Adequate Intake

    Sources of Vitamin A

    Preformed vitamin A is found only in foods from animals, with the liver being the richest source because that’s where vitamin A is stored. Plant sources (dark green, orange, and deep yellow fruits and vegetables) provide beta-carotene which can be converted to vitamin A. See (Table \(\PageIndex{2}\))3 for a list of foods and their vitamin A content (given in mcg of Retinol Activity Equivalent).

    Table \(\PageIndex{2}\): Vitamin A Content of Various Foods
    Food Vitamin A (mcg RAE) % Daily Value
    Beef liver (3 ounces) 6,582 731
    Sweet potato (1 whole) 1,403 156
    Spinach, boiled (1/2 cup) 573 64
    Ricotta cheese (1 cup) 263 29
    Milk, fat-free (1 cup) 149 17

    Learn even more about this nutrient by reading the Vitamin A Fact Sheet from the National Institutes of Health, Office of Dietary Supplements.

    Key Takeaways

    • Vitamin A is a fat-soluble vitamin that plays a major role in vision.
    • Vitamin A can be found in animal foods and plant foods. Preformed vitamin A is abundant in many animal-derived foods. Carotenoids are found in high concentrations in yellow and orange colored fruits and vegetables and some oils.

    References

    1. Sommer A. Vitamin A Deficiency and Clinical Disease: An Historical Overview. The Journal of Nutrition. 2008;138(10):1835–1839. https://doi.org/10.1093/jn/138.10.1835.
    2. Summary Report of the Dietary Reference Intakes. nationalacademies.org. www.nationalacademies.org/our-work/summary-report-of-the-dietary-reference-intakes. Accessed June 29, 2020.
    3. Vitamin A - Health Professional Fact Sheet. ods.od.nih.gov. https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/. Accessed June 29, 2020.

    8.3: Vitamin A and Vision is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.

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