Vitamin A deficiency is the leading preventable cause of childhood blindness worldwide. Vitamin A is also called the "anti-infective vitamin" as it’s critically important for immune function. In addition to its effects on vision and immune function, vitamin A plays a role in other processes in the body. This article will discuss vitamin A's role in the body, how to test vitamin A levels, and how to ensure you're getting proper amounts in your diet.
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What is Vitamin A?
Vitamin A is an umbrella term encompassing a group of molecules called retinoids. Retinol, retinal, and retinoic acid are the active retinoids found in the body. These fat-soluble molecules are derived from two main sources: preformed vitamin A and provitamin A carotenoids. Preformed vitamin A includes retinol and retinyl esters and is primarily found in animal products. Provitamin A carotenoids are found in plants, often giving them their color, and are converted into retinoids in the intestines. Beta-carotene, alpha-carotene, and beta-cryptoxanthin are the most common provitamin A carotenoids. Most vitamin A consumption is in the form of preformed vitamin A.
What is Vitamin A's Role in The Body?
Vitamin A plays an essential role in eyesight, regulation of gene expression, immunity, development, and red blood cell production.
Eyesight
Vitamin A is essential for proper functioning in many parts of the eye, including the cornea, conjunctival membranes, and retina.
Gene Expression
Vitamin A has various roles in gene expression. Retinoic acid has the potential to modulate over 500 different genes, including vitamin D, steroids (including sex hormones), thyroid hormones, and more. Genes in the eye, muscle, and white adipose tissue seem responsive to retinol. Finally, retinal has a modulatory role in lipid, or fat, metabolism.
Immunity
Retinoic acid is found in the mucosal lining of the pulmonary system, digestive tract, and urinary tract. This lining is the body's first defense against foreign invaders. Retinoic acid is also found in the lymph nodes of the areas mentioned above.
Development
Vitamin A is essential for the formation of various organs, including the heart, lungs, and eyes. Additionally, vitamin A is important for the development of limbs.
Red Blood Cell Production
Red blood cells (RBCs) are responsible for carrying oxygen throughout the body. Vitamin A has the ability to transfer iron into the developing red blood cell, where it will be utilized to make hemoglobin. Hemoglobin is where oxygen is carried on the RBC.
How to Test Vitamin A Levels
Vitamin A deficiencies are rare in the United States. However, they still occur in areas where consumption of provitamin A and preformed vitamin A foods are low, primarily due to the population's diet or low socioeconomic status. Vitamin A deficiency rates are 44% and 48% in South Asia and sub-Saharan Africa, respectively. Children and pregnant women in the middle to lower economic areas seem to be most at risk for vitamin A deficiencies. Populations at risk for lower levels of vitamin A include preterm babies, people with cystic fibrosis and gastrointestinal disorders, and certain populations that live in middle to low-income areas, including infants, children, and pregnant and lactating women.
Xerophthalmia is the most common sign of a vitamin A deficiency. Xerophthalmia is a condition of the eye that occurs when vitamin A stores are low and begins with difficulty seeing at night but can eventually lead to permanent blindness. For children, vitamin A deficiency is one of the most common causes of preventable blindness.
Vitamin A deficiency increases the risk of poor outcomes for children against viral diseases, specifically measles and infectious diarrhea. Chronic vitamin A deficiency may lead to lung developmental problems, pneumonia, and other respiratory diseases. People with chronic vitamin A deficiency are also at an increased risk for anemia and death.
Hypervitaminosis A, or vitamin A toxicity, if chronic, can lead to symptoms of fatigue, depression, joint and muscle pain, and dry skin. Elevated liver enzymes will be seen on blood work as vitamin A is stored in the liver and thus can damage it at high levels. Acute hypervitaminosis A, occurring days to weeks after an excessive intake of vitamin A, can cause nausea, dizziness, blurred vision, severe headaches, muscle aches, and balance issues. Severe cases of acute hypervitaminosis A can cause increased brain pressure and may lead to drowsiness, coma, and death. High levels of vitamin A can be achieved through supplementation and certain medications.
Pregnant women are often advised to avoid Vitamin A supplements as taking higher amounts of preformed vitamin A can lead to congenital birth defects, including malformations of the skull, eye, lungs, and heart.
Beta-carotene consumption does not seem to carry the same risk of preformed vitamin A. Excessive intake of beta-carotene may cause carotenodermia, a condition where the skin turns orange. This condition is reversible once the person stops consuming beta-carotene. However, there is evidence that excessive beta-carotene supplementation in male smokers may increase lung cancer risk and mortality. Additionally, a study assessing beta carotene paired with retinyl palmitate in males who currently or formerly smoked and some men who have been exposed to asbestos can increase the risk and mortality of lung cancer.
How to Make Sure You Are Getting Enough Vitamin A in Your Diet
The Food and Nutrition Board at the National Academies of Sciences, Engineering, and Medicine created dietary recommendations for vitamins and minerals called Dietary Reference Intakes (DRIs). One such DRI is the Recommended Daily Allowance or the amount of a vitamin or mineral that is sufficient to reach nutritional adequacy in most healthy adults. For Vitamin A, RDA units are in retinol activity equivalents (RAEs), encompassing both retinol and the provitamin A carotenoids, as the carotenoids will eventually be converted into retinol. One microgram (mcg) RAE equals one mcg of retinol, two mcg beta-carotene from supplements, 12 mcg beta-carotene from foods, 24 mcg alpha-carotene, or 24 mcg beta-cryptoxanthin.
Vitamin A RAE
Requirements for pregnant and lactating women often differ from the general population due to increased nutritional demands. The RAE for pregnant women aged 14-18 is 750 mcg, and 770 mcg RAE for 19+ years old. For lactating women, the RAE is 1,200 mcg for 14-18 year-olds and 1,300 mcg RAE for 19+ year-olds.
For those under one year, insufficient evidence to form RDAs has led to the formation of Adequate Intakes (AIs). AIs are only created when there is not enough evidence to form RDAs. AIs are levels assumed to meet nutritional requirements for a given population. The AI for birth to six months is 400 mcg RAE, and for seven to 12 months is 500 mcg RAE for both males and females.
Vitamin A can be sourced from food and supplements.
High Food Sources of Vitamin A
It's important to note that the Food and Drug Administration (FDA), the organization responsible for food label regulation, does not require a product to list vitamin A content unless it has been added to the food. Foods at 20% or higher are considered high sources of vitamin A.
Supplements
Vitamin A is available as a stand-alone supplement and is usually in multivitamins. The most common forms of vitamin A seen in supplements are retinyl palmitate, and retinyl acetate, with an absorption rate between 70-90%, and beta carotene, with an absorption rate of 8.7 to 65%.
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Health Benefits of Vitamin A
Vitamin A is an essential vitamin from conception to the end of life. With direct effects on the immune system, vision, gene expression, and oxygenation of the tissues, vitamin A plays a significant role in health quality. Thus, vitamin A testing can help assess healthy bodily functioning.
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