How does Vitamin A benefit health?
Vitamin A is a fat-soluble vitamin essential for a number of functions in the body such as eye health, immune function and growth. It is readily available in foods and supplements in different forms that are converted by the body into vitamin A for use. In this article, we discuss the importance of vitamin A to health alongside deficiency symptoms, food sources and supplementation options.
Vitamin A is readily found in animal foods as retinol and in plant foods as β-carotene (also known as provitamin A). In addition, there are some synthetic vitamin A forms used for therapeutic or medical purposes, some of which are applied topically. Oral synthetic forms and β-carotene are converted in the intestines into retinol. Retinol is mixed with bile acids in the intestines before being absorbed and transported to the liver. In the liver, vitamin A is either redirected to cells or moved into storage. The liver is the main storage organ, but vitamin A can also be stored in other organs such as kidneys, adrenal glands, lungs and adipose (fat) tissue. In cells, vitamin A becomes active to support gene activity, enzyme processes and vision. Notably, vitamin A plays a key role in converting light that enters the eyes into a visual image. Once used, vitamin A is eliminated mainly via the kidneys in urine.1-3
Symptoms of deficiency
Vitamin A deficiency symptoms can take a long time to manifest due to our ability to store vitamin A in organs across the body.1
The most characteristic consequence of vitamin A deficiency is impaired vision - especially in reduced light - which, if not corrected, can lead to permanent total blindness.
Vitamin A deficiency is also noticeable as increased inflammation and susceptibility to infection, due to reduced immune function. Deficiency particularly affects epithelial cells, the cells that make up the lining of surfaces in, for example, the lungs, intestines and blood vessels. This can lead to bronchial and lung problems, dry hard skin, itchy rashes, hardened sweat glands, weight loss and changes in urogenital and reproductive function, as well as in gastro-intestinal function and health. Vitamin A deficiency may also show as loss of taste and/or smell, changes in nerve function, inadequate tooth enamel formation and abnormalities in bone formation. Deficiency affects iron levels in the body and can cause anaemia.1
Health Benefits of Vitamin A
The eye retina is responsible for visual perception. It is sensitive to light levels and vitamin A is required for the signalling between the eye and the brain to create a visual image. Vitamin A deficiency leads to low-light vision impairment (night blindness), which is reversible once plasma vitamin A levels are corrected.4 The ability of our eyes to adapt to darkness is affected by age and poor removal of used vitamin A from eye’s cells. Both effects can lead to retinal degeneration and loss of sight.5 Vitamin A deficiency is also associated with dry eye disease, reduced tear quality, poor wound healing and increased inflammation.6,7
Vitamin A plays an important role in ensuring our genes function correctly to maintain health. It influences genes in tasks such as tissue development and regeneration; cell formation, division and turnover; and the creation of enzymes and other proteins the body needs.1,8
Vitamin A is essential for reproduction. It is involved in sperm quality and quantity, as well as embryo formation and foetal development. Vitamin A deficiency in the pregnant mother can lead to a range of structural, cardiovascular and nervous system abnormalities, collectively known as vitamin A deficiency syndrome. Babies are born vitamin A deficient and develop their reserves via breast milk or fortified formula milk.1,9,10
Vitamin A is involved with cell growth and tissue development as well as the removal of old or faulty cells, meaning it has the potential to drive cancer or inhibit it. Vitamin A may prevent the proliferation of lymphoma and non-melanoma skin cancer, lung, liver and solid ovarian tumour cells. Both vitamin A and β-carotene act as antioxidants in the body, mopping up damaging “free radicals", helping to prevent potential cancerous cell development. Vitamin A and its derivatives have recently been used in cancer treatment.11–14
Immunity and inflammation
Vitamin A plays an important role in immune function and the control of inflammation. It is involved in epithelial barrier development (the cell layer forming the intestinal lining, skin, lung) and isan integral part of mucous membrane structures, promoting mucin secretion. Vitamin A plays a critical role in activating the inflammatory response to an infection, pathogen, allergen, tissue wound or autoimmunity. It regulates the supply of immune cells from bone marrow via the thymus gland and controls healing and repair of any damage, ultimately downgrading the immune response as appropriate.1,15,16
Vitamin A deficiency is associated with increased infections, diarrhoea and poor response to measles in children. Deficiency in vitamin A can worsen a range of conditions such as inflammatory bowel diseases (e.g., Crohn’s, colitis), skin conditions (e.g., psoriasis, skin ageing), brain diseases (e.g., Alzheimer’s, multiple sclerosis), lung conditions (e.g., asthma, rhinitis) and obesity.15–17
Vitamin A is involved in the life cycle of cells, including cells required for bone formation and bone loss. It works in conjunction with vitamin D in bone metabolism, affecting bone mineral density and development of osteopenia and osteoporosis.1,18
Obesity, Type 2 Diabetes, Liver and Heart diseases
Vitamin A status affects blood cholesterol and lipid levels in the body, which in turn may influence the development of atherosclerosis and non-alcoholic fatty liver disease. Vitamin A is also involved in the development and regulation of the pancreas, therefore indirectly affecting insulin resistance, blood glucose control and Type 2 diabetes. Vitamin A may have an anti-obesity action through its effect on energy production in skeletal muscles and in the mitochondria (our cells’ energy batteries).19–23
Recommended daily intake of vitamin A
The UK recommended daily intake values for Vitamin A are:24
AGE &RECOMMENDED DAILY INTAKE
- Children aged 1-6*: 400µg RAE** / 1333IU
- Children aged 7-10: 500µg RAE** / 1333IU
- Children aged 11-15: 600µg RAE** / 1333IU
- Males aged 15 and above: 700µg RAE** / 1333IU
- Females aged 15 and above: 600µg RAE** / 1333IU
*Children aged 6 months to 5 years are recommended to take a vitamin supplement containing vitamins A, C and D as a precautionary measure as the diet may not provide sufficient intake. Infants who are bottle-fed should not be supplemented unless they are receiving less than 500ml (1 pint) of infant formula per day.24
**Retinol Activity Equivalent (1IU Vitamin A = 0.3µg RAE) is used to consolidate all forms of vitamin A and their ability to provide vitamin A to the body into a single unit of measure.
Top Food Sources of Vitamin A
Vitamin A is available in both animal and plant foods.25
Vitamin A from animal sources (retional) is almost completely absorbed, especially in the presence of fats and zinc. Retinol levels depend on the season, feed and breed of animals. Examples of animal sources are (µg retinol per 100g edible portion):25
- Milk, whole 36µg
- Cheese, Cheddar 364µg
- Butter, unsalted 958µg
- Liver, calf, fried 25200µg
- Kidney, lamb, fried 110µg
- Cod liver Oil 18000µg
- Egg, whole, boiled 120µg
- Fish liver oil 18000µg
Plant foods provide β-carotene, which is less available for use by the body. As little as 1/3 of β-carotene consumed may be converted to retinol for use in the body and there are great variations among individuals. The conversion rateis dependent on the quantity of β-carotene in the food consumed, retinol levels in the body and the fat content of the diet.1 Examples of plant sources are (µg β-carotene per 100g edible portion):25
- Red peppers, raw 580
- Carrot, raw 11800
- Pumpkin, boiled 955
- Spinach, raw 3540
- Tomatoes, raw 349
- Watercress, raw 2520
- Sweet potato, boiled 3960
- Kale, boiled 3380
- Cabbage, green, raw 454
- Broccoli, boiled 598
- Green Beans, boiled 143
- Mangetout, boiled 665
- Apricots, ready-to-eat 545
- Melon, cantaloupe, flesh 1770
- Mango, flesh, raw 696
- Papaya, flesh, raw 810
- Plums, stewed 315
- Nectarine 114
Note: herbs and spices often contain high levels of β-carotene but are only used in small quantities. They can be useful additions to meals, however. For example, 100g basil contains 3950µg β-carotene.
Do we need to supplement with Vitamin A?
Causes of Vitamin A deficiency
The primary cause of vitamin A deficiency is lack of dietary intake, found especially in developing countries, which leads to malnutrition. It can also occur in developed countries in low-fat diets or when the diet is lacking vegetables and meat. Alcohol consumption can interfere with vitamin A absorption, leading to deficiency, and this is of particular importance during pregnancy as deficiency may lead to embryonic malformations (vitamin A deficiency syndrome).1
The secondary cause of vitamin A deficiency is lipid (fat) malabsorption and chronic diseases such as Crohn’s, cirrhosis, pancreatic insufficiency, chronic diarrhoea and biliary tract problems.1
Vitamin A deficiency is defined as a plasma level of less than 0.52 µM or liver concentration below 5-20µg/g.
Supplementation options and cautions
As Vitamin A is a fat-soluble vitamin that can be stored in the body, especially the liver, excess intake can be harmful to health - see comments below regarding toxicity. Individuals with kidney or liver disease or who drink heavily should not supplement with vitamin A. It is crucial that you seek the advice of your GP or a healthcare practitioner before you start supplementing vitamin A.
Vitamin A is found in many multivitamin formulations as well as in stand-alone supplements. It can be obtained in liquid, capsule and tablet options that usually contain retinyl acetate, retinyl palmitate, β-carotene or a combination of these. β-carotene will need to be converted to useable vitamin A – retinol – in your body, so the amount of retinol you obtain may be lower than from supplements containing retinyl compounds. Dose levels vary considerably but you should always check that you are not going to exceed your recommended daily intake limit (see the guidance above) from both supplemental and dietary intake, unless advised by your GP.
Vitamin A supplementation can also be in topical form for use with skin and eye conditions.
Vitamin A is stored in our organs (such as the liver, pancreas, eyes) and absorption of retinol from foods, dietary supplements or topical applications is not regulated by the body’s control mechanisms. A build-up of stored vitamin A can therefore lead to toxicity, known as hypervitaminosis A, which is also dependent on age and liver health. Prolonged excess vitamin A intake over several weeks can raise triglyceride levels in the body and potentially lead to irreversible liver damage.1,26 β-carotene is considered safer and excess intake usually shows as orange/yellowish skin colouration which disappears after intake reduction.
- Chronic Vitamin A toxicity can occur after intakes of 10mg /day over several months in adults (7.5-15mg/day in children)
- Acute Vitamin A toxicity can occur after more than 500mg/day in adults (100mg/day in children, 30mg/day in infants), less in the event of alcoholism or kidney failure
The main symptoms of acute vitamin A toxicity are nausea, irritability, reduced appetite, vomiting, blurred vision, headaches, hair loss, muscle pain, bleeding, weakness, drowsiness and altered mental status.
Routine excess oral intake can lead to cracked lips, headache, flushing, stomach pain, dizziness and loss of coordination. Routine excess topical application can lead to a reduction in sebum production in the skin as well as skin redness, peeling and discomfort.1
Discontinuation of any supplementation (oral or topical) helps to resolve symptoms, although this may take several months and, in some cases, irreversible changes may have occurred.1
Vitamin A is an essential fat-soluble vitamin involved in key body functions of vision, gene regulation, immune function, growth and reproduction. It isavailable in both animal and plant foods. Supplementation can be an option when food sources are unavailable, limited or symptoms of deficiency are detected. However, care needs to be taken not to exceed the daily recommended intake levels as this can be harmful to health.
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- Sajovic J, et al. The Role of Vitamin A in Retinal Diseases. Int J Mol Sci. 2022 Feb 1;23(3).
- Zhang D, et al. Vitamin A cycle byproducts impede dark adaptation. J Biol Chem [Internet]. 2021 Sep 1;297(3). Available from: https://pubmed.ncbi.nlm.nih.gov/34391781/
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- Fogagnolo P, et al. A review of topical and systemic vitamin supplementation in ocular surface diseases. Nutrients [Internet]. 2021 Jun 1;13(6). Available from: https://pubmed.ncbi.nlm.nih.gov/34200595/
- Barber T, et al. Vitamin A deficiency and alterations in the extracellular matrix. Nutrients. 2014 Nov 10;6(11):4984–5017.
- Cho K, et al. Retinaldehyde dehydrogenase inhibition-related adverse outcome pathway: Potential risk of retinoic acid synthesis inhibition during embryogenesis. Toxins (Basel) [Internet]. 2021 Nov 1;13(11). Available from: https://pubmed.ncbi.nlm.nih.gov/34822523/
- Cañete A, et al. Role of vitamin a/retinoic acid in regulation of embryonic and adult hematopoiesis. Nutrients [Internet]. 2017 Feb 20 [cited 2022 Apr 11];9(2). Available from: https://pubmed.ncbi.nlm.nih.gov/28230720/
- Hada M, et al. Serum retinol and risk of overall and site-specific cancer in the ATBC study. Am J Epidemiol [Internet]. 2020 Jun 1;189(6):532–42. Available from: https://pubmed.ncbi.nlm.nih.gov/31612201/
- Doldo E, et al. Vitamin A, cancer treatment and prevention: The new role of cellular retinol binding proteins. Biomed Res Int [Internet]. 2015 [cited 2022 Apr 8];2015. Available from: https://pubmed.ncbi.nlm.nih.gov/25879031/
- Tratnjek L, et al. Vitamin A and retinoids in bladder cancer chemoprevention and treatment: A narrative review of current evidence, challenges and future prospects. Int J Mol Sci. 2021 Apr 1;22(7).
- Everts HB, Akuailou EN. Retinoids in cutaneous squamous cell carcinoma. Nutrients [Internet]. 2021 Jan 1;13(1):1–18. Available from: https://pubmed.ncbi.nlm.nih.gov/33466372/
- Huang Z. et al. Role of vitamin A in the immune system. J Clin Med [Internet]. 2018 Sep 6;7(9). Available from: https://pubmed.ncbi.nlm.nih.gov/30200565/
- de Medeiros PHQS, et al. Modulation of intestinal immune and barrier functions by vitamin A: Implications for current understanding of malnutrition and enteric infections in children. Nutrients. 2018 Sep 1;10(9).
- Oliveira LDM, Teixeira FME, Sato MN. Impact of Retinoic Acid on Immune Cells and Inflammatory Diseases. Mediators Inflamm. 2018;2018.
- Yee MMF, et al. Vitamin a and bone health: A review on current evidence. Molecules [Internet]. 2021 Mar 2;26(6). Available from: https://pubmed.ncbi.nlm.nih.gov/33801011/
- Zhou F, et al. β-Carotene conversion to vitamin A delays atherosclerosis progression by decreasing hepatic lipid secretion in mice. J Lipid Res. 2020 Nov 1;61(11):1491–503.
- Olsen T, Blomhoff R. Retinol, Retinoic Acid, and Retinol-Binding Protein 4 are Differentially Associated with Cardiovascular Disease, Type 2 Diabetes, and Obesity: An Overview of Human Studies. Adv Nutr [Internet]. 2020 May 1;11(3):644–66. Available from: https://pubmed.ncbi.nlm.nih.gov/31868199/
- Zhou Y, et al. Vitamin A and Its Multi-Effects on Pancreas: Recent Advances and Prospects. Front Endocrinol (Lausanne) [Internet]. 2021;12:620941. Available from: http://www.ncbi.nlm.nih.gov/pubmed/33679618
- Zhao Y, et al. Retinoic acid exerts sexually dimorphic effects on muscle energy metabolism and function. J Biol Chem. 2021 Sep 1;297(3).
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