The ABCs of Vitamins: Vitamin A
Vitamins are involved in so many processes in the human body that we can sometimes forget just how important they are to maintaining our body in an optimal working state. In this series I will be discussing the ins and outs of vitamins, what they do, what happens if you have too much or too little in your diet, and new research into their effectiveness on various conditions. This week I will be looking at a fat-soluble, sightseeing powerhouse known as vitamin A.
Vitamin A is a fat-soluble vitamin found in many foods that is involved in immune function, vision, reproduction, and cellular communication. Vitamin A comes in many forms, but only from two sources. One group is found in animal-based foods. These are called retinoids. The other group is found in plant-based foods. These are called carotenoids and include beta-carotene. Beta-carotene in the body is converted to vitamin A.
One of vitamin A's major functions is helping with vision. This is because the human retina has four photo pigments that store vitamin A. Rhodopsin is one of these pigments and is located in the rod cells of the retina. Rhodopsin is responsible for the rod cells detecting small amounts of light and allows us to see at night and in dimly-lit conditions. Retinal, the aldehyde version of vitamin A, is used to manufacture rhodopsin and is also involved in the reactions that cause visual excitation by light hitting the rod cells of the eye.
Vitamin A also supports cell growth, playing a critical role in the normal formation and maintenance of the heart, lungs, and kidneys. It also helps form and maintain healthy skin, teeth, skeletal and soft tissue, and mucus membranes. It may also be needed for reproduction and breast-feeding.
Foods Rich in Vitamin A
There are many foods rich in vitamin A and beta-carotene. Vitamin A comes from animal sources, such as eggs, meat, milk, cheese, cream, liver, kidney, and cod. If you drink skim milk it should be noted that generally it will be fortified with vitamin A.
Sources of beta-carotene include bright orange and yellow fruits such as cantaloupe, pink grapefruit, and apricots, and vegetables such as carrots, pumpkin, sweet potatoes, and squash. Other good sources of beta-carotene include broccoli, spinach, and most other dark green and leafy vegetables. Just remember that the more intense the colour of a fruit or vegetable, the higher the beta-carotene content.
Vitamin A Deficiency
Vitamin A deficiency due to a poor diet is quite rare in developed countries. Deficiency could be attributed to inflammatory diseases like Crohn's and celiac because the digestive track becomes damaged and this affects the absorption of vitamin A. Zinc deficiency, alcoholism, and pancreatic diseases can also affect vitamin A levels in the body. Nutrients that can help with vitamin A absorption in the body include vitamins B2, B3, B12, C, D, and E, as well as magnesium, selenium, manganese, potassium, phosphorus, carotenoids, iodine, tyrosine, and zinc.
As vitamin A is an important component for normal vision, vitamin A deficiency can cause night-blindness, or at the very least a decreased ability to see in dim light. Vitamin A deficiency can also manifest as diminished immune system function, which will cause your body to have difficulty fighting infections and healing wounds.
General deficiency symptoms can include dry eyes, night blindness, diarrhea, and skin problems. If you have vitamin A deficiency symptoms you need to see a health care provider who can order blood tests to determine if a vitamin A deficiency is the problem or if there are other causes.
Vitamin A Side Effects
If you get too much vitamin A, you can become sick. Large doses of vitamin A can also cause birth defects. Acute vitamin A poisoning can occur when several hundred thousand IUs of vitamin A is taken randomly or is over supplemented. Symptoms of chronic vitamin A poisoning may occur in adults who regularly take more than 25,000 IU a day. Babies and children are more sensitive to vitamin A, and can become sick after taking smaller doses of vitamin A or vitamin A-containing products.
On the other side large amounts of beta-carotene will not make you sick. Consuming large amounts of beta-carotene can turn the skin yellow or orange, though. The change in skin color will return to normal once the intake of beta-carotene is reduced.
Scientists are studying vitamin A to understand how it affects health. Here are some examples of what this research has shown:
Cancer: People who consume large amounts of food containing beta-carotene might have a lower risk of certain kinds of cancer, such as lung cancer or prostate cancer. Studies to date have not shown that supplementing with vitamin A or beta-carotene can help prevent cancer or lower the chances of dying from this disease.
Age-Related Macular Degeneration: Age-related macular degeneration is one of the most common causes of vision loss in older people. Among people afflicted, a supplement containing large doses of beta-carotene combined with other antioxidants, zinc, and copper has shown promise for slowing down the rate of vision loss.
Measles: When children with vitamin A deficiency get measles, the disease tends to be more severe. In these children, taking supplements with high doses of vitamin A can shorten the fever and diarrhea caused by measles. These supplements can also lower the risk of death in children with measles who live in developing countries where vitamin A deficiency is common.
It is recommended to get the following amounts of vitamin A per day:
- Infants: 400 micrograms (mcg) a day up to 6 months and 500 mcg up to 12 months
- Children: 300mcg a day up to 3 years, 400mcg a day up to 8 years, and 600mcg a day up to 13 years
- Adolescents and adults: 900mcg a day for males 14 and older, 700mcg a day for females 14 and older
- Women who are pregnant or breastfeeding: You need more, and you should ask the doctor what is best for you in this case.
As you can "see" literally, vitamin A is an important vitamin to include in your diet, but with any type of supplementation always consult your health care provider to make sure you are taking the correct doses. Just remember that you don't want to be caught orange-faced over a lack of vitamin A.
1. Osiecki, Henry , The Nutrient Bible 8th Edition, Bio Concepts Pub, Kelvin Grove QLD
2. "Vitamin A." Office of Dietary Supplements, National Institute of Health. July 2012.
3. "Vitamin A (Retinol)." University of Maryland Medical Center. June 2011.
4. "Vitamin A." Mediline Plus. February 2013.
5. Hennekens CH, Buring JE, Manson JE, Stampfer M, Rosner B, Cook NR, et al. Lack of effect of long-term supplementation with beta carotene on the incidence of malignant neoplasms and cardiovascular disease. The New England journal of medicine 1996;334:1145-9.
6. Neuhouser ML, Barnett MJ, Kristal AR, Ambrosone CB, King IB, Thornquist M, et al. Dietary supplement use and prostate cancer risk in the Carotene and Retinol Efficacy Trial. Cancer Epidemiol Biomarkers Prev 2009;18:2202-6.
7. Age-Related Eye Disease Study Research Group. A randomized, placebo-controlled, clinical trial of high-dose supplementation with vitamins C and E, beta carotene, and zinc for age-related macular degeneration and vision loss: AREDS report no. 8. Arch Ophthalmol 2001;119:1417-36.
8. Yang HM, Mao M, Wan C. Vitamin A for treating measles in children. Cochrane Database Syst Rev 2011;2005.
9. Sudfeld CR, Navar AM, Halsey NA. Effectiveness of measles vaccination and vitamin A treatment. Int J Epidemiol 2010;39 Suppl 1:i48-55.
Photos courtesy of Shutterstock.