Vitamin A is one in a group of fat-soluble micronutrients, and part of the retinoids.
These include retinoic acid, retinol, retinal, and retinyl esters.
There are two forms of Vitamin A that can be obtained in the diet.
is preformed vitamin A and is found in foods coming from animal sources, such as meat and fish, as well as dairy products.
is provitamin,or carotenoids. The most well-known is beta carotene; others include alpha-carotene and beta-cryptoxanthin.
It is important to know that both preformed and provitamin A need to be intracellularly metabolized into retinoic and retinal acids, which are the active forms of vitamin A. In performing this function, the vitamin is able to carry out its important biological functions. The remaining carotenoids that can be found in food, such as lutein and lycopene are not converted into vitamin A.
- Retinal binds to a protein called opsin in rod cells. This occurs in the eye, once light strikes the retina and retinol is converted to retinal, subsequently shuttled to rods. This aids in night vision or being able to see in the dark.
- Retinoic acid is necessary to regulate the highly vital regulation of gene transcription. On a daily basis, this form of vitamin A and proteins carry out this function.
- Vitamin A is needed for iron to be incorporated into hemoglobin. It is also a component of proper red blood cell formation, specializing in the creation of stems into the cells.
- Necessary for proper formation of white blood cells, which protect the body against illness and infection.
- Vitamin A supports immune system health through retinol. This form is used by the skin and the linings of several internal organs. Retinol is used in white cells, which in turn protect the skin and are a barrier for the internal organs against infection. In this way, immune health is supported.
Daily requirements are based upon daily intakes for healthy individuals. They do vary by age and gender, and are based upon recommendations developed the Food and Nutrition Board (FNB) at the Institute of Medicine of the National Academies (formerly National Academy of Sciences).
It is important to note that the RDA's for vitamin A are (see Table 1). Because the body converts all dietary sources of vitamin A into retinol, 1 mcg of physiologically available retinol is equivalent to the following amounts from dietary sources: 1 mcg of retinol, 12 mcg of beta-carotene, and 24 mcg of alpha-carotene or beta-cryptoxanthin. From dietary supplements, the body converts 2 mcg of beta-carotene to 1 mcg of retinol. Currently, vitamin A is listed on food and supplement labels in international units (IUs) even though nutrition scientists rarely use this measure. Conversion rates between mcg RAE and IU are as follows: 1 IU retinol = 0.3 mcg RAE 1 IU beta-carotene from dietary supplements = 0.15 mcg RAE 1 IU beta-carotene from food = 0.05 mcg RAE 1 IU alpha-carotene or beta-cryptoxanthin = 0.025 mcg RAE("Dietary supplement fact," 2012).1
|0–6 months2||400 mcg RAE||400 mcg RAE|
|7–12 months2||500 mcg RAE||500 mcg RAE|
|1–3 years||300 mcg RAE||300 mcg RAE|
|4–8 years||400 mcg RAE||400 mcg RAE|
|9–13 years||600 mcg RAE||600mcg RAE|
|14–18 years||900 mcg RAE||700 mcg RAE3|
|19–50 years||900 mcg RAE||700 mcg RAE3|
|51+ years||900 mcg RAE||700 mcg RAE|
1U.S. Department of Health and Human Services, National Institutes of Health. (2012). Dietary supplement fact sheet: Vitamin a. Retrieved from Office of Dietary Supplements website: http://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
2Adequate Intake (AI), equivalent to the mean intake of vitamin A in healthy, breastfed infants.
3Pregnancy & Lactation values 750 mcg RAE; 1,200 mcg RAE and 770 mcg RAE; 1,300 mcg RAE for age 14-18 and 19-50 age groups respectively.
Deficiency is not typical in developed countries, but most commonly seen in countries where people have limited access to animal-based food sources containing preformed vitamin A and are frequently unable to consume quantities of foods with beta-carotene because they are in poverty-stricken locations. This begins early on, usually with infants and is seen in those who suffer with malabsorption disorders ("Micronutrient deficiencies: Vitamin," 1998). Signs of the deficiency most commonly present with a condition known as xerophthalmia, and its hallmark symptom is night blindness. Vitamin A deficiency also puts people at higher risk for severity of infections and mortality, especially measles.
Studies have been conducted to collect information with regard to vitamin A toxicity in both adults and children. Acute toxicity, which can occur when adults consume occur greater than 100% of the RDA and children consume more than 20% over a brief period (hours or days), is not as dangerous as those who have a more chronic toxicity. A longer term toxicity occurs when large amounts are consumed of vitamin A in preformed type over a period of time of months or even years. If daily intakes are greater than 1500 IU/kg of body weight, it is reported that such high levels lead to toxicity. The greatest concern in toxicity is bone alteration in children and bone loss in adults. Large levels of consumption causes an alteration in bone metabolism, altered skeletal development, and consistent occurrences of spontaneous bone fractures (Penniston & Tanumihardjo, 2006) among those diagnosed with hypervitaminosis.
Healthy people who already eat enough proformed vitamin A in their diets naturally through consumption of leafy green vegetables, orange and yellow vegetables, fruits, tomato products, and some oils need not take a daily vitamin A supplement. Most of these supplements contain a form of beta-carotene and the rest is preformed vitamin A. Taking too much of this vitamin in the diet can lead to toxic levels since this is stored by the body and not excreted in the urine. Taking mega doses of vitamin A (15,000-25,000 IU) per day can lead to severe headache, vomiting, suppressed appetite, abnormalities in vision, hair loss, bone pain, dry skin, and sleep problems. In the most troubling cases, liver damage can occur from the toxicity.
Other Benefits of Vitamin A
There are some studies that show vitamin A (beta-carotene) supplements are beneficial in treating and/or preventing several illnesses. Some forms of cancer appear to respond fairly well to higher amounts of proformed vitamin A (Mufti & Basul, 2001) than those who didn’t receive it. There is also some evidence that supports vitamin A is helpful in preventing age-related macular degeneration. Lastly, some children infected with measles were given doses of vitamin A for several days at the onset. It seemed to improve their chance of recovery, giving some basis to the thought that vitamin A can also help reduce mortality caused from measles in developing countries.
Vitamin A is an important micronutrient to support optimal daily health. Remembering to consume only what is needed is key. Making sure to get as much of those retinoids from the foods we eat is the best way to ensure proper supplementation of this crucial nutrient.