What is Vitamin A?
Vitamin A is one of the four fat-soluble vitamins. The terms vitamin A and pre-formed vitamin A are used to refer to the retinoids—retinol, retinal, and retinoic acid. Provitamin A is a term that refers to certain red, orange, and yellow plant pigments called carotenoids. Of the 400 or more carotenoids found in nature, about 10 percent can be converted to vitamin A in the body. These carotenoids are called provitamin A, as they are considered to be a precursor source of retinoids. The most abundant and important provitamin A carotenoid is beta-carotene.
Retinoids and carotenoids are broken down, then combine with other dietary fats into micelles, which can then be absorbed by the body. Retinol absorption tends to be much more efficient than carotenoid absorption. It is estimated that 70 to 90 percent of retinol is absorbed, while perhaps 20 to 50 percent of carotenoids are absorbed.1 Absorption is also rapid, with the majority of uptake occurring within six hours of digestion.
Once absorbed, most retinoids are carried to target cells or to the liver (for storage or excretion). The liver can store nearly a year’s supply of vitamin A, so deficiencies can take time to develop. Stored retinol can be mobilized whenever the body needs it. Carotenes can be stored in fat tissue anywhere in the body and can be converted to vitamin A on an as-needed basis if body stores of retinol become low.
Vitamin A has numerous and diverse functions in human health. In the eyes, vitamin A is important for maintaining the health of the retina and transmitting light that comes into the eye into nerve signals. Retinal, formed from retinol through a zinc-dependent process, is particularly important to the function of rod cells. Rods transmit low light signals, accounting for much of a person’s night vision. Because of this, one of the serious effects of vitamin A deficiency is night blindness. Normal immunity is vitamin A-dependent, and vitamin A is especially important to mucosal immunity in the lungs, sinuses, gastrointestinal system, urinary tract, and skin. Retinoids also assist in the differentiation of white blood cells and lymphocyte activation. Red blood cells (RBCs) depend on vitamin A for differentiation as well. Furthermore, vitamin A plays a role in the transport of iron into the hemoglobin of developing RBCs. Some of the most critical physiologic actions of retinol are hormonal. In conjunction with vitamin D and thyroid hormone, the retinoic acid form of vitamin A works to both stimulate and inhibit some of the genes involved in cellular differentiation. In this way, growth and development are also vitamin A-dependent—both too much and too little vitamin A can cause defects of limbs, heart, eyes, and ears in fetal development. Regulation of growth hormone production is similarly dependent on vitamin A signaling. Retinoids and carotenoids also have the ability to act as antioxidants.
Signs and Symptoms of Vitamin A Deficiency
Vitamin A deficiency can result in serious health problems. Pathology of the eye is the best recognized sign of retinol depletion. Xerophthalmia is the general term that describes manifestations of vitamin A deficiency in the eye. Patients with vitamin A deficiency may experience loss of night vision and extreme eye dryness. They may also report stumbling over things in the dark, a need to turn on lights earlier in the day, or difficulty driving after dusk. However, because of the abundance of electrical lighting, this symptom might go unnoticed. Patients with vitamin A deficiency may also suffer from low tear production. Severe vitamin A deficiency can cause total blindness.
Those with vitamin A deficiency may have problems with skin and mucous membranes due to keratin accumulation. These can include dry skin, dry hair, broken nails, follicular hyperkeratosis (especially of the arms and thighs), itching, and generally rough or bumpy skin. Increased incidence of infection (especially of the respiratory, genitourinary, and gastrointestinal systems) and poor wound healing are signs of impaired vitamin A-mediated immunity. Additionally, low sperm counts in men (impaired spermatogenesis) and history of spontaneous abortion in women can be signs of a subclinical deficiency. Vitamin A deficiency can also cause increased deposition of periosteal bone and possible increased incidence of kidney stone formation.
Deficiency can be treated with high doses of retinol for several days, followed by dose reduction and maintenance, supervised by a physician. Xerophthalmia should be treated as a medical emergency because of the serious risk of blindness. Progression to corneal involvement is generally irreversible.
High-dose vitamin A treatment must be closely monitored, as it can cause significant side effects. Acute toxicity can occur within hours to a few days of a large bolus dose. Symptoms may include nausea, vomiting, blurred vision, headache, and vertigo. Symptoms will usually resolve rapidly, within 1 to 2 days. Peeling skin, hair loss, and bone pain may develop a few days later. Chronic toxicity can produce a range of symptoms including hair loss, anorexia, bone pain, liver and spleen enlargement, cheilitis, rashes/peeling, pseudotumor cerebri, and hyperpigmentation. Osteoporosis is also a risk of chronic vitamin A overload.
Due to the known teratogenic nature of retinol, great caution should be taken in pregnancy or possible pregnancy. Daily levels of vitamin A over 10,000IU could cause potential harm to a fetus, although the risk of doses between 10,000 and 30,000IU appears small.2 Low levels have the potential to harm fetal development as well.
Recommended Intake of Vitamin A
The RDA for vitamin A is 3,000IU (900mcg). The tolerable upper limit (UL) is set at 10,000IU (3,000mcg). However, chronic ingestion of over 5,000IU per day is associated with increased risk of osteoporosis in individuals without malabsorption.3 There is no upper limit for beta-carotene, as toxicity has not been observed even at very high doses over long periods of time. Multivitamin preparations containing up to 10,000IU are readily available in many forms.
Preformed vitamin A is not common in foods other than liver, fortified milk, egg yolks, and fortified breakfast cereals. Beta-carotene and other carotenoids are found throughout the plant world in colorful fruits and vegetables (e.g., carrots, broccoli, apricots). Although it is important for general health to include these foods in the diet, individuals with malabsorption cannot fully depend on these foods for maintaining vitamin A levels in the body.
Vitamin A Interactions
Numerous drugs interact with retinol. Oral contraceptives and statins can increase serum levels of vitamin A with long-term use. Isotretinoin, a synthetic derivative of vitamin A, should not be taken with retinol without close physician supervision. Caution should also be used with topical retinoids. Medroxyprogesterone, methyltestosterone, oral corticosteroids, bile acid sequestrants, orlistat, and some anticonvulsants and antibiotic classes can all lower vitamin A levels. Patients taking these drugs who are also at risk for vitamin A deficiency may require closer monitoring.
Other nutrients also interact with vitamin A. Both transport and use of vitamin A require adequate protein status. Therefore, low protein can contribute to vitamin A deficiency. Vitamin A interacts with the other three fat-soluble vitamins in different ways. Low vitamin E can decrease the conversion of beta-carotene to retinol, and high intake of vitamin A can reduce absorption of vitamin K and interfere with vitamin D-dependent calcium uptake.4 The conversion of retinol in the body to retinal is dependent on zinc. Since the formation of retinal from retinol requires zinc, low zinc status can alter night vision and visual acuity without signs of xerophthalmia. Low vitamin A can also contribute to iron deficiency. Finally, regular alcohol intake increases the potential toxicity of vitamin A. Patients taking high-dose vitamin A should be advised to avoid alcohol intake.
Editor’s note. Consult with your physician or a certified dietitian/nutritionist to determine a diet best suited to your individual needs.
Source
This article was adapted with permission from Jacques J. Vitamin A (retinol). Micronutrition For The Weight Loss Surgery Patient. Edgemont, PA: Matrix Medical Communications; 2005:57–63.
Other sources
- Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, DC: National Academies Press, 2000.
- Miller RK, Hendrickx AG, Mills JL, et al. Periconceptional vitamin A use: How much is teratogenic? Reprod Toxicol. 1998;12(1):75–88.
- Michaelsson K, Lithell H, Vessby B, Melhus H. Serum retinol levels and the risk of fracture. N Engl J Med. 2003;348(4):287–294.
- Johansson S, Melhus H. Vitamin A antagonizes calcium response to vitamin D in man. J Bone Miner Res. 2001;16(10):1899–1905.
