“Vitamin A” is not a single compound, it is the blanket term for retinoids, biologically active compounds that occur naturally in both plant and animal tissues, including retinol and its derivatives (retinal and retinyl esters). The vitamin A that comes from animal sources is in the form of retinoic acid, retinal and retinol.
Two forms of vitamin A are available in the human diet: preformed vitamin A and provitamin A carotenoids.
- Preformed vitamin A (retinol and its esterified form, retinyl ester) is found in foods from animal sources, including dairy products, fish, and meat (especially liver). Because these retinoids are very bioavailable and stored in our tissues (most of them in the liver), too much vitamin A can build up in the body and become toxic.
- Provitamin A carotenoid is the most important beta-carotene; other provitamin A carotenoids are alpha-carotene and beta-cryptoxanthin. The body converts these plant pigments into vitamin A.
Both provitamin A and preformed vitamin A must be metabolized intracellularly to retinal and retinoic acid, the active forms of vitamin A, to support the vitamin’s important biological functions.
Other carotenoids found in food, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A. Still, many that cannot be converted to vitamin A have healthful effects.
Vitamin A plays a vital role in bone growth, reproduction, cellular communication and immune system health. It is critical for vision as an essential component of rhodopsin, a protein that absorbs light in the retinal receptors, and because it supports the normal differentiation and functioning of the conjunctival membranes and cornea.
Vitamin A also supports cell growth and differentiation, playing a critical role in the normal formation and maintenance of the heart, lungs, kidneys and other organs. It also helps the skin and mucous membranes repel bacteria and viruses more effectively.
Topical and oral retinoids are common prescription treatments for acne and other skin conditions, including wrinkles. Oral vitamin A is also used as a treatment for measles and dry eye in people with low levels of vitamin A.
Vitamin A deficiency
Vitamin A deficiency is common in developing countries, but is quite rare in the United States. One of the earliest signs of a deficiency is night blindness. Permanent blindness can result if the deficiency is left unchecked. Vitamin A deficiency also allows opportunistic infectious diseases such as measles and pneumonia to become deadly.
Vitamin A deficiency is a major cause of preventable blindness in the world. It is most prevalent among children and women of childbearing age. Vitamin A deficiency is associated with an increased susceptibility to infections, as well as to thyroid and skin disorders.
Alcoholics may develop vitamin A deficiencies, and should consequently include rich food sources of vitamin A in their diets (while concurrently sharply curtailing or eliminating alcohol consumption). Supplements may not be wise for alcoholics, however, because vitamin A is stored in the liver, and existing liver damage could make them more susceptible to vitamin A toxicity. In such cases, a doctor’s supervision is critical.
Food sources of vitamin A
Concentrations of preformed vitamin A are highest in liver and fish oils. Other sources of preformed vitamin A are milk and eggs, which also include some provitamin A. Most dietary provitamin A comes from leafy green vegetables, orange and yellow vegetables, tomato products, fruits, and some vegetable oils. The top food sources of vitamin A in the U.S. diet include dairy products, liver, fish, and fortified cereals; the top sources of provitamin A include carrots, broccoli, cantaloupe, and squash .
The next table shows many dietary sources of vitamin A. The foods from animal sources contain primarily preformed vitamin A, the plant-based foods have provitamin A, and the foods with a mixture of ingredients from animals and plants contain both preformed vitamin A and provitamin A.
|Food||mcg RAE per
|Sweet potato, baked in skin, 1 whole||1,403||28,058||561|
|Beef liver, pan fried, 3 ounces||6,582||22,175||444|
|Spinach, frozen, boiled, ½ cup||573||11,458||229|
|Carrots, raw, ½ cup||459||9,189||184|
|Pumpkin pie, commercially prepared, 1 piece||488||3,743||249|
|Cantaloupe, raw, ½ cup||135||2,706||54|
|Peppers, sweet, red, raw, ½ cup||117||2,332||47|
|Mangos, raw, 1 whole||112||2,240||45|
|Black-eyed peas (cowpeas), boiled, 1 cup||66||1,305||26|
|Apricots, dried, sulfured, 10 halves||63||1,261||25|
|Broccoli, boiled, ½ cup||60||1,208||24|
|Ice cream, French vanilla, soft serve, 1 cup||278||1,014||20|
|Cheese, ricotta, part skim, 1 cup||263||945||19|
|Tomato juice, canned, ¾ cup||42||821||16|
|Herring, Atlantic, pickled, 3 ounces||219||731||15|
|Ready-to-eat cereal, fortified with 10% of the DV for vitamin A, ¾–1 cup (more heavily fortified cereals might provide more of the DV)||127–149||500||10|
|Milk, fat-free or skim, with added vitamin A and vitamin D, 1 cup||149||500||10|
|Baked beans, canned, plain or vegetarian, 1 cup||13||274||5|
|Egg, hard boiled, 1 large||75||260||5|
|Summer squash, all varieties, boiled, ½ cup||10||191||4|
|Salmon, sockeye, cooked, 3 ounces||59||176||4|
|Yogurt, plain, low fat, 1 cup||32||116||2|
|Pistachio nuts, dry roasted, 1 ounce||4||73||1|
|Tuna, light, canned in oil, drained solids, 3 ounces||20||65||1|
|Chicken, breast meat and skin, roasted, ½ breast||5||18||0|
Vitamin A supplements
Curently some food and supplements labels list vitamin A in internation uits (IUs) or i miligrams. Using the folowing information you can find out how much vitamin A do you need, and how much does your supplement provide.
The Recommended Dietary Allowance (RDA)
Food and Nutrition Board of the US Institute of Medicine, has established recommended dietary allowance for vitamin A: 700 micrograms of retinol activity equivalents (μg RAE)/day for women and 900 μg RAE/day for men. Average daily dietary is an intake level of a nutrient sufficient to meet the requirements of nearly all healthy individuals in a specific life stage and gender group.
Retinol Activity Equivalents (RAE)
Vitamin A can be obtained from food as preformed vitamin A in animal products or as provitamin A carotenoids in fruit and vegetables. Yet, while preformed vitamin A is effectively absorbed, stored, and hydrolyzed to form retinol, provitamin A carotenoids like β-carotene are less easily digested and absorbed, and must be converted to retinol and other retinoids by the body after uptake into the small intestine. The efficiency of conversion of provitamin A carotenes into retinol is highly variable, depending on factors such as food matrix, food preparation, and one’s digestive and absorptive capacities.
The most recent international standard of measure for vitamin A is retinol activity equivalents (RAE), which represent vitamin A activity as retinol. It has been determined that 2 micrograms (μg) of β-carotene in oil provided as a supplement could be converted by the body to 1 μg of retinol giving it an RAE ratio of 2:1. However, 12 μg of β-carotene from food are required to provide the body with 1 μg of retinol, giving dietary β-carotene an RAE ratio of 12:1. Other provitamin A carotenoids in food are less easily absorbed than β-carotene, resulting in RAE ratios of 24:1. RAE ratios are shown in Table 1
|Quantity Consumed||Quantity Bioconverted to Retinol||RAE Ratio|
|1 μg of dietary or supplemental vitamin A||1 μg of retinol*||1:1|
|2 μg of supplemental β-carotene||1 μg of retinol||2:1|
|12 μg of dietary β-carotene||1 μg of retinol||12:1|
|24 μg of dietary α-carotene||1 μg of retinol||24:1|
|24 μg of dietary β-cryptoxanthin||1 μg of retinol||24:1|
|*1 IU is equivalent to 0.3 microgram (μg) of retinol, and 1 μg of retinol is equivalent to 3.33 IU of retinol.|
|Life Stage||Age||Males (μg/day)||Females (μg/day)|
|Infants||0-6 months||400 (AI)||400 (AI)|
|Infants||7-12 months||500 (AI)||500 (AI)|
|Adults||19 years and older||900||700|
|Pregnancy||18 years and younger||–||750|
|Pregnancy||19 years and older||–||770|
|Breast-feeding||18 years and younger||–||1,200|
|Breast-feeding||19 years and older||–||1,300|
Vitamin A international units (IUs)
Vitamin A on some food and supplement labels is listed in international units. Contrary to RAE, the number of IUs of vitamin A does not reflect the bioavailability of vitamin A from different food sources.
Conversion rates between IUs and μg RAE are set as follows:
• 1 IU of retinol is equivalent to 0.3 μg RAE
• 1 IU of supplemental β-carotene is equivalent to 0.15 μg RAE
• 1 IU of dietary β-carotene is equivalent to 0.05 μg RAE
• 1 IU of α-carotene or β-cryptoxanthin to 0.025 μg RAE
Can vitamin A be harmful?
Overconsumption of preformed vitamin A can be highly toxic and is especially contraindicated prior to and during pregnancy as it can result in severe birth defects. The tolerable upper intake level for vitamin A in adults is set at 3,000 μg RAE/day. The UL does not apply to vitamin A derived from carotenoids
Excessive, chronic intake of some forms of vitamin A can be toxic. Avoid taking supplemental vitamin A as retinol or retinoic acid, and instead use plant-derived vitamin A precursors such as beta-carotene (in addition to other mixed carotenoids). Also avoid concentrated animal sources such as cod liver oil (although some forms of cod liver oil are vitamin A reduced and are safe – check the label). Warning symptoms of overdose include hair loss, confusion, liver damage and bone loss.