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In this article, we describe:
- the major purposes of this specific nutrient in the human body,
- its experimentally confirmed health uses,
- conventional ways to estimate nutrient status,
- nutrient’s toxicities and deficiencies,
- experimentally confirmed and approved levels of the nutrient intake for different demographics,
- dietary sources of the nutrient.
Two forms of vitamin A are available in the human diet: preformed vitamin A (retinol) and provitamin A carotenoids. Preformed vitamin A is found in foods from animal sources, including dairy products, fish, and meat (especially liver). By far the most important provitamin A carotenoid is 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.
Retinol and carotenoid levels inadequacy are typically measured in plasma. However, they do not decline until vitamin A levels in the liver are almost depleted. Liver vitamin A reserves can be measured indirectly through the relative dose-response test, in which plasma retinol levels are measured before and after the administration of a small amount of vitamin A. A plasma retinol level increase of at least 20% indicates an inadequate vitamin A level. For clinical practice purposes, plasma retinol levels alone are sufficient for documenting significant deficiency.
Premature infants, pregnant and lactation women, patients with cystic fibrosis, cancer, age-related macular degeneration (AMD) are at risk for vitamin A inadequacy (deficiency or adverse effects). Excess preformed vitamin A can have significant toxicity (known as hypervitaminosis A) leading to increased intracranial pressure (pseudotumor cerebri), dizziness, nausea, headaches, skin irritation, pain in joints and bones, coma, and even death.
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Recommended Dietary Allowances (RDAs) for Vitamin A
RDAs (Recommended Dietary Allowance) for vitamin A are given as mcg of retinol activity equivalents (RAE) to account for the different bioactivities of retinol and provitamin A carotenoids. 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
However, under FDA’s new labeling regulations for foods and dietary supplements that take effect by January 1, 2020, vitamin A will be listed only in mcg RAE and not IUs. An RAE cannot be directly converted into an IU without knowing the source(s) of vitamin A. For example, the RDA of 900 mcg RAE for adolescent and adult men is equivalent to 3,000 IU if the food or supplement source is preformed vitamin A (retinol). However, this RDA is also equivalent to 6,000 IU of beta-carotene from supplements, 18,000 IU of beta-carotene from food, or 36,000 IU of alpha-carotene or beta-cryptoxanthin from food. So, a mixed diet containing 900 mcg RAE provides between 3,000 and 36,000 IU of vitamin A, depending on the foods consumed.
|0–6 months*||400 mcg RAE||400 mcg RAE|
|7–12 months*||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||600 mcg RAE|
|14–18 years||900 mcg RAE||700 mcg RAE||750 mcg RAE||1,200 mcg RAE|
|19–50 years||900 mcg RAE||700 mcg RAE||770 mcg RAE||1,300 mcg RAE|
|51+ years||900 mcg RAE||700 mcg RAE|
* Adequate Intake (AI), equivalent to the mean intake of vitamin A in healthy, breastfed infants.
Dietary 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 vegetable oils. The top food sources of vitamin A in Western diet include dairy products, liver, fish, and fortified cereals; the top sources of provitamin A include carrots, broccoli, cantaloupe, and squash.
The table below suggests 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.
Selected Food Sources of Vitamin A
|Food||mcg RAE per|
|Beef liver, pan fried, 3 ounces||6,582||731|
|Sweet potato, baked in skin, 1 whole||1,403||156|
|Spinach, frozen, boiled, ½ cup||573||64|
|Pumpkin pie, commercially prepared, 1 piece||488||54|
|Carrots, raw, ½ cup||459||51|
|Ice cream, French vanilla, soft serve, 1 cup||278||31|
|Cheese, ricotta, part skim, 1 cup||263||29|
|Herring, Atlantic, pickled, 3 ounces||219||24|
|Milk, fat-free or skim, with added vitamin A , 1 cup||149||17|
|Cantaloupe, raw, ½ cup||135||15|
|Peppers, sweet, red, raw, ½ cup||117||13|
|Mangos, raw, 1 whole||112||12|
|Breakfast cereals, fortified with 10% of the DV , 1 serving||90||10|
|Egg, hard boiled, 1 large||75||8|
|Black-eyed peas (cowpeas), boiled, 1 cup||66||7|
|Apricots, dried, sulfured, 10 halves||63||7|
|Broccoli, boiled, ½ cup||60||7|
|Salmon, sockeye, cooked, 3 ounces||59||7|
|Tomato juice, canned, ¾ cup||42||5|
|Yogurt, plain, low fat, 1 cup||32||4|
|Tuna, light, canned in oil, drained solids, 3 ounces||20||2|
|Baked beans, canned, plain or vegetarian, 1 cup||13||1|
|Summer squash, all varieties, boiled, |
|Chicken, breast meat and skin, roasted, ½ breast||5||4|
|Pistachio nuts, dry roasted, 1 ounce||4||0|
*DV = Daily Value. The DV for vitamin A is 900 mcg RAE for adults and children age 4 years and older, where 1 mcg RAE = 1 mcg retinol, 2 mcg beta-carotene from supplements, 12 mcg beta-carotene from foods, 24 mcg alpha-carotene, or 24 mcg beta-cryptoxanthin. Foods providing 20% or more of the DV are considered to be high sources of a nutrient, but foods providing lower percentages of the DV also contribute to a healthful diet.
Foods providing 20% or more of the DV are considered to be high sources of a nutrient, but foods providing lower percentages of the DV also contribute to a healthful diet.