Vitamin A

 “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.

 Selected Food Sources of Vitamin A
Food mcg RAE per
serving
IU per
serving
Percent
DV*
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

Table 1. Retinol activity equivalents (RAE) Ratios for Preformed Vitamin A and Provitamin A Carotenoids
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.
Table 2. Recommended Dietary Allowance (RDA) for Vitamin A as Preformed Vitamin A (micrograms [μg] of Retinol Activity Equivalents [RAE]/day) by  Food and Nutrition Board (FNB) of the US Institute of Medicine (IOM).
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)
Children 1-3 years 300 300
Children 4-8 years 400 400
Children 9-13 years 600 600
Adolescents 14-18 years 900 700
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.

Advertisements

2 thoughts on “Vitamin A

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s