CLINICAL

  nutrition

Vitamin A

The myth, benefits, dangers and prescriptions

KIMBERLY K. REED, O.D., F.A.A.O.

This column is the first in a series on individual nutrients often found in multivitamin/mineral supplements (MVMs) and their link with eye health. This month’s focus: Vitamin A, a fairly generic term for several substances, such as beta-carotene (BC), that is most identified with raw carrots — believed to improve night vision.

Here, I discuss this belief, how much Vitamin A is recommended and the importance of prescribing specific MVMs.

Improving vision at night?

Vitamin A plays a critical role in the regulation of gene expression; the differentiation of red blood cells from stem cells; embryonic development of the heart, eyes and ears; in the functioning of the immune system; and in the complex metabolic pathway to form rhodopsin, the primary rod photoreceptor pigment, making it vital for vision at low-light levels. Therefore, if one doesn’t get enough vitamin A, he or she experiences “night blindness.”

On the other hand, no research reveals that eating a lot of raw carrots improves night vision. This fallacy came from the British military in WWII. In an effort to keep secret a new radar technology that allowed them to identify and repel enemy fighters, they let it “leak” that their soldiers were able to “see” the enemy’s planes at night thanks to eating an abundance of carrots.

How much?

The answer to this question is surprisingly complicated. The most recent international standard is known as Retinol Activity Equivalents or RAE, although the older International Units (IU) is still widely used.

Unlike the IU, the RAE takes into account the various absorption rates of vitamin A in supplements and foods.

For example, to get 1 microgram (mcg) of retinol, you need 2mcg of BC in supplement form, but 12mcg in dietary (food) form. Alpha-carotene and beta-cryptoxanthin have even lower conversion rates of 24:1.

Taking this into account, the Recommended Dietary Allowance (RDA) for adult males is 900mcg RAE, 700mcg RAE for females and 750mcg RAE for pregnant females. (Much of the RDA levels are still based upon 1968 data, which is often thought outdated.)

So, how does the carrot stack up? One half cup of a raw, chopped carrot has 1,793IU of vitamin A, which translates to 538mcg of RAE (more than half of the RDA for males and females). A half cup of cooked spinach has nearly the same amount, at 1,572IU and 472mcg RAE. One teaspoon of cod liver oil has a whopping 1,350mcg RAE.

The importance of prescribing

Vitamin A-containing vegetables, such as carrots, butternut squash and spinach, contain carotenoids, in the form of provitamin A. Because the body only converts provitamin A substances into retinol on an as-needed basis, carotenoids in foods are not associated with vitamin A toxicity, or hypervitaminosis A. Preformed vitamin A (retinol), found in some MVMs, however, is. This can result in an increased risk of osteoporosis (1,500IU per day), especially in older adults. In severe cases, liver damage and coma can occur. (These outcomes generally do not occur at levels less than 10 times the RDA of 8,000mcg to 10,000mcg/day). As the tolerable upper intake level (UL) for preformed vitamin A (retinol) is 3,000 mcg (10,000 IU) per day for adults, you must prescribe specific products and dosages, taking into account the patient’s current nutritional intake. As Vitamin A is found in a variety of foods, it may not be necessary or even advisable to supplement it in some patients.

Something else to consider: MVMs may contain BC alone or in combination with preformed vitamin A as retinyl palmitate or retinyl acetate. An increased risk of lung cancer in current or former smokers is associated with BC-containing supplements, say several large studies, including AREDS and AREDS2. AREDS2 data reveal that lutein and zeaxanthin are appropriate substitutes for BC in a supplement. Therefore, prescribe specific products that don’t contain BC to these patients.

As always, it’s best to meet nutritional needs with food, including BC-rich foods, like spinach and carrots. However, when additional vitamin A is needed to supplement a vitamin A-poor diet, I recommend an MVM that has at least 700mcg RAE of preformed vitamin A (retinyl palmitate or retinyl acetate), bearing in mind the UL of 3,000mcg RAE. OM

DR. REED IS AN ASSOCIATE PROFESSOR AT THE NOVA SOUTHEASTERN UNIVERSITY COLLEGE OF OPTOMETRY IN FORT LAUDERDALE, FLA., A MEMBER OF THE OCULAR NUTRITION SOCIETY AND AUTHOR OF NUMEROUS ARTICLES ON OCULAR NUTRITION, DISEASE AND PHARMACOLOGY. SHE IS ALSO A FREQUENT CONTINUING EDUCATION LECTURER. TO COMMENT ON THIS COLUMN, E-MAIL DR. REED AT KIMREED@NOVA.EDU, OR VISIT TINYURL.COM/OMCOMMENT TO COMMENT ON THIS ARTICLE.