The Best Dietary Source Of Preformed Vitamin A

Vitamin A is "the messiest of all vitamins" when it comes to understanding its terminology, metabolism and daily requirements, according to "Staying Healthy with Nutrition: Vitamin A." In 1913, when vitamin A was discovered, scientists were trying to isolate a fat-soluble nutrient in butterfat. This was probably retinyl palmitate or retinol, both of which are now called "preformed vitamin A," a term that includes all animal sources of vitamin A. A 2011 review in "Nutrients" uses the term "retinoid" to encompass all forms of preformed vitamin A.

Is It Preformed Vitamin A or Provitamin A?

You can obtain dietary vitamin A in several forms. Preformed vitamin A, which is typically a retinyl ester or retinol, is acquired from animal sources, such as liver or eggs. Retinyl esters are converted to retinol in your gut. Substances called carotenoids -- the source of yellow and orange pigmentation in fruits and vegetables -- are additional dietary sources of vitamin A. Carotenoids, such as beta-carotene, are all considered "provitamin A." Provitamin A can also be converted to retinol in your intestine or, if your body requires additional vitamin A, by other tissues.

The Vitamin A Shuffle

Vitamin A can be converted from one form to another to meet your body's needs. Retinol, a form of vitamin A absorbed from animal foods, serves as a storage form of vitamin A. A 2009 study in "Biochimica et Biophysica Acta" showed that retinyl esters are also stored in special lipid droplets in your liver. Retinol and retinyl esters can both be converted to retinal, which is necessary for normal vision. Retinal can be converted back to retinol, or it can be transformed into retinoic acid, which is the form of vitamin A that participates in growth and development. Plant-based provitamin A is converted to retinol, which can then follow the same metabolic pathways as preformed vitamin A.

Food Sources of Preformed Vitamin A

Liver, especially calf liver, and fish liver oils, such as cod liver oil, are the best sources of preformed vitamin A, but excess consumption of such foods can lead to vitamin A toxicity. Egg yolks, whole milk, cream and butter are also good sources of preformed vitamin A. Vegetarians and others who prefer to avoid these foods can take provitamin A, usually in the form of beta-carotene, if they wish to take supplemental vitamin A. Carotenes, which do not cause toxicity, are abundant in leafy greens and orange fruits and vegetables.

Considerations and Recommendations

It is no wonder that people are confused by the terminology and dosage recommendations for the various forms of vitamin A. According to "Staying Healthy with Nutrition: Vitamin A," beginning in 2000, the National Academy of Sciences began expressing vitamin A doses in terms of micrograms of retinol equivalents, or mcg REs. However, many supplements still express vitamin A content in international units, or IUs. To convert from IUs to mcg REs, 3.33 IUs are equal to 1 mcg RE. Recommended daily intake of vitamin A varies from 300 mcg RE for toddlers to 900 mcg RE for lactating females. Since excess intake of preformed vitamin A can lead to toxicity, ask your doctor what form and dosage is best for you.