Test Your Knowledge

The term vitamin A is often used to refer to a number of substances aside from its three usable forms (retinol, retinal, retinoic acid). To understand these forms, it is important to know how they are created in the body. Vitamin A itself is not found in foods. Instead, the body converts chemical precursors found in dietary sources once they are metabolized in the body.

Retinyl esters (found in meat and other animal products) are the most efficient vitamin A precursor for bioavailability. The body can also convert some carotenoids (plant pigments) into vitamin A, including beta carotene, alpha carotene and beta cryptoxanthin.

Once in the body, vitamin A precursors can be converted into three forms of vitamin A. Each form plays a specific role in the body and which form is created depends on what the body needs at that time. Retinyl esters, together with the three usable forms of vitamin A, belong to a group called retinoids. The forms of vitamin A are:

• Retinol. The primary form of vitamin A in the body. Both retinyl esters and carotenoids can be changed into retinol in the intestines. Retinol is then either transported to the liver to be stored or to body cells to be used or converted into retinal. Once in the liver, retinol-binding protein must be present for retinol to be transported out of the liver into body cells. Once converted, retinol functions the same way, no matter its original source.
  
  
• Retinal. Retinyl esters must first be converted to retinol. Carotenoids can be converted directly into retinal, if needed. Whatever its original source, retinol can be converted into retinal in body cells when needed. Excess amounts can be further converted into retinoic acid or be changed back into retinol for storage.
  
  
• Retinoic acid. An irreversible form of vitamin A that can be made from retinal in body cells when needed. However, excess cannot be converted back into retinal or retinol for storage. Therefore any retinoic acid produced in body cells must be used. Neither retinyl esters nor carotenoids can be directly converted into retinoic acid.

Much greater amounts of carotenoids than retinyl esters are needed to produce the same amount of vitamin A. In addition, certain carotenoids are more efficient when taken in supplement form than when ingested through diet. To better compare the efficiency of different sources of vitamin A available from various foods and supplements, the U.S. National Academy of Sciences developed retinol activity equivalents (RAEs). For example, to produce 1 RAE, the following amounts of vitamin A sources would be needed:

• 1 microgram retinol
• 2 microgram beta carotene (from supplements)
• 12 microgram beta carotene (from food)
• 24 microgram alpha carotene or beta cryptoxanthin

In supplements, vitamin A can be present in two forms, either separately or together:

• Preformed retinol. A synthetic form of retinol.
  
  
• Beta carotene. A plant-based precursor for vitamin A. Beta carotene in supplement form is used more efficiently by the body than when ingested through food.