Preformed vitamins are those that you obtain in the same form that your body uses. They may require further processing in your body, as with vitamin D, to form hormones or other substances, but they are already the completed animal form of the vitamin. For example, the vitamin D made by your skin is chemically the same as that found in fish liver oils. It is a preformed vitamin. However, most of what you read about preformed vitamins refers to vitamin A.
Preformed Vitamin A
Vitamin A can be obtained from animal sources in a preformed state. In this form, it is ready to use but can be highly toxic with overdose. Even with more moderate intake, the preformed vitamin A may be harmful to bone health in older people according to the National Institutes of Health. Preformed vitamin A can be a valuable addition to your diet, but must be used with caution.
Types of Vitamin A
Dr. Jane Higdon of Oregon State University explains that the two basic types of pre-formed vitamin A are retinol and retinal. Your body converts retinal to the retinoic acid used for gene transcription. These three forms are called retinoids and all are of animal origin. Your body can synthesize vitamin A from plant substances called carotenoids, which includes carotene. Plants make hundreds of forms of carotene, but only about ten percent of these are in the previtamin A form you need. One of the most useful plant forms is beta carotene that your body can make into retinol. Beta carotene is considered non toxic.
Expert Insight
Vitamin A deficiency is a world wide concern. According to Dr. Higdon, children that have even a mild deficiency of vitamin A have a higher incidence of respiratory disease and diarrhea and are more likely to die from infectious disease than children with adequate intake. Dr. Higdon also wrote that vitamin A deficiency is the most common preventable cause of blindness in developing countries. Deficiency of vitamin A also makes you less able to utilize iron and zinc.
Considerations
In order to assure that you get the proper amount of vitamin A that your body can easily use, it was necessary to find a way to compare the animal and plant forms and express these in a way that would be useful. The standard of measurement is retinol and vitamin A is now described with Retinol Activity Equivalents as well as International Units. This sounds a little confusing, but is intended to make it possible to compare apples to apples for your body's needs.
Nutrition
The recommended daily allowance for vitamin A is 3500 International Units. For a woman 31-50 years old, the daily allowance expressed as Retinol Activity Equivalents is up to 1000 RAE, or RE. One microgram of retinol is considered one RAE. Six micrograms of beta carotene are needed to equal one RAE , so if that were your only source you would need 3000-6000 microgram RAE of beta carotene. Using the microgram RAE equivalent to 3.33 IU, that would calculate to almost 10,000 IU of vitamin A as beta carotene, but less than 1700 IU of preformed vitamin A. You can safely get all your vitamin A as beta carotene if you choose to avoid animal products. One large raw carrot contains a day's supply.
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