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Vitamin E is a fat-soluble vitamin. It is an important anti-oxidant in the body. Vitamin E is also referred to as tocopherol.
Vitamin E is a family of 8 compounds, consisting of 4 tocopherols and 4 tocotrienols. The 4 tocopherols are alpha-tocopherol, beta-tocopherol, gamma-tocopherol and delta-tocopherol. The 4 tocotrienols are alpha-tocotrienol, beta-tocotrienol, gamma-tocotrienol and delta-tocotrienol.
Only alpha-tocopherol is considered to have any Vitamin E activity in humans.
There are several forms of alpha-tocopherol. The 2 main forms that are usually listed are naturally occurring d-alpha-tocopherol and synthetic dl-alpha-tocopherol.
The body absorbs d-alpha-tocopherol differently than it does dl-alpha-tocopherol. A unit called International Units (IU) is used to compare the activity of d-alpha-tocopherol and dl-alpha-tocopherol. 1 mg (milligram) of d-alpha-tocopherol is equivalent to 1.49 IU. 1 mg of dl-alpha-tocopherol is equivalent to 1.1 IU. The following table summarizes these relationships:
The other tocopherols (other than alpha-tocopherol) and the tocotrienols are included for reference, but do not contribute to the Total Vitamin E values.
Historical Measures of Vitamin E
Earlier versions of the USDA Standard Nutrition Reference (US Department of Agriculture) considered the four types of tocopherols as contributors to Vitamin E activity. They used a unit called Alpha Tocopherol Equivalents (αTE or aTE) to combine the quantities of tocopherols.
Current research (as of 2004) indicates that only the alpha-tocopherols contribute to Vitamin E activity. For this reason the beta-, gamma- and delta-tocopherols are no longer considered to contribute to Vitamin E activity, and Alpha Tocopherol Equivalents are no longer used.
The United States Pharmacopoeia (USP) defined the International Unit (IU) for Vitamin E in 1979, based on a lab test called the rat fetal resorption assay. Based on these studies, 1 mg of d-alpha-tocopherol was defined as 1.49 IU, and 1 mg dl-alpha-tocopherol was defined as 1.1 IU.
Recently, researchers at the Food and Nutrition Board of the National Academy of Sciences (NAS) have concluded that the Vitamin E activity of synthetic dl-alpha-tocopherol is only half that of naturally occurring d-alpha-tocopherol. They have recommended that the RDI of Vitamin E be specified in terms of mg of equivalent d-alpha-tocopherol. Until the FDA (US Food and Drug Administration) revises the nutrition labeling regulations, the RDI (Reference Daily Intake) of Vitamin E is still defined in terms of IU.
At the time of this writing (2004), the RDI (Reference Daily Intake) of Vitamin E is 30 IU.
Because the conversion from mg to IU is different for d-alpha-tocopherol and dl-alpha-tocopherol, you cannot convert mg to IU (or IU to mg) for Total Vitamin E unless you know the proportion of d-alpha-tocopherol to dl-alpha-tocopherol in the sample.
For example, knowing that you have 20 IU of Total Vitamin E is not enough information to determine how much d-alpha-tocopherol and dl-alpha-tocopherol you have. 20 IU of Total Vitamin E can come from 13.4 mg of d-alpha-tocopherol, or from 18.2 mg of dl-alpha-tocopherol, or from 6 mg d-alpha-tocopherol plus 10.055 mg dl-alpha-tocopherol, or from many other combinations of d-alpha-tocopherol and dl-alpha-tocopherol. The following table summarizes:
In general, if a food is naturally occurring and has not been fortified or enriched with dl-alpha-tocopherol, you can assume that all of the Vitamin E is d-alpha-tocopherol.
Entering d-alpha-tocopherol or dl-alpha-tocopherol Values
When you enter mg for d-alpha-tocopherol or dl-alpha-tocopherol, nutraCoster automatically calculates the IU and %RDI values for the nutrient.
When you enter IU for d-alpha-tocopherol or dl-alpha-tocopherol, nutraCoster automatically calculates the mg and %RDI values for the nutrient.
When you enter %RDI for d-alpha-tocopherol or dl-alpha-tocopherol, nutraCoster automatically calculates the mg and IU values for the nutrient.
When you change any of the values for d-alpha-tocopherol or dl-alpha-tocopherol, nutraCoster checks the Total Vitamin E value. If the Total Vitamin E value was the sum of the d-alpha-tocopherol and dl-alpha-tocopherol values before the change, nutraCoster recalculates the Total Vitamin E value to be the sum of the new d-alpha-tocopherol and dl-alpha-tocopherol values.
If the Total Vitamin E value was not the sum of the d-alpha-tocopherol and dl-alpha-tocopherol values before the change, nutraCoster does not update the Total Vitamin E value.
Entering Total Vitamin E Values
When you enter Total Vitamin E values for mg, IU or %RDI, nutraCoster cannot calculate the values for d-alpha-tocopherol or the dl-alpha-tocopherol unless it knows the proportions of d-alpha-tocopherol and dl-alpha-tocopherol (see the section IU Details above for why this is so).
When you enter mg for Total Vitamin E, nutraCoster checks to see if the Total Vitamin E mg value was the sum of the d-alpha-tocopherol and dl-alpha-tocopherol mg values before the change. If so, nutraCoster recalculates the d-alpha-tocopherol and dl-alpha-tocopherol mg values so the sum of the values is equal to the new Total Vitamin E mg value, while keeping the proportions the same
If nutraCoster recalculates the d-alpha-tocopherol and/or dl-alpha-tocopherol mg values as a result of a change to the Total Vitamin E mg value, it will also recalculate the IU and %RDI values for the d-alpha-tocopherol and dl-alpha-tocopherol.
Finally, nutraCoster will recalculate the IU and %RDI values of Total Vitamin E based on the new values for d-alpha-tocopherol and dl-alpha-tocopherol.
Example: Suppose you have 10 mg Total Vitamin E, which is composed of 6 mg d-alpha-tocopherol and 4 mg dl-alpha-tocopherol. The following table lists the values for the example:
Now change the Total Vitamin E to 15 mg. nutraCoster performs the following steps:
nutraCoster performs a similar series of steps when you enter IU or %RDI values for Total Vitamin E.