Before 2001, all forms of vitamin E contributed to the RDA, using a measure called alpha-tocopherol equivalents. In 2001, the Dietary Reference Intake (DRI) committee decided only 2R forms of alpha-tocopherol should be used to estimate the requirement, because these forms bind to alpha-TTP. Thus, other forms of vitamin E (gamma-tocopherol, tocotrienols etc.) do not count toward the requirement and the unit is now mg of alpha-tocopherol. As a result, soybean, corn, and flaxseed oils, which are good sources of gamma-tocopherol, are no longer considered to be good sources of vitamin E. The figure below is a reminder of the tocopherol content of different nuts and oils.
Figure 9.251 Only the yellow bars count toward the 2001 DRI requirement1
Another level of complexity is added by the introduction of international units (IU). IUs are a unit that are used to describe the bioactivity of different compounds, including 4 vitamins: A, D, E, and C. It would be less confusing if these units were not used. However, most supplements use IUs. IUs are not as common on food items.
For vitamin E, IUs are specific for alpha-tocopherol and adjusted for the molecular weight of the different forms (alpha-tocopherol acetate etc.). The conversion factors for converting IU to mg of alpha-tocopherol are:
0.67 for RRR-alpha-tocopherol (and its esters)
0.45 for all-rac-alpha-tocopherol (and its esters)
Here are some example calculations showing how to use these conversion factors:
Example 1. For a supplement containing 100 IU of RRR-alpha tocopherol:
100 IU X 0.67 = 67 mg alpha-tocopherol
Example 2. For a supplement containing 100 IU of all-rac-alpha tocopherol:
100 IU X 0.45 = 45 mg alpha-tocopherol2,3
References & Links
- Wagner KH, Kamal-Eldin A, Elmadfa I. (2004) Gamma-tocopherol--an underestimated vitamin? Ann Nutr Metab 48(3): 169-188.
- DRI (2000) Dietary reference intakes for vitamin C, vitamin E, selenium, and carotenoids.