TABLE 6-1 Factors for Converting International Units of Vitamin Ea to α-Tocopherolb (mg) to Meet Recommended Intake

 

USP Conversion Factorsc

Molar Conversion Factorsd

α-Tocopherol Conversion Factorse

 

IU/mg

mg/IU

µmol/IU

mg/IU

Synthetic Vitamin E and Esters

 

dl-α-Tocopheryl acetate

1.00

1.00

2.12

0.45

dl-α- Tocopheryl succinate

0.89

1.12

2.12

0.45

dl-α-Tocopherolf

1.10

0.91

2.12

0.45

Natural Vitamin E and Esters

 

d-α-Tocopheryl acetate

1.36

0.74

1.56

0.67

d-α-Tocopheryl succinate

1.21

0.83

1.56

0.67

d-α-Tocopherolg

1.49

0.67

1.56

0.67

a Vitamin E supplements are historically and incorrectly labeled d- or dl-α-tocopherol. Vitamin E compounds include the all racemic (all rac)-α-tocopherol (dl-α-tocopherol [RRR-, RRS-, RSR-, RSS-, SSS-, SRS-, SSR-, and SRR-] or synthetic) form and its esters and the RRR-α-tocopherol (d-α-tocopherol or natural) form and its esters. All of these compounds of vitamin E may be present in fortified foods and multivitamins. Not all stereoisomers function to meet vitamin E requirements in humans.

b α-Tocopherol as defined in this report to meet recommended intakes includes RRR-α-tocopherol (historically and incorrectly labeled d-α-tocopherol) the only form of α-tocopherol that occurs naturally in foods, and the other 2R-stereoisomeric forms of α-tocopherol (RSR-, RRS-, and RSS-α-tocopherol) that are synthesized chemically and thus are found in fortified foods and supplements (Figure 6-2).

c Official United States Pharmacopeia (USP) conversions where one IU is defined as 1 mg of all rac-α-tocopheryl acetate (USP, 1979, 1999). All of the conversions are based on rat fetal resorption assays that were conducted in the 1940s. The amounts of the free and succinate forms have been adjusted for their different molecular weights relative to the all rac-α-tocopheryl acetate (incorrectly labeled dl-α-tocopheryl acetate).

d To convert mg to µmol divide the mg by the molecular weight of the vitamin E compound (α-tocopheryl acetate = 472; α-tocopheryl succinate = 530; α-tocopherol = 430) and multiply by 1,000. Because the amount of free and succinate compounds are adjusted for their different molecular weights relative to α-tocopheryl acetate, these forms have the same conversion factors as the corresponding tocopherol compounds.

e To convert the µmol of the vitamin E compound to mg of α-tocopherol, multiply the µmol by the molecular weight of α-tocopherol (430) and divide by 1,000. The activities of the three synthetic α-tocopherol compounds have been divided by 2 because the 2S-stereoisomers contained in synthetic-α-tocopherol are not maintained in the blood.

fdl-α-Tocopherol = all rac-(racemic) α-tocopherol = synthetic vitamin E; all rac-α-tocopherol = RRR-, RRS-, RSR-, RSS-, SSS-, SRS-, SSR-, and SRR-α-tocopherol isomers.

gd-α-Tocopherol = RRR-α-tocopherol = natural vitamin E.



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