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Vitamin E

Tocopherols, Tocotrienols and Their Natural Sources

Vitamin E comprises of three homologous series: tocopherols, tocotrienols and tocomonoenols.  Both tocopherols and tocotrienols have four homologues each: -, -, - and -tocopherols, and -, -, - and -tocotrienols respectively whereas there are only two isomers for -tocomonoenol and a -tocomonoenol reported.  -Tocopherol has the highest bioavailability in the plasma irrespective of the vitamin E contents in food intake, consequently -tocopherol was considered as the only significant vitamin E and other members of the vitamin E family was often ignored. 

 Fig. 1: Structures of tocopherols and tocotrienols

The molecular structures of tocopherols and tocotrienols are very similar, sharing the same chroman ring (as indicated by the purple circle in Figure 1).  The difference between the two is that tocopherol has saturated side chain whereas the remaining acylic side chain of tocotrienol comprises three isoprene units, therefore has three double bonds at the side chain.  The difference has the following implications:

  • a CCC single bond (154 pm) is longer than a C=C double bond (133 pm).

  • Saturated phytyl side chain has regular tetrahedral structure whereas the regular structure in geranylgeranyl side chain is disrupted by the plane trigonal structure at the double bonds. 

  • The bond angles at the olefinic carbons are larger (about 120º) as compared to that at the saturated carbons (about 109.5º). 

  • Double bonds are more rigid than single bonds as the additional bond restrict the free rotation of bond. 

The 3-dimensional conformation of side chain (depends on bond lengths and bond angles), the electron density and rigidity shall affect the binding affinities of tocotrienols with the -tocopherol transfer protein (with specific ligand-binding sites) [1, 2] and hence their bioavailability in humans.

Figure 1 also reveals that tocopherols have three chiral centers and hence have eight stereoisomers each whereas tocotrienols have only one chiral center and hence have only two stereoisomers.

Natural Sources of Tocotrienols

Table 1.  Composition of Tocopherols and Tocotrienols from Selected Sources

Source

-T

-T

-T

-T

Total T

-T3

-T3

-T3

-T3

Total T3

50mg T3

Palm oil

182

 

 

 

182

170

25

301

122

618

81g

Rice bran oil

129

6

23

3

161

57

8

78

2

145

345g

Oat

14.9

3.0

0.4

 

18.3

56.4

5.4

 

 

61.8

809g

Barley

8.6

0.9

5.6

0.7

15.8

40.3

8.7

10.4

0.9

60.3

829g

Soft wheat

15.9

9.5

 

 

25.4

6.4

42.5

 

 

48.9

1022g

Durum wheat

8.4

4.8

 

 

13.2

6.9

39.6

 

 

46.5

1075g

Spelt

10.3

7.0

 

 

17.3

5.5

32.7

 

 

38.2

1309g

Triticale

13.6

6.5

 

 

20.1

6.2

32.0

 

 

38.2

1309g

Rye

14.4

3.5

0.5

 

18.4

16.9

13.0

0.3

0.2

30.4

1645g

Maize

3.7

0.2

45.0

1.0

49.9

5.3

 

11.3

0.4

17.0

2941g

Data for cereals:  Panfili et al. 2003 [3]; Ryynänen et al. 2004 [4].

 

In contrast to ubiquitous tocopherols, natural sources of tocotrienols are very scarce.  Cereals contain small quantities of tocotrienols but polished rice does not contain tocotrienols after removal of the rice bran.  Other than palm oil and rice bran oil, vegetable oils such as soybean, rapeseed and sunflower oils, practically do not contain tocotrienols.  The ratio of tocopherols to tocotrienols in palm oil is about 23:77 whereas that for rice bran oil varies widely from 23:52 to 43:46. 

Table 1 indicates that in order to have 50 mg equivalent of tocotrienols, one has to consume about 81 g of crude palm oil, 345 g of rice bran oil or ranging from one to three kg of cereals/grains per day.  A recent study indicates that fruits and vegetables contain very little tocotrienols [5] and even larger daily consumption is needed in order to achieve 50 mg equivalent of tocotrienols. Even at that large quantity of food intake, the bioavailability of tocotrienols is expected to be depressed significantly by the presence of tocopherols. 

Tocotrienols are essential for chemoprevention of degenerative diseases.  Currently perceived balance diet is unachievable, in terms of tocotrienols.

Global annual production of palm oil exceeds 40 million tons whereas that for rice bran oil is only 3 million tons, out of which 2 million tons are unsuitable for human consumption.  Therefore palm oil is the best and most reliable natural source for tocotrienols.  Although palm oil is the best natural source for tocotrienol, it contains too high -tocopherol, causing the tocotrienol-rich fraction extracted from palm oil to be far from ideal.  However, appropriate processing technology can reduce the -tocopherol content in tocotrienol-rich fraction.

 

 

References

[1]    Meier R, Tomizaki T, Schulze-Briese C, Baumann U and Stocker A (2003).  The molecular basis of vitamin E retention: Structure of human -tocopherol transfer protein.  J Mol Biol 331(3): 725-734.

[2]    Min C K, Kovali R A and Hendrickson W A (2003).  Crystal structure of human -tocopherol transfer protein bound to its ligand: Implications for ataxia with vitamin E deficiency.  PNAS 100(25): 14713-14718.

[3]    Panfili G, Fratianni A and Irano M (2003).  Normal phase high-performance liquid chromatography method for the determination of tocopherols and tocotrienols in cereals.  J Agric Food Chem 51(14): 3940-3944.

[4]    Ryynänen M, Lampi A-M, Salo-Väänänen P, Ollilainen V and Piironen (2004).  A small-scale sample preparation method with HPLC analysis for determination of tocopherols and tocotrienols in cereals.  J Food Composition and Analysis 17(6): 749-765.

[5]    Chun J, Lee J, Ye L, Exler J and Eitenmiller R R (2006).  Tocopherol and tocotrienol contents of raw and processed fruits and vegetables in the United States diet.  J Food Composition and Analysis 19(2-3): 196-204.

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