There are eight forms of vitamin E: four tocopherols (alpha, beta, gamma, delta) and four tocotrienols (alpha, beta, gamma, delta). Alpha-tocopherol (alpha-Toc) is the main source found in supplements and in the European diet, while gamma-tocopherol (gamma-Toc) is the most common form in the American diet.
Tocotrienols are only minor components in plants, although several sources with relatively high levels include palm oil, cereal grains and rice bran.
While the majority of research on vitamin E has focused on alpha-Toc, studies into tocotrienols account for less than one per cent of all research into the vitamin.
In a new in vitro study, published in Biochemical and Biophysical Research Communications (Vol. 339, pp. 949-955), Japanese researchers studied the effects of all eight forms of vitamin E on the inhibition of mammalian DNA polymerase, the enzyme that assists DNA replication.
"The four tocopherols did not influence the activities of mammalian polymerases at all. On the other hand, the alpha- and delta-tocotrienols inhibited polymerase lambda activity," reported lead author Yoshiyuki Mizushina from the Kobe-Gakuin University.
The researchers reported that the inhibition of the polymerase enzyme was dose-dependent, with a dose of 18.4 micromoles needed to achieve 50 per cent inhibition.
The spread of cancer cells (cell proliferation) was also considered by the research team. Again, the alpha- and delta-tocotrienols inhibited the spread of cells, while the tocopherols had no effect.
The significance of these results, taken together, is the indication that angiogenesis (the growth of new blood vessels) is inhibited by this form of vitamin E. Also by stopping cell proliferation, the spread of the cancer to other parts of the body is hindered, a process called metastasis.
"These results led us to hypothesise that the inhibition of tumour growth by vitamin E in part by its polymerase lambda inhibitory activity and antioangiogenic activity," said Mizushina.
Whether the in vitro study is applicable in the real world is highly debatable and significant further work is needed before the same in vivo conclusions can be drawn.
Despite these big limitations, the news has been welcomed by WH Leong, vice president for Carotech, one of the world' major tocotrienol producers.
"Since angiogenesis is essential for tumour growth, its strong inhibition by tocotrienols but not tocopherols, could very well provide us with a safe dietary means to prevent a cancer from becoming metastatic," said Leong.
The results of this study fit in with other claims for the benefits of tocotrienols, reviewed in an on-line article in Life Sciences (doi: 10.1016/j.lfs.2005.12.001) by Chandon Sen, Savita Khanna and Sashwati Roy from Ohio State University Medical Center.
After reviewing the majority of studies for tocotrienols, the list of benefits for this form of vitamin E included neuroprotection, reduction of cholesterol, as an antioxidant, and other anti-cancer studies.
The reviewers concluded: "The current state of knowledge warrants strategic investment into the lesser known forms of vitamin E with emphasis on uncovering the specific conditions that govern the function of vitamin E molecules in vivo.
Outcomes studies designed in light of such information would yield lucrative returns."