Their findings lend weight to the growing support for a mixture of vitamin E forms, over single form alpha-tocopherol, in supplements.
Gamma-tocopherol, found naturally in walnuts, sesame seeds and corn, was found to hold back the proliferation of lab-cultured human prostate and lung cancer cells, reports the team from Purdue University in the 13 December online edition of the Proceedings of the National Academy of Sciences.
Previous research by the same team found that gamma-tocopherol inhibits inflammation, which had already been implicated in cancer development. The researchers theorized that it might retard the progress of cancer and cardiovascular disease, and to test their hypothesis they exposed cultures of cancerous prostate and lung cells to the vitamin.
Normal prostate epithelial cells were used as a control group.
"We discovered that as we increased the quantity of gamma-tocopherol, the cancer cells grew more slowly," lead author Qing Jiang said. "But the normal prostate cells were not affected and grew normally. This could indicate that the vitamin could be used to target lung and prostate cancer cells without the damaging side effects of chemotherapy."
The study also revealed that gamma-tocopherol caused cell death by interrupting synthesis of fatty acid molecules called sphingolipids.
"This is also a novel discovery," Jiang said. "Although there have been prior indications that some form of vitamin E may cause cell death in some mouse cell lines, we are the first to provide a mechanism for such an effect."
However she added: "Although this discovery is promising, we do not yet know whether gamma-tocopherol has any effect on cancer in living creatures. We hope that future research not only will clarify whether gamma-tocopherol could have applications in human cancer treatment, but also will show how we might supplement the body with the vitamin to prevent cancer from developing in the first place."
Scientists have been studying vitamin E for more than three-quarters of a century, but most efforts have focused largely on alpha-tocopherol, one of eight known forms in the vitamin's family.
Alpha-tocopherol was found early on to have the most beneficial effects on laboratory animals fed diets deficient in vitamin E, and also is the major form found in body tissues. For these reasons, it has been nearly the only form of the vitamin to be included in most manufactured nutritional supplements.
"Since then, alpha-tocopherol has justifiably earned a good reputation as an antioxidant, which helps to fight against damage caused by unwanted free radicals," Jiang said. "But its familiarity has perhaps attracted research away from the other seven forms of vitamin E."
Specialist supplement manufacturers in the US, including major brands like GNC, are beginning to offer 'full spectrum' or 'complete complex' vitamin E products in order to provide all seven forms of the vitamin in a supplement.
Jiang said the next step for her research team would be testing the effect of gamma-tocopherol and mixed forms of vitamin E on animal cancers.