The Boston-based scientists based their conclusions on the results of a randomised, double-blind, placebo-controlled study of 6377 women (average age of 66 at baseline) taking part in the Women's Health Study.
The women were assigned to receive either 600 IU of alpha-tocopherol acetate every other day for ten years or placebo. No measurements of cognitive function were taken until the sixth year of intervention.
"In this randomised placebo-controlled trial among more than 6000 healthy women, vitamin E supplementation did not provide overall cognitive benefits or reduce cognitive decline over four years," wrote lead author Jae Hee Kang in the Archives of Internal Medicine.
Kang and co-workers report that overall there was no apparent difference Cognitive function was assessed by telephone interviews that adapted the Mini-Mental State Examination (MMSE) and the East Boston Memory Test.
After ten years of intervention and four years of follow-up, no significant difference was observed between the vitamin E supplemented groups and placebo. The lack of information during the initial years of the trial is not considered a serious limitation by the researchers, who said that since cognitive impairment occurs slowly over decades, it was unlikely that short-term benefits would have been observed.
Despite this study reporting that vitamin E supplements did not produce any significant effects, the researchers do report that women with low dietary vitamin E intake did experience slower cognitive decline as a result of supplementation than women with similar levels assigned to placebo, and there was also a suggestion that women without diabetes also benefited from the supplements.
Moreover, reports from other studies that antioxidant and vitamin E-rich foods may offer protection means that the door is not closed on this area of research.
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 a European diet, while gamma-tocopherol (gamma-Toc) is the most common form in the American diet.
In an accompanying editorial, Mark Espeland and Victor Henderson from Wake Forest University School of Medicine said that the results of the "well-conducted and well-powered" trial were "disappointing but not surprising" given that other clinical trials had reported null results.
Writing in the Archives of Internal Medicine, Espeland and Henderson pointed out however that a potential link between vitamin E from food and slowed cognitive decline reported by other studies (for example, the Chicago Health and Aging Project (CHAP), Neurology, Vol. 67, pp. 1370-1376) raised the possibility that "dietary antioxidants other than RRR-alpha-tocopherol could be important."
"Some have speculated that other vitamin E compounds (eg. gamma-tocopherol or alpha-tocotrienol) may have greater benefit than alpha-tocopherol or that antioxidants in general can modify outcomes only when therapy is initiated at a relatively young age before oxidative damage has crossed a certain threshold," they said.
Both Kang and co-workers and Espeland and Henderson agreed that several questions remained, most notably whether mixed tocopherols and tocotrienols or increased intake in younger subjects could offer neuroprotective effects.
"It is likely that cognitive interventions will vary in effectiveness depending on an individual's genetic background, current cognitive status, cognitive trajectory, concomitant disorders, and other exogeneous risk factors," said Espeland and Henderson.
Sources: Archives of Internal Medicine Volume 166, Pages 2462- 2468 "A randomised trial of vitamin E supplementation and cognitive function in women" Authors: J.H. Kang, N. Cook, J Manson, J.E. Buring, F. Grodstein
Editorial: Volume 166, Pages 2433-2434 "Preventing cognitive decline in usual aging" Authors: M.A. Espeland and V.W. Henderson