*Limitations*

This review has certain limitations including that research articles focused on dementia, which have explored relationship of vitamin E on mild cognitive impairment but not necessarily mentioned MCI in the title or abstract, those articles might have left from inclusion. Authors utilized phrase searching, however, due to the lack of truncation rules for some terms of dementia, some articles might have been missed. Therefore, future studies using a systematic literature review with a well-defined and peer-reviewed search strategy can be conducted. Six animal studies are included in this review and comparison were made between findings in human and animals. Comparison of effect of an element between human and animal specially on cognitive function and process is extremely complex. In simpler terms, behavioral studies to investigate cognitive decline among humans offer higher inferential benefits than those being conducted among animals due to differences in their baseline intelligence levels and capabilities. Next, studies included in this review were heterogenous in terms of type of measures used to detect the cognitive decline and these measures had varied threshold criteria. These restricted our ability to generate pooled estimates, which otherwise might have helped us to quantify the association between vitamin E and neuroprotection in the form of appropriate effect sizes (such as odds ratios). In addition, understanding trajectories of change in cognitive function will be difficult with such a heterogeneity. On the other hand, the observation of physiological process relatively easier in animal population compared to human due to ethical consideration. The findings of animal studies showing favorable outcome of vitamin E on cognition process may not be rewarding when compared to human. The database such as LILACS were not explored. Few studies included in this review has a very small sample size or their results were mixed up with some other compound which might have impacted on the conclusion of this review.

Despite the limitations, the study has implications for practice. Vitamin E supplementation should be monitored closely when in combination with Vitamin E nutrients from food sources. Other than alpha tocopherol, no other component of vitamin was found helpful in slowing the process of cognitive decline. It is not known if the other form of vitamin E or its component has negative effect on cognitive process. Therefore, caution needs to be practiced in optimizing level of vitamin E that way the other components of vitamin E do not affect the properties of alpha tocopherol and its impact on MCI. In addition, this review indicates favorable physiological and neurochemical benefits of vitamin E in protecting or delaying cognitive decline process, however, effects may be different among animals and human-beings. These differences may be due to the varied vitamin E requirements and physiological mechanisms among these groups. Therefore, further research to understand associations between vitamin E requirements, consumption, and effects at organismal levels would be critical to unfold interactions of vitamin E with body mechanisms.
