Safety and Neuroprotective Efficacy of Palm Oil and Tocotrienol-Rich Fraction from Palm Oil: A Systematic Review

Background: Several natural products have been reported to elicit beneficial effects against neurodegenerative disorders due to their vitamin E contents. However, the neuroprotective efficacy of palm oil or its tocotrienol-rich fraction (TRF) from the pre-clinical cell and animal studies have not been systematically reviewed. Methods: The protocol for this systematic review was registered in PROSPERO (CRD42019150408). This review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines. The Medical Subject Heading (MeSH) descriptors of PubMed with Boolean operators were used to construct keywords, including (Palm Oil[Mesh]) AND Nervous System[Mesh], (Palm Oil[Mesh]) AND Neurodegenerative Diseases[Mesh], (Palm Oil[Mesh]) ANDBrain[Mesh], and (Palm Oil[Mesh]) ANDCognition[Mesh], to retrieve the pertinent records from PubMed, Scopus, Web of Science and ScienceDirect from 1990 to 2019, while bibliographies, ProQuest and Google Scholar were searched to ensure a comprehensive identification of relevant articles. Two independent investigators were involved at every stage of the systematic review, while discrepancies were resolved through discussion with a third investigator. Results: All of the 18 included studies in this review (10 animal and eight cell studies) showed that palm oil and TRF enhanced the cognitive performance of healthy animals. In diabetes-induced rats, TRF and alpha-tocotrienol enhanced cognitive function and exerted antioxidant, anti-apoptotic and anti-inflammatory activities, while in a transgenic Alzheimer's disease (AD) animal model, TRF enhanced the cognitive function and reduced the deposition of beta-amyloid by altering the expression of several genes related to AD and neuroprotection. In cell studies, simultaneous treatment with alpha-tocotrienols and neurotoxins improved the redox status in neuronal cells better than gamma- and delta-tocotrienols. Both pre-treatment and post-treatment with alpha-tocotrienol relative to oxidative insults were able to enhance the survival of neuronal cells via increased antioxidant responses. Conclusions: Palm oil and its TRF enhanced the cognitive functions of healthy animals, while TRF and alpha-tocotrienol enhanced the cognitive performance with attenuation of oxidative stress, neuroinflammation and apoptosis in diabetes-induced or transgenic AD animal models. In cell studies, TRF and alpha-tocotrienol exerted prophylactic neuroprotective effects, while alpha-tocotrienol exerted therapeutic neuroprotective effects that were superior to those of gamma- and delta-tocotrienol isomers.