Evidence lacking for polyphenols' brain benefits: expert

By Stephen Daniells

- Last updated on GMT

Related tags: Polyphenols, Antioxidant

Years of research lie ahead before science can confidently support
the apparent benefits of polyphenols for brain health, according to
a new review from Canada.

It would be "unwise"​ to assume that the same protective effects occur for Alzheimer's disease (AD) and other human disorders, despite more than 200 laboratory and animal studies documenting the effects of polyphenols in protecting nerves from damage, stated a review published in the Journal of Agricultural and Food Chemistry​. "In view of their multiple biological activities, polyphenols hold great promise as potential therapeutic/prophylactic agents in neurodegenerative diseases,"​ wrote Charles Ramassamy and colleagues from INRS-Institut Armand-Frappier and Laval University. A vast body of epidemiological studies has linked increased dietary intake of antioxidants from fruits, vegetables wine, chocolate, coffee, tea, and other foods to reduced risks of a range of diseases including cancer, cardiovascular disease and diabetes. And when it comes to brain health, and particularly Alzheimer's disease, the in vitro​ and in vivo​ studies may not translate to humans. "It is not at all clear whether these compounds reach the brain in sufficient concentrations and in a biologically active form to exert beneficial effects,"​ wrote the authors. "On the other hand, it has become clear that the mechanisms of action of these polyphenols go beyond their antioxidant activity and the attenuation of oxidative stress. "Therefore, there is a need for more research on their intracellular and molecular targets as special pathways underlying distinct polyphenol-induced neuroprotection,"​ they wrote. Polyphenols are not created equal "One of the major difficulties of elucidating the beneficial effects of polyphenols is the large number of polyphenolic compounds found in fruits, vegetables, and beverages and the even larger numbers of their metabolites,"​ wrote the authors. In addition, the chemical structure and the consumers themselves affect the bioavailability, both factors which further muddy the waters. Polyphenols can be split into two groups - flavonoids and non-flavonoids. The former includes anthocyanins found in berries, flavonols from a variety of fruit and vegetables, flavones from parsley and thyme, for example, flavanones from citrus, isoflavones from soy, mono- and poly-meric flavonols like the catechins in tea, and proanthocyanidins from berries, wine and chocolate. The non-flavonoids include phenolic acids, lignans, and stilbenes such as resveratrol. The polyphenol story is further compounded by the "extreme difficultly" in estimating daily average intakes, said the reviewers. All of this adds up to the need for "more detailed studies… to determine [polyphenols] absorption, bioavailibity, and ability to cross the blood-brain barrier,"​ wrote Ramassamy and co-workers. "Their neuroprotective activity in various models of neurodegenerative diseases in vitro and in vivo have been documented, but it would be unwise to extrapolate these results to the human situation without proper clinical trials in patients suffering from irreversible and extensive neuronal loss,"​ they concluded. Call to arms ​ Earlier this year, Ming Hu from the University of Houston issued "a call to arms"​ for more relevant research into the bioavailability and utilisation of the antioxidants, particularly polyphenols, in order to help "the successful development of polyphenols as chemopreventive agents in the future".​ In an editorial in the journal Molecular Pharmaceutics​, Hu asked: "A critically important scientific question is then: are these flavonoids and polyphenols as effective as people believe?"​ Answering his own question, he said that insufficient research has been done with respect to the bioavailability of these compounds, and possible interactions with pharmaceuticals. The Houston-based researcher highlighted the need for mechanistic studies to determine the activities and functions of related compounds, and how the metabolites of the polyphenols are transported across biological membranes. Source: Journal of Agricultural and Food Chemistry​ Volume 56, Issue 13, Pages 4855-4873, doi: 10.1021/jf0735073 "Challenges for Research on Polyphenols from Foods in Alzheimer's Disease: Bioavailability, Metabolism, and Cellular and Molecular Mechanisms" ​Authors: M. Singh, M. Arseneault, T. Sanderson, V. Murthy, C. Ramassamy

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