Advances in Polyphenol Research: A Journal of Agricultural and Food Chemistry Virtual Issue

The term polyphenols embraces several thousand compounds ranging from small compounds that contain only a single aromatic ring bearing one to three hydroxyls up to those containing multiple such structural units with molecular masses exceeding 2000 Da.Polyphenols are commonly divided into flavonoids and non-flavonoids.Flavonoids, characterized by a C6-C3-C6 skeleton, include anthocyanins, chalcones, dihydrochalcones, dihydroflavonols, flavanols (flavan-3-ols or catechins), condensed tannins or proanthocyanidins, flavanones, flavones, flavonols, and isoflavones.Non-flavonoids include simple phenols, phenolic acids, phenolic amino acids, curcuminoids, stilbenes, lignans, and hydrolyzable gallo-and ellagitannins.Polyphenols are widespread and are almost ubiquitous in unprocessed plant material.Polyphenols are much less common in animals, generally occurring as metabolites and catabolites of dietary polyphenols.Crustaceans and molluscs, which have significant contents of phenolic amino acids, are notable exceptions.The polyphenol profile of foods of plant origin varies extensively with source and tissue within a source and total content from mg/kg to some 100 g/kg of dry mass.Processing of foods produces yet more polyphenols through various transformations.These transformations may be purely chemical in nature, as seen during coffee and cocoa roasting.Enzyme-driven oxidative changes may be desirable, as in black tea production, where many thousand thearubigins are produced, or undesirable, as in cut fruits and vegetables, i.e., through browning reaction.Matured wines, both red and white, contain polyphenol transformation products, arising both enzymatically and chemically during maceration and aging.In processed food, native and transformed polyphenols contribute critically to color, astringency, and odor.Scientists have investigated polyphenols since at least the start of the 19th century, and these compounds continue to be the subject of intense activity, with some 500 papers published in just the Journal of Agricultural and Food Chemistry (JAFC) between January 2016 and July 2017.Early studies on polyphenols focused on structure determination, and while structural characterization of the more complex polyphenols continues, since the end of the 20th century, there has been an increasing emphasis on biological activity, both in planta and after consumption by humans and domesticated animals.Studies on biological activity have been driven by epidemiological studies associating better health with the greater long-term consumption of fruits and vegetables as well as beverages, such as tea, coffee, and red wine.A serious limitation of this line of study is the difficulty of accommodating the variation of polyphenols in a dietary portion of such fruits and vegetables, because this variation might be 10-fold as a result of plant variety, agronomical, food processing, and storage conditions.Recent studies are making it increasingly clear that plants are extremely variable in their polyphenol content, which hinders the interpretation of epidemiologic correlations.Therefore, a better understanding of the causes and magnitude of such variation is essential if valid correlations between diet and health are to be achieved.Catabolites/metabolites excreted in urine have been sought as a possible means to circumvent this problem, but major individual differences clearly indicate a non-normal, probably bimodal distribution and significantly constrain this approach.Such variation seems most pronounced with the gut microbiota catabolism, as clearly illustrated by Guttierez-Diaz et al., but human genomic factors are also to be expected.Much has been learned through in vitro and animal studies about the mechanisms and extent of intestinal absorption of polyphenols.Clinical trials have defined the basic metabolic pathways for polyphenols and have provided some indication of the effect of different ways of inclusion of those polyphenols in the diet on their absorption and distribution in the body; however, more data are still required.Also needed are data for repeat consumption at short intervals, as in tea and coffee drinking, and on long-term consumption of vegetables and fruits as part of a normal diet.Intestinal absorption of plant polyphenols is low, sometimes below 0.1% of dose, and the majority of the dietary polyphenols pass to the large bowel, where they are transformed by the gut microbiota.