Phenolic compounds : potential health benefits and toxicity

Phenolic compounds are probably the most explored natural compounds due to their potential health benefits as demonstrated in a number of studies (Del Rio et al. 2010). Generic terms ‘phenolic compounds’, ‘phenolics’ or ‘polyphenolics’ refer to more than 8,000 compounds found in the plant kingdom and possessing at least an aromatic ring with one or more hydroxyl substituents, including functional derivatives like esters, methyl ethers, glycosides, etc. (Ho 1992, Cartea et al. 2011). These are plant secondary metabolites produced via shikimic acid pathway (Cartea et al. 2011, Talapatra and Talapatra 2015). Phenolic compounds regulate the various metabolic functions including structure and growth, pigmentation and are resistant to different pathogens in plants (Naumovski 2015). The organoleptic properties of the plant food (fruits, vegetables, cereals, legumes, etc.) and beverages (tea, coffee, beer, wine, etc.) are also partially ascribed to phenolic compounds (Dai and Mumper 2010). For instance, the interactions between phenolic compounds (such as procyanidins) and the glycoprotein present in our saliva contribute to the bitterness and astringency of fruit and juices (Dai and Mumper 2010). These phenolics have varied chemical structures ranging from simple molecules (i.e., phenolic acids) to more complex polymerized compounds (i.e., proanthocyanidins) (Galleano et al. 2010). They also help in defense against ultraviolet radiation, insects and predators (Dai and Mumper 2010). Phenolics derived from various natural sources are linked to antioxidant, anti-inflammatory, anti-allergic, anti-carcinogenic, antihypertensive, cardioprotective, anti-arthritic and antimicrobial activities (Rauha et al. 2000, Penna et al. 2001, Puupponen-Pimia et al. 2001, Wang and Mazza 2002, Liu et al. 2004, Dai and Mumper 2010). Studies 28on natural antioxidants has developed significantly in the last few years due to restrictions on the use of synthetic antioxidants and enhanced public awareness of health-related issues (Vázquez et al. 2012). Because of their potential health benefits, natural antioxidants are considered to be a better alternative than the synthetic ones (Fu et al. 2010). Hence, the identification of novel antioxidants from natural sources is one of the main research focuses in natural product development these days. Various studies validate the positive correlation between phenolic content and the antioxidant activity (Dimitrios 2006, Galleano et al. 2010, Bhuyan et al. 2015). Free radicals play an important role in the development of cancer, diabetes, neurodegenerative, ageingrelated and cardiovascular diseases. Therefore, antioxidants, such as flavonoids and other phenolics have gained more attention in recent years as potential agents for preventing and treating a number of oxidative stress-related and chronic diseases (Rice-Evans et al. 1996, Stanner et al. 2004, Dimitrios 2006, Fu et al. 2010, Galleano et al. 2010, Gharekhani et al. 2012). The antioxidant activity of phenolics is primarily attributed to their redox properties that enable them to act as singlet oxygen quenchers, reducing agents and hydrogen donors (Rice-Evans et al. 1996, Galleano et al. 2010, Gharekhani et al. 2012). The hydroxyl (–OH) groups of phenolics are good H-donating antioxidants that disrupt the cycle of new radical generation by scavenging reactive oxygen species (ROS) (Castellano et al. 2012).