Besides one should also consider that all

Besides, one should also consider that all antioxidant compounds (free soluble or insoluble bounds) exist all together in colon, where radicals and antioxidant compounds react continuously with each other. At this point, after the consumption of antioxidants bound to dietary fiber, they reach the colon without being digested. They scavenge the radicals that are naturally found in the colon and become bound antioxidant radicals themselves. Fortunately, if free soluble antioxidant compounds are also available in the colon, then these soluble antioxidants regenerate the bound antioxidant radicals thus helping them to exert their antioxidant action longer. In a word, the balance between oxidative radical species—that is, between soluble and insoluble antioxidant compounds in the digestive system—provides a healthy antioxidant environment to protect the gastrointestinal digestive system from diseases (Cömert & Gökmen, 2017). These findings have important implications for improving the knowledge about physiological effects of antioxidant compounds and for developing functional foods. With these developments, it was emphasized that antioxidant potential in colon should also be considered to determine the antioxidant activity in human body. Serrano et al. (2007) highlighted that antioxidant capacity of compounds (especially in cereals and legumes) determined by the sum of bioaccessible part in small and large intestine were higher than those determined by classical chemical assays. Rufián-Henares and Delgado-Andrade (2009) emphasized that when determining the antioxidant activity of food during in vitro digestion, both the soluble (digestible) and insoluble fractions (represented colon phase) must be considered to evaluate the exact antioxidant activity of bound antioxidants. This study provided a basis for further studies to predict the exact physiological activity of antioxidants using the QUENCHER approach combined with enzymatic Metaphit synthesis for both insoluble and soluble fractions (Pastoriza et al., 2011, Rufián-Henares and Delgado-Andrade, 2009) and even their combination with fermentation procedure (Pérez-Burillo et al., 2018). In addition to simulation of in vitro gastrointestinal digestion models, their combination with dialysis method have been also used to understand the bioavailability and antioxidant potential of active compounds (Liang et al., 2012, You et al., 2010). However, one should also keep in mind that intestinal absorption involved biochemical events along with the physical and chemical processes in vitro models must contain all of these events. Fernández-García et al. (2012) provided valuable information about the experimental models focusing on all events (digestibility or absorption efficiency, structural modifications during digestion or the intestinal metabolism, mechanisms involved in the absorption or inhibition of absorption) occurred during each stage of the carotenoid digestion process. This type of studies must be expanded for all antioxidant structures to evaluate the mechanism of antioxidant activity in the body. It has been regarded necessary to extend our knowledge concerning metabolism, absorption, bioavailability and the mechanism of phenolic compounds and their bioactive properties. To date, numerous studies concerning the gut absorption of phenolic compounds have been performed although most have used in vitro models; such as intestinal absorption through Caco-2 cell simulations (Fernández-Ochoa et al., 2017). Moreover, plasma samples after taken from in situ or in vivo assays have been also rarely used to verify the absorption and an in-depth study of the bioavailability of phenolic compounds, as they and their metabolites could be absorbed across the gut barrier to reach the blood stream (Fernández-Ochoa et al., 2017). Nevertheless, although the number of these types of studies has recently increased, there is a lack of information on absorption and bioavailability of complex compounds for mechanistic understanding.