There is an increased interest in secondary plant metabolites, such as polyphenols and carotenoids, due to their proposed health benefits. Much attention has focused on their bioavailability, a prerequisite for further physiological functions. As human studies are time consuming, costly, and restricted by ethical concerns, in vitro models for investigating the effects of digestion on these compounds have been developed and employed to predict their release from the food matrix, bioaccessibility, and assess changes in their profiles prior to absorption. Most typically, models simulate digestion in the oral cavity, the stomach, the small intestine, and, occasionally, the large intestine. A plethora of models have been reported, the choice mostly driven by the type of phytochemical studied, whether the purpose is screening or studying under close physiological conditions, and the availability of the model systems. Unfortunately, the diversity of model conditions has hampered the ability to compare results across different studies. For example, there is substantial variability in the time of digestion, concentrations of salts, enzymes, and bile acids used, pH, the inclusion of various digestion stages; and whether chosen conditions are static (with fixed concentrations of enzymes, bile salts, digesta, and so on) or dynamic (varying concentrations of these constituents). This review presents an overview of models that have been employed to study the digestion of both lipophilic and hydrophilic phytochemicals, comparing digestive conditions in vitro and in vivo and, finally, suggests a set of parameters for static models that resemble physiological conditions.

Diet has shaped microbiota profiles through human evolution. Traditional gut microbiomes are described to be driven by high levels of Prevotella. In the present, however, it is consistently described a lower microbial richness in urban industrialized populations compared with individuals living in rural settings, Bacteroides being predominant among urban-industrial gut microbiomes. Components of diet are highly influential in shaping the gut microbiota, being fiber, fat, proteins, polyphenols and micronutrients differentially metabolized by generalist and specialized microorganisms alone or through the phenomenon of cross-feeding. The progressive loss of microbial diversity over generations in industrialized societies along with the emerging increase of chronic non-transmissible diseases have been related to the decline in the consumption of dietary fiber. Diet and derived microbial metabolites have strong implications with the development of food associated diseases such as obesity and metabolic syndrome, malnutrition and eating disorders, intestinal inflammatory diseases and colorectal cancer, among others. Still, there is a need of further studies in order to identify microbiota-related biomarkers of risk for these disorders. In turn, healthy diets and specific nutritional interventions, including increase of dietary fiber and the consumption of probiotics and prebiotics, could be valuable for restoration of beneficial bacteria and microbiota diversity capable to shift from disease to health promoting states.