Current in vitro digestion systems for understanding food digestion in human upper gastrointestinal tract

Food digestion rate and location within the gastrointestinal (GI) tract are important for human health. Ideally, food digestion studies should be performed in vivo but this is not always technically, ethically and financially possible. Thus, various in vitro digestion systems have been developed, from static mono-compartmental to dynamic multi-compartmental models, to simulate food digestive behaviors within the GI tract. In this review, food digestion process along the GI tract is briefly described. The current in vitro digestion systems with regards to the human GI physiology, and their advantages versus limitations in the understanding of various food digestion processes in the upper GI tract are critically discussed. There is an emphasis on the "near real" dynamic rat (DRSD) and human (DHSI) gastric-intestinal systems, which not only mimic the peristaltic movements and biochemical conditions found in vivo, but also incorporate the gastric morphology and anatomical structures. Although some in vitro digestion systems reported in literature can be statistically correlated with certain perspectives of food digestion processes in vivo, many physiological, anatomical and geometrical factors that play important roles in determining the digestion rate and extent have been overlooked. The DRSD and DHSI are advantageous in terms of being able to resemble the gastric morphology and anatomy in the rats and humans, respectively. It is of importance that the upper GI anatomy and morphology along with the related biochemical environments and peristaltic movements occurring in vivo should be considered in the development of more advanced and biologically relevant in vitro digestion systems.