Relevance and challenges in modeling human gastric and small intestinal digestion Aure ´ lie Guerra 1* , Lucie Etienne-Mesmin 1, 2* , Vale ´ rie Livrelli 2 , Sylvain Denis 1 , Ste ´ phanie Blanquet-Diot 1 , and Monique Alric 1 1 Clermont Universite ´ , Universite ´ d’Auvergne, Centre de Recherche en Nutrition Humaine Auvergne, EA 4678 CIDAM, ‘Conception, Inge ´ nierie et De ´ veloppement de l’Aliment et du Me ´ dicament’, BP10448, F-63000 Clermont-Ferrand, France 2 Clermont Universite ´ , Universite ´ Auvergne, Centre de Recherche en Nutrition Humaine Auvergne, M2iSH, ‘Microbes, Intestin, Inflammation et Susceptibilite ´ de l’Ho ˆ te’, UMR Inserm/Universite ´ d’Auvergne U1071, USC-INRA 2018, BP 10448, F-63000 Clermont-Ferrand, France Gastric and small intestinal (GSI) models are increasing- ly used as an alternative to in vivo assays to answer many questions raised by industry and researchers. A broad range of in vitro systems is available, from static mono- compartmental to dynamic multicompartmental mod- els. However, these models require a compromise between technological complexity and biological signif- icance. Further efforts and technological innovations are still needed to improve in vitro models and meet grow- ing demands in the areas of nutrition and health. This review describes the models available to date for the human stomach and small intestine and highlights their relevance in nutritional, toxicological, pharmaceutical, and microbiological studies. Limitations and challenges facing artificial digestion technology are also discussed. In vitro models: taking digestion out of the body Human digestion is a complex process essential for health wherein ingested food is broken into nutrients that can be used by the body for growth, cell maintenance, and fuel. During human digestion, two main processes occur simul- taneously: (i) mechanical transformations that reduce the size of the food particles; and (ii) enzymatic transformations where macromolecules are hydrolyzed into smaller consti- tuents that are absorbed into the bloodstream. Food disin- tegration mainly occurs in the mouth and stomach, whereas enzymatic digestion and absorption of nutrients and water take place mainly in the small and large intestine. The digestive system is central to numerous questions raised by researchers and industrials in various fields such as nutrition, toxicology, pharmacology, and microbiology [1–3]. Unfortunately, studying the complex multistage process of human digestion is technically difficult, costly, and limited by ethical constraints when potentially harm- ful substances are involved (such as xenobiotics or patho- genic microorganisms). There is consequently a real need for in vitro models that closely mimic the physiological processes occurring during human digestion. These models should be flexible, accurate, and reproducible. Many attempts to model the human stomach and small intestine have been made in the past two decades. Most of these in vitro tools are static, include a limited number of simulated parameters, and are dedicated to a particular application [4]. However, to simulate the complex physio- logical and physicochemical events occurring within the upper human digestive tract, it is crucial to expose a meal to each step of digestion with realistic transit time, pH, and enzymatic conditions. Consequently, a few dynamic bicom- partmental [5–7] or multicompartmental [8] models have been developed and applied in a large number of studies. Despite their complexity, the GSI models described so far remain simplified compared to the in vivo situation: they do not include feedback mechanisms, resident microbiota, immune system, or specific hormonal controls. Further efforts and technological innovations are therefore needed to improve in vitro models and keep up with the growing interest of industry and researchers. This review provides an overview of the GSI models developed to date and their main applications, and discusses the limitations and chal- lenges facing this in vitro technology for adequate simula- tion of human digestive functions. Mastication simulators, which have specific scope and purposes (such as food texture or flavor release studies [9,10]), are not discussed here, and nor are colonic models, which have been recently reviewed [11,12]. Region specificity and complexity of human gastrointestinal digestion Digestion starts with chewing food in the mouth (Figure 1). Mastication is a short but important step with a significant influence on the overall digestive process and particularly on gastric emptying rate [13]. The food bolus resulting from mechanical and enzymatic degradations in the mouth is transported through the esophagus to the stomach by the mechanism of peristalsis. The gastric digestion is a regionalized dynamic step [14]. In the proximal part of the stomach, fundus and body act as a reservoir for food and initiate the contact between Review Corresponding author: Blanquet-Diot, S. (stephanie.blanquet@u-clermont1.fr) Keywords: human digestion; stomach and small intestine; in vitro models. * These authors contributed equally to this manuscript. 0167-7799/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tibtech.2012.08.001 Trends in Biotechnology, November 2012, Vol. 30, No. 11 591