During the last decade, there has been a growing interest in understanding food's digestive fate in order to strengthen the possible effects of food on human health. Ideally, food digestion should be studied in vivo on humans but this is not always ethically and financially possible. Therefore, simple in vitro digestion models mimicking the gastrointestinal tract have been proposed as alternatives to in vivo experiments. Thus, it is no surprise that these models are increasingly used by the scientific community, although their various limitations to fully mirror the complexity of the digestive tract. Therefore, the objective of this article was to call upon the collective experiences of scientists involved in Infogest (an international network on food digestion) to review and reflect on the applications of in vitro digestion models, the parameters assessed in such studies and the physiological relevance of the data generated when compared to in vivo data. The authors provide a comprehensive review in vitro and in vivo digestion studies investigating the digestion of macronutrients (i.e., proteins, lipids, and carbohydrates) as well as studies of the bioaccessibility and bioavailability of micronutrients and phytochemicals. The main conclusion is that evidences show that despite the simplicity of in vitro models they are often very useful in predicting outcomes of the digestion in vivo. However, this has relies on the complexity of in vitro models and their tuning toward answering specific questions related to human digestion physiology, which leaves a vast room for future studies and improvements.

Understanding and manipulating how emulsion structure impacts on fat digestion is an important step towards understanding the role of fat in our diet. This article reports on the nature of emulsion structuring within the digestive tract and how it affects the dynamics of fat digestion. Emulsions were designed a priori to have specific structuring behaviours (stable, coalesced, partially coalesced and fully broken) under gastrointestinal conditions, through careful emulsifier selection and control of solid fat composition. The impact these structures had on lipolysis was then assessed in vitro using a digestion model and in vivo by measuring the postprandial change in blood triglyceride concentration as a marker of fat absorption. The major factor controlling the rate of fat digestion in vitro was the droplet surface area available for lipase adsorption, which was governed by emulsion instability. The rate of fat absorption in vivo was only affected by large changes in the droplet surface area, and only if these changes remained until the droplets reached the small intestine. This was most evident in emulsions that had undergone extensive partial coalescence under gastric conditions. Partial coalescence resulted in a dramatic reduction in triglyceride absorption, in part because the network of fat crystals provided the agglomerates with an internal scaffold to resist re-dispersion as they passed through the pylorus. The differences in fat absorption profile achieved by controlling emulsion structural stability during digestion provide a basis for examining the physiological effects of food structure on lipid metabolism, which will be the subject of a follow-up clinical paper.

The prevalence of lifestyle-related diseases is increasing in developing countries with the causes for death starting to follow the same pattern in the developed world. Lifestyle factors including inadequate dietary intake of fruits and vegetables and over consumption of nutrient-poor processed foods, are considered to be major causal risk factors associated with increased susceptibility to developing certain diseases (Alldrick, 1998 Alldrick, A. J. (1998). Functional foods: Assuring quality. In: Functional foods - The consumer, the products, the evidence, pp. 135–142. Sadler, M. J. and Saltmarsh, M. Ed. Royal Society of Chemistry, Cambridge . [Google Scholar] ; Kiani, 2007 Kiani, L. (2007, 13/03/2008). “Natural miracles: What functional foods can do for you?” ProQuest Discovery Guides. from http://www.csa.com/discoveryguides/discoveryguides-main.php. [Google Scholar] ). Recent epidemiological evidence confirms a strong association between dietary fiber and reduced all-cause mortality risk, as well as a risk reduction for a number of non-communicable diseases (Chuang et al., 2012 Chuang, S.-C. , Norat, T. , Murphy, N. , Olsen, A. , Tjonneland, A. , Overvad, K. , Boutron-Ruault, M. C. , Perquier, F. , Dartois, L. , Kaaks, R. , Teucher, B. , Bergmann, M. M. , Boeing, H. , Trichopoulou, A. , Lagiou, P. , Trichopoulos, D. , Grioni, S. , Sacerdote, C. , Salvatore, P. , Palli, D. , Tumino, R. , Peeters, P. H. M. , Bueno-de-Mesquita, B. , Ros, M. M. , Brustad, M. , Asli, L. A. , Skeie, G. , Quiros, J. R. , Gonzalez, C. A. , Sanchez, M.-J. , Navarro, C. , Aicua, E. A. , Dorronsoro, M. , Drake, I. , Sonestedt, E. , Ingegerd, J. , Hallmans, G. , Key, T. , Crowe, F. , Khaw, K.-T. , Wareham, N. , Ferrari, P. , Slimani, N. , Romieu, I. , Gallo, V. , Riboli, E. and Vineis, P. (2012). Fiber intake and total and cause-specifics mortality in the European prospective investigation into cancer and nutrition cohort. Am. J. Clin. Nutr. 96:164–174. [Crossref], [PubMed], [Web of Science ®] [Google Scholar] ). The relationship between dietary fiber and mortality has been described as “convincing observations that call for mechanistic investigations” (Landberg, 2012 Landberg, R. (2012). Dietary fiber and mortality: convincing observatins that call for mechanistic investigations. Am. J. Clin. Nutr. 96:3–4. [Crossref], [PubMed], [Web of Science ®] [Google Scholar] ). In particular, the health protective roles played by dietary fibers of different origin are not well understood. Whilst Hippocrates was the earliest known physician to study the health benefits of fiber derived from grains (Burkitt, 1987 Burkitt, D. P. (1987). Dietary fibre historical aspects. Scand. J.Gastroentero 22(S129):10–13. [Taylor & Francis Online], [Web of Science ®] [Google Scholar] ), the functionality of fruit and vegetable fiber, especially in association with other compounds such as polyphenols and carotenoids, is an area of more recent interest. Hence the objective of this review is to assess the complexity and health-related functional role of plant cell wall (PCW) fibers from fruits and vegetables with a particular emphasis on interactions between cell walls and phytonutrients.