On the kinematics and efficiency of advective mixing during gastric digestion – A numerical analysis
Introduction
The delivery of improved human health and well-being through food innovation relies on a better understanding of the conditions to which foods are exposed upon ingestion. Among them, the role of gastric digestion on the final delivery of optimal nutrition is being increasingly recognized (Wickham et al., 2012).
Second only to the mouth, the stomach is the main site for food disintegration. Changes in the compliance of the proximal wall allow the stomach to receive and store the ingested meal, while its disintegration is facilitated by a series of antral contraction waves (ACWs) that periodically compress, shear and mix gastric contents during the process (Schulze, 2006).
The use of advanced imaging technologies has recently demonstrated how the physical properties of the meal can modulate satiety and the rate of nutrient delivery to the small bowel, by simply modifying the intragastric distribution of the meal (Boulby et al., 1999, Faas et al., 2002, Kunz et al., 2005, Marciani et al., 2001, Marciani et al., 2007, Marciani et al., 2012). However, up to now little is known about the mechanisms underpinning the distribution and mixing of different digesta systems within the stomach.
Computational modeling has been recently used as an alternative approach to investigate the dynamics of gastric contents (Pal et al., 2004, Kozu et al., 2010, Ferrua and Singh, 2010, Ferrua et al., 2011, Imai et al., 2013). In general, these studies have focused on the behavior of different Newtonian fluids at the time of the most occluded ACW, and little has been done to characterize their mixing abilities. A pioneer work in this area, Pal et al. (2004) analyzed the dispersion of small particles within a 2D model of the human stomach and found that mixing was largely confined to its distal region. In close agreement, Imai et al. (2012) showed that the dispersion of tracer particles within a 3D model of the stomach was always enhanced by postures that completely fill this region. While informative, these two studies investigated only the advective properties of a single Newtonian fluid (1 Pa s) and based their analysis on the sole dispersion of discrete and inert particles within the domain. While associated to the bulk motions, the simple dispersion of particles within the domain cannot fully describe the advective properties of the flow (as for instance, particles trapped inside an eddy will experience little dispersion). In addition, the results obtained cannot provide a universal characterization of the advective performance of the flow, as the dispersion of the particles is highly dependent on the geometrical dimensions of the system.
From a fundamental perspective, the efficiency of advective mixing can be ultimately traced to the amount of stretching that differential material elements experience while being transported and deformed by the flow (Ottino, 1990). The faster and more chaotic their stretching growth within the domain, the faster and more efficient the mixing process. In this study, a numerical model (previously developed and validated) was used to characterize the kinematics and efficiency of advective mixing during gastric digestion by analyzing the stretching properties of gastric flows with different rheological properties.
Section snippets
Gastric geometry
A simplified 3D model that reproduces the shape and averaged dimensions of a human stomach was developed (Fig. 1a). The shape of the model was created using a series of 89 auxiliary circles that extended between the lesser and greater curvatures of a crossectional image of the human stomach (MedlinePlus). The model was then scaled to reproduce the average dimensions of a human stomach (Geliebter et al., 1992, Keet, 1993, Schulze, 2006).
A Cooper scheme was used to mesh the gastric domain (0.3
Dynamics of gastric fluids (Eulerian analysis)
Gastric flows were generally characterized by two regions with widely different behaviors and time scales (Fig. 2). The faster and more irregular fluid motions always developed within the distal region of the stomach, while the dynamics of the proximal region was characterized by slower and more ordered bulk motions.
Gastric rheology significantly affected the velocity and overall behavior of gastric flows. The large retropulsive and vortex structures, commonly used to describe the dynamics of
Conclusions
By analyzing the stretching ability of different gastric flows, this study provided a more fundamental and universal characterization of the kinematics and efficiency of advective mixing within the stomach.
Similar to previous studies, the formation of two distinct flow regions of significantly different advective properties was determined. Advection was always enhanced within the distal region of the stomach by the more irregular behavior of the flow. Despite the significant effect of gastric
Conflict of interest statement
The authors declare no conflict of interest.
Acknowledgment
This research was partially supported by USDA-NRI Contract C00029256-1.
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