This paper uses computational fluid dynamics to simulate and analyze intragastric fluid motions induced by human peristalsis. We created a two-dimensional computational domain of the distal stomach where peristalsis occurs. The motion of the gastric walls induced by an antral contraction wave (ACW) on the wall of the computational domain was well simulated using a function defined in this study. Retropulsive flow caused by ACW was observed near the occluded region, reaching its highest velocity of approximately 12 mm/s in the narrowest region. The viscosity of the model gastric contents applied in this study hardly affected the highest velocity, but greatly affected the velocity profile in the computational domain. The shear rate due to gastric fluid motion was calculated using the numerical output data. The shear rate reached relatively high values of approximately 20 s −1 in the most occluded region. The shear rate profile was almost independent of the fluid viscosity. We also simulated mass transfer of a gastric digestive enzyme (pepsin) in model gastric content when peristalsis occurs on the gastric walls. The visualized simulation results suggest that gastric peristalsis is capable of efficiently mixing pepsin secreted from the gastric walls with an intragastric fluid.

A novel in vitro gastric device, the Gastric Digestion Simulator (GDS), was developed for the direct observation and quantitative analysis of the food digestion process in the human stomach. In addition to simulating the chemical digestion environment, this device provides a physical digestion environment comparable to that found in the stomach by simulating peristalsis, which is assumed to contribute to solid food disintegration. The GDS was successfully used to directly observe the disintegration process of Tofu (bean curd) as a typical solid food containing protein. The size distribution and protein content of Tofu particles during the digestion experiments were investigated. The results demonstrated the difference in particle disintegration between GDS and flask shaking experiments, which may be due to the lack of peristalsis in the latter case. Moreover, the size distribution of Tofu particles after the GDS experiments was affected by the physical properties of Tofu, thus revealing the usefulness of GDS for food digestion analysis.Keywords: GI tract, gastric digestion, in vitro gastric device, peristalsis, direct observation, solid food IntroductionIn the human gastrointestinal (GI) tract, foods are digested by a combination of physical and chemical processes. Chemical digestive processes are catalyzed by digestive enzymes secreted in the stomach and small intestine, and disintegrate foods down to the molecular scale. However, physical digestive processes, which are induced mainly by peristalsis, mix and empty gastric and intestinal contents, and thus play an important role in promoting food digestion in the GI tract. Peristalsis in the GI tract is caused by peristaltic wall motion (e.g., antral contraction waves (ACWs)) in the stomach. Periodically generated ACWs induce the motion of the gastric contents, mix the gastric contents, and grind bulk solid foods to reduce their particle size (Kong and Singh, 2008) Food digestion in the human GI tract has been studied using in vivo, in vitro, and in silico approaches. MRI has been used for in vivo studies on gastric peristaltic motion and the physical properties of gastric contents. The speed of ACWs and the ACW amplitude in the human stomach were calculated using data obtained by real-*To whom correspondence should be addressed. time MRI (King et al., 1984;Pallotta et al., 1998). The timedependent change of the viscosity of the gastric contents was measured using echo planar MRI (Marciani et al., 2000). This study seeks to develop a new in vitro gastric digestion device that focuses mainly on the physical digestion environment of the human stomach, and to assess the device's performance. The gastric digestion simulator (GDS) that we developed here can simulate peristaltic motion in the human stomach and enables direct observation of the digestion process in real time. In this study, two different types of Tofu were used as model solid foods.We investigated food digestion characteristics using GDS to quantitatively understand the dynamic digestion processes in t...

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This study quantitatively analyzed the flow phenomena in model gastric contents induced by peristalsis using a human gastric flow simulator (GFS). Major functions of the GFS include gastric peristalsis simulation by controlled deformation of rubber walls and direct observation of inner flow through parallel transparent windows. For liquid gastric contents (water and starch syrup solutions), retropulsive flow against the direction of peristalsis was observed using both particle image velocimetry (PIV) and computational fluid dynamics (CFD). The maximum flow velocity was obtained in the region occluded by peristalsis. The maximum value was 9 mm s(-1) when the standard value of peristalsis speed in healthy adults (UACW = 2.5 mm s(-1)) was applied. The intragastric flow-field was laminar with the maximum Reynolds number (Re = 125). The viscosity of liquid gastric contents hardly affected the maximum flow velocity in the applied range of this study (1 to 100 mPa s). These PIV results agreed well with the CFD results. The maximum shear rate in the liquid gastric contents was below 20 s(-1) at UACW = 2.5 mm s(-1). We also measured the flow-field in solid-liquid gastric contents containing model solid food particles (plastic beads). The direction of velocity vectors was influenced by the presence of the model solid food particle surface. The maximum flow velocity near the model solid food particles ranged from 8 to 10 mm s(-1) at UACW = 2.5 mm s(-1). The maximum shear rate around the model solid food particles was low, with a value of up to 20 s(-1).

This study investigates the ef fect of the texture of agar gel as a model solid food on gastric digestion using a human Gastric Digestion Simulator (GDS). The GDS, which simulates gastric peristalsis, can investigate physical digestion phenomena such as disintegration of solid foods. Agar gels with different fracture stresses (56 to 219 kPa) and constant fracture stain (29%) were prepared by var ying the agar concentration (1.5 to 4.5 wt%). Direct obser vation demonstrated that agar gel particles initially cut to a 5.0 mm cube gradually reduced in size and broke down into random shapes during simulated gastric peristalsis. The size distribution of the agar gel particles after the digestion experiment was analyzed using the sieving method. The fraction larger than 2.36 mm, which corresponds to the pylorus size, decreased with time: the wet weight ratio of that fraction to the total amount of agar gel particles was 18.0% at 180 min in the case of 1.5 wt% agar gel. The agar gel concentration did not affect the size distribution after 180 min, which shows that fracture strain plays a more important role in agar gel digestion. Our results provide useful information about the relationship between solid food texture and gastric digestion.

We developed a beverage that forms a gel containing gas bubbles in the stomach and induces satiety. In a preliminary experiment, we confirmed that when a carbonated beverage containing any one of three types of ionic polysaccharides was mixed with artificial gastric juice, it resulted in the formation of a gel containing gas bubbles. Among the three types of polysaccharides referenced above, low methoxyl pectin (LM pectin) was identified as being the optimal for preparation of the test beverage in this study. Both static evaluation using a vessel and dynamic evaluation using the Gastric Digestion Simulator (GDS) revealed that the volume of the bubble-containing gel remained relatively stable with time. Presence of the bubble-containing gel in the stomach following consumption of the carbonated beverage containing LM pectin was confirmed in a clinical Magnetic Resonance Imaging (MRI) study. Consumption of this test beverage resulted in a greater increase of the intragastric volume than consumption of the same amount of water. Moreover, in the satiety questionnaire, the subjects reported a higher degree of satiety following consumption of the beverage than following consumption of an equal amount of water. These results indicate that when this test beverage, a carbonated beverage containing LM pectin that forms a bubble-containing gel in the stomach, is consumed, the stomach becomes distended, inducing a feeling of satiety.