Modeling the Effect of Sleeve Gastrectomy on Gastric Digestion in the Stomach: Insights from Multiphase Flow Modeling

Jung-Hee Seo; Rajat Mittal; Sharun Kuhar; Weixuan Li

REVIEW 3 major objections 6 minor 26 references

Modeling the Effect of Sleeve Gastrectomy on Gastric Digestion in the Stomach: Insights from Multiphase Flow Modeling

T0 review · 3 major / 6 minor · reviewed 2026-08-12 · deepseek-v4-flash

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Pith's one-line read Sleeve gastrectomy accelerates gastric emptying through higher intragastric pressure and faster volume change, with modeled liquid emptying 33–87 percent faster.

desk verdict A legitimate first computational flow study of sleeve gastrectomy, but the headline emptying numbers are largely inherited from prescribed pressure inputs, and an internal inconsistency in the extreme-sleeve ACW length muddies the mechanism. read the letter →

arxiv 2411.18586 v2 pith:6JEFKIVG submitted 2024-11-27 physics.flu-dyn physics.bio-phphysics.comp-ph

Weixuan Li , Sharun Kuhar , Jung-Hee Seo , Rajat Mittal This is my paper · ORCID

classification physics.flu-dynphysics.bio-phphysics.comp-ph

keywords sleevegastrectomygastricemptyingmultiphaseflowcomputationalfluiddynamicsimmersedboundarymethodstomachmotilityintragastricpressureSim

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper uses a computational model of the stomach, built from imaging data, to ask how sleeve gastrectomy changes the way a liquid meal is emptied and mixed. It claims that removing stomach tissue accelerates gastric emptying: total liquid emptying is 33 percent faster in a moderate sleeve that keeps 70 percent of the original volume and 87 percent faster in an extreme sleeve that keeps 45 percent, while the food bolus itself empties 106 and 210 percent faster. The acceleration is attributed to two linked mechanisms: the smaller sleeve raises intragastric pressure during the phase when the pylorus is open, and antral contraction waves reach full strength sooner, so the stomach volume drops faster. The paper also claims that when motility is impaired by moving the resection closer to the pylorus, total emptying is 21 percent faster but bolus emptying is 49 percent slower, because slower mixing leaves the bolus less ready to leave. A sympathetic reader cares because these predictions give a mechanistic, patient-specific route to understanding a surgery whose emptying outcomes are measured mainly by half-emptying times.

What carries the argument

The central object is StomachSim, an imaging-based multiphase computational model of the stomach in which a liquid meal of viscosity 0.1 Pa·s moves through a water-like gastric solvent inside a deformable geometry driven by antral contraction waves and a fundic 'piston' boundary condition. The piston enforces a fundic pressure $p_o$ set to 0.075 mmHg pre-operatively and to 0.097 and 0.125 mmHg in the moderate and extreme sleeves, following published pressure–volume ratios. The model then computes emptying flux through the pylorus and an intensity-of-segregation mixing index. The key relationship carrying the argument is that emptying rate tracks the simultaneous evolution of intragastric pressure and volume variation rate during the 7-second pylorus-open window; this is what converts resected volume into faster emptying.

What would settle it

Measure intragastric pressure and gastric emptying together in patients after sleeve gastrectomy, with sleeve volume quantified by imaging, and check whether emptying speed scales with pressure during the pylorus-open period as the model predicts. A simpler computational falsifier is to rerun the same simulations with $p_o$ set by a patient-specific pressure–volume curve; if the bolus acceleration disappears, the central claim is an artifact of the chosen boundary pressure.

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Extended reading notes

Core claim

On the paper's own terms, the central discovery is that sleeve gastrectomy accelerates gastric emptying through a pressure–volume mechanism rather than through a simple loss of storage capacity. In simulations of a liquid meal, the reduced post-surgery stomach develops higher average intragastric pressure while the pylorus is open—0.32 mmHg for the moderate sleeve and 0.44 mmHg for the extreme sleeve, versus 0.17 mmHg pre-operatively—and shows a faster rate of volume decrease, because antral contraction waves originate closer to the pylorus and reach their maximal amplitude sooner. The combined effect is a monotonic increase in emptying rate with resection volume: total liquid emptying rises by 33 percent and 87 percent, and bolus emptying by 106 percent and 210 percent. A second finding is that motility impairment and size reduction pull in opposite directions for the bolus: with the resection extended to 2 cm from the pylorus, total emptying is 21 percent faster than the moderate sleeve while bolus emptying is 49 percent slower, since the loss of proximal motility degrades mixing. The authors take these results to show that computational fluid dynamics can resolve emptying dynamics, not just half-emptying times, and can distinguish effects of anatomy from effects of motility.

Load-bearing premise

The load-bearing premise is that the simulated post-surgery stomachs, including their prescribed fundic pressures, faithfully represent real sleeve-gastrectomy anatomy and tone; if those inputs are unrepresentative, the reported emptying acceleration is an artifact of the boundary conditions.

Editorial extensions

If this is right

If the pressure–volume mechanism is right, post-LSG emptying speed should scale with retained volume, and surgeons could use modeled pressure–volume curves to anticipate which patients will empty too fast.
The 106–210 percent bolus acceleration predicts that liquid nutrients reach the duodenum sooner after surgery, consistent with the GLP-1 and reduced-absorption pathway the paper cites for weight loss.
The dissociation between total and bolus emptying in the reduced-motility case implies that half-emptying time alone can hide nutrient-specific emptying deficits.
The model's validation against measured emptying rates of about 4.1–4.26 ml/min suggests the same solver could be applied to other bariatric geometries or meal viscosities before clinical trials.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

A testable extension would be to run the same model with fundic pressure derived from patient-specific post-surgery pressure–volume measurements instead of Toniolo et al.'s ratios; if the emptying acceleration persists, the geometry effect is real, and if it vanishes, the piston pressure is the true driver.
Because the bolus emptying rate is 49 percent slower when the resection reaches 2 cm from the pylorus, the model implicitly predicts that preserving antral length is what protects bolus emptying—a quantity surgeons could weigh against weight-loss goals.
The same framework could be applied to solid or semi-solid meals, where mixing impairment is likely to matter more than for liquids; this would connect the model to the observed variability in post-LSG emptying outcomes.
One could test the retrograde-jet claim directly by imaging antro-duodenal flow after LSG: the paper predicts attenuated retrograde jets when motility is reduced, which would appear as weaker backflow on time-resolved MRI.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Here's my take on arXiv:2411.18586, the Li et al. StomachSim sleeve gastrectomy paper. What's actually new: they extend their previously validated multiphase flow model of the stomach to simulate laparoscopic sleeve gastrectomy, looking at flow, mixing, and emptying rather than just wall stress. That's a real gap in the literature, and they fill it. They also find something non-obvious: in the reduced-motility case, total emptying speeds up while bolus (nutrient) emptying slows down—a decoupling that emerges from the flow and mixing, not from the prescribed inputs. That part is worth attention. The validation is modest but reasonable: the VOF method is checked against a falling drop, and their pre-operative model matches published emptying rates from their earlier work. For a follow-on application paper, that's adequate. Now the soft spots. First, the central quantitative result is largely hardwired. The fundic pressure po is a prescribed boundary condition, taken from Toniolo et al.'s pressure-volume ratios (0.097 and 0.125 mmHg for the two sleeves). Equation (1) shows that higher po directly pushes more fluid through the system. So the 33% and 87% acceleration numbers are not independent predictions—they follow from the chosen inputs. The paper acknowledges this in Section 2.2, but the conclusions still present the acceleration as a finding. Without a sensitivity analysis or a control case that changes geometry/motility while holding po fixed, we can't tell how much of the effect is the boundary condition. Second, there's a clear internal inconsistency. Section 3.2 says the extreme sleeve has ACW starting length lA = 2 cm, while Table 1 and Fig. 4(c) say the resection ends at P1, 6 cm from the pylorus—same as the moderate sleeve. The proposed mechanism for the faster volume variation in the extreme sleeve (ACWs reaching maximal amplitude sooner) is therefore wrong for that case. This needs fixing before the mechanism discussion can be trusted. Third, no code or data are made available, which makes it harder to check the quantitative claims. The limitations section mentions simplified motility and liquid meals, but doesn't address the direct dependence of the headline result on the prescribed pressures. That omission matters. Who is this for? People working in computational gastroenterology and bariatric surgery biomechanics. It's a useful proof-of-concept, and the qualitative trends align with clinical observations. But I would not rely on the specific percentages. Recommendation: send it to peer review, not desk reject. A serious referee can push for the sensitivity analysis and the correction of the lA inconsistency. The core approach is sound, and the topic is relevant. Overall: worth a careful look, but the advertised numbers need to be interpreted with caution.

Referee Report

3 major / 6 minor

Summary. This paper uses an immersed-boundary multiphase flow solver (StomachSim/ViCar3D) to simulate the emptying and mixing of a high-viscosity liquid meal in a pre-operative MRI-derived stomach model and in two Blender-edited post-sleeve-gastrectomy geometries (moderate, 70% retained volume; extreme, 45% retained volume). Four cases vary resection endpoint and motility; fundic pressure p_o is imposed at the fundic inlet through Eq. (1), with values 0.075, 0.097, and 0.125 mmHg taken from Toniolo et al.'s pressure-volume ratios. The authors report that total liquid emptying is 33% and 87% faster and bolus emptying 106% and 210% faster in the moderate and extreme sleeves, and that a reduced-antral-motility variant empties total liquid faster but bolus more slowly. They attribute the acceleration to increased intragastric pressure and faster volume variation after surgery.

Significance. If the acceleration were an emergent model outcome, this would be a useful demonstration of CFD for bariatric surgical planning, with strengths including an imaging-derived pre-operative geometry, a documented pre-operative emptying-rate comparison (4.48 vs 4.1 and 4.26 ml/min), a VOF validation in Appendix A, and a well-controlled motility comparison (Cases 2 and 4 share retained volume and p_o). However, the central quantitative claim is partially hard-wired because p_o is a prescribed input, not a computed quantity; this makes the headline accelerations a consequence of the chosen boundary conditions unless a control or sensitivity analysis shows otherwise. The manuscript is therefore informative as a modeling framework but does not yet establish the stated mechanism.

major comments (3)

[Section 2.2, Eq. (1), Table 1, Section 3.2] The fundic pressure p_o is a prescribed boundary condition, and Eq. (1) makes p_o directly drive the normal velocity at the fundic inlet. Table 1 increases p_o from 0.075 to 0.097 to 0.125 mmHg across the pre-operative, moderate-sleeve, and extreme-sleeve cases, using factors (1.3 and 1.67) taken from Toniolo et al.'s pressure-volume ratios. The simulated average intragastric pressures in Table 3 (0.1745, 0.3192, 0.4432 mmHg) therefore inherit this ordering, and the reported 33% and 87% total-emptying accelerations (and the 106% and 210% bolus accelerations) are at least partly a direct consequence of the imposed inputs rather than an emergent prediction of the model. The manuscript contains no control case in which geometry and motility are changed while p_o is held fixed, and no sensitivity analysis on p_o. The statement in Section 2.2 that p_o 'can be prescribed to not just achieve the desired emptying rate' further suggests that the pre-operative calibration against the literature emptying rate is not an independent test of the post-operative predictions. I therefore do not consider the central quantitative claim to be established as presented.
[Section 3.2 vs Table 1 and Fig. 4(c)] There is a direct internal inconsistency in the definition of the extreme-sleeve case. Section 3.2 states that the ACWs begin at l_A = 2 cm from the pylorus for the extreme sleeve stomach, but Table 1 and Fig. 4(c) specify that the extreme-sleeve resection ends at P1, 6 cm from the pylorus (l_A = 6 cm), identical to the moderate sleeve; the l_A = 2 cm value belongs to Case 4 (moderate sleeve with reduced motility). Because the explanation of the extreme sleeve's faster volume-variation rate relies on ACWs reaching maximal amplitude sooner, this inconsistency removes the stated mechanistic support for the extreme-sleeve result.
[Section 2.2 and Section 3.2] The 'different sleeve size' comparison conflates two changes: the geometry/retained volume and the prescribed p_o. The paper's own mechanism discussion in Section 3.2 attributes the emptying acceleration both to higher intragastric pressure (which is largely imposed) and to volume-variation rate (which for the extreme case rests on the contradicted l_A value). The post-operative geometries are also hand-built in Blender from a single pre-operative anatomy, with no comparison to post-operative imaging. Without a factorial decomposition (e.g., varying geometry with p_o fixed, and p_o with geometry fixed) or at least a sensitivity study over the resection shape and retained volume, the relative contributions of sleeve size and fundic pressure cannot be separated, and the conclusion that the reduction in sleeve size and the concomitant increase in fundic pressure causes the acceleration is not quantitatively supported.

minor comments (6)

[Section 3, References] Reference [23] is cited in the text as Marciani et al., but the reference list entry [23] is Padalino et al., 'Effects of transportation on gastric pH and gastric ulceration in mares'; the intended Marciani reference appears to be missing.
[Section 2.3 vs Section 3.3 and Fig. 5] Section 2.3 defines Phase 1 as the gastric solvent and Phase 2 as the liquid meal, but Section 3.3 and the Fig. 5 caption treat Phase 1 as the liquid meal; the phase numbering should be made consistent throughout.
[Section 2.3, Eq. (5)] Equation (5) includes a diffusion coefficient D_i but no value is specified anywhere; please state whether this is a physical coefficient or a numerical diffusion parameter and give its value.
[Appendix A] The VOF validation uses a density ratio of 5 and equal kinematic viscosities, whereas the stomach simulations use a density ratio of 1.02 and a viscosity ratio of 100; the validation therefore does not exercise the high-viscosity-contrast regime central to the meal model, which should be acknowledged.
[Fig. 7 caption and Table 3] The caption of Fig. 7 says the duodenal outlet pressure is set to zero Pascal, while the text and Table 3 use mmHg; the units should be unified.
[Section 3.2] The statement that 'the motility pattern is kept similar while the sleeve size varies' is imprecise because the pre-operative case has full proximal motility with l_A = 8 cm, whereas both sleeve cases have no sleeve-tube motility with l_A = 6 cm; more precise wording would help.

Circularity Check

1 steps flagged · score 6.0 of 10

Predicted emptying acceleration largely reproduces prescribed fundic-pressure ratios

self definitional [Section 2.2 (fundic pressure model, Eq. (1), Table 1) and Section 3.2 / Conclusions]
"We note that in the model of fundic tone above, po, which represents the pressure exerted by the fundus, affects the flux at the fundic inlet, and consequently at the duodenal outlet. The fundic pressure po can therefore be prescribed to not just achieve the desired emptying rate into the duodenum, but also to model the effect of sleeve gastrectomy on the gastric tone."

The fundic boundary condition (Eq. 1: d u_n/dt = (po - pbar_A)/m_p) makes the inlet flux, and hence the downstream duodenal outflow, a direct response to the prescribed po. Table 1 sets po = 0.075, 0.097, and 0.125 mmHg for pre-operative, moderate, and extreme sleeves, giving increases of about 29% and 67% that are taken from Toniolo et al.'s pressure-volume ratios. The paper then reports total emptying increases of 33% and 87% and concludes that the 'increase in fundic pressure following LSG leads to a significant increase in gastric emptying rate.' That conclusion restates the boundary condition: the fundic pressure increase is an input chosen to drive emptying, not an emergent model prediction.

full rationale

The model's CFD core (immersed-boundary Navier-Stokes, VOF, gastric motility) is independently implemented and validated against Fakhari & Rahimian and prior in vivo emptying rates, so the paper is not globally circular. However, the headline quantitative claim that LSG accelerates emptying by 33%/87% because of reduced sleeve size and increased fundic pressure is substantially hard-wired: the fundic pressures po are prescribed inputs (Table 1) that directly drive fundic inflow and, per the paper's own statement, can be set 'to achieve the desired emptying rate into the duodenum.' The reported total-emptying ratios closely track the prescribed pressure ratios (1.33/1.87 vs. 1.29/1.67), indicating that the central result is largely a transduction of the input. The motility comparison (Case 2 vs Case 4, same po) does contain emergent content, which prevents full circularity. There is also an internal inconsistency between Section 3.2's claim that the extreme sleeve has lA = 2 cm and Table 1 / Fig. 4(c), which list P1 at 6 cm; this is a correctness issue, not a circularity. No load-bearing uniqueness theorem or self-citation chain is invoked.

Assumptions & free parameters 6 free parameters · 6 assumptions · 0 invented entities

The central results depend on several imposed inputs: fundic pressure values, hand-built sleeve geometries, and chosen motility patterns. No new physical entities are introduced, but the modeling assumptions replace what would ideally be patient-specific measured inputs. The free parameters and axioms listed above are the load-bearing choices that determine the emptying and mixing outcomes.

free parameters (6)

Fundic pressure po, pre-operative model = 0.075 mmHg
Imposed inlet pressure at the fundus; directly controls outflow and is taken from prior model calibration/literature rather than derived in this paper.
Fundic pressure po, moderate sleeve = 0.097 mmHg
Set to 1.3 times the pre-operative value based on Toniolo et al.'s pressure-volume relationship for 70% retained volume.
Fundic pressure po, extreme sleeve = 0.125 mmHg
Set to 1.67 times the pre-operative value based on Toniolo et al.'s pressure-volume relationship for 45% retained volume.
Retained stomach volume for sleeve models = 70% and 45%
Representative resection volumes chosen from the literature range; the geometries were created by editing the pre-operative model in Blender, not from post-operative imaging.
Antral contraction wave starting distance lA = 8, 6, 6, and 2 cm for the four cases
Modeling choice for pacemaker impairment and resection endpoint; this directly affects volume variation and emptying. The lA value for the extreme sleeve is inconsistent between text and table.
Liquid meal volume and viscosity = 5 mL, 0.1 Pa·s
A high-viscosity liquid meal (honey-like) is used; solid and semi-solid foods are not considered, as the paper acknowledges.

assumptions (6)

domain assumption Incompressible Navier-Stokes and volume-of-fluid equations describe gastric contents flow
Equations (2) to (5) assume incompressible, immiscible fluids with a diffusion term for volume fraction; gastric juice and liquid meal are treated as Newtonian fluids.
domain assumption The single-subject VPL geometry modified to a postprandial shape is representative of a human stomach
Section 2.1: geometry is from the Virtual Population Library 'Duke' model, modified based on public MRI data; it is not patient-specific.
ad hoc to paper Blender-edited geometries capture actual post-sleeve-gastrectomy anatomy
Section 2.2: no post-operative imaging was used; the sleeve shapes are constructed from the pre-operative model and may not reflect real surgical outcomes.
ad hoc to paper Motility simplification: complete loss of peristalsis in the sleeve tube with preserved antral motility
Section 2.2 and limitations: based on Baumann et al., but actual post-LSG motility varies among patients and is not always fully absent in the sleeve.
domain assumption The fundic piston boundary condition models fundic tone and emptying drive
Equation (1) and ref. [21]: the piston is a simplified representation of fundic contraction; the pressure po is imposed externally.
domain assumption Toniolo et al.'s pressure-volume ratios apply to the modeled sleeve geometries
Section 2.2: the pressure ratios from a different computational study are used to set po for the moderate and extreme sleeves.

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Cite this review

Pith. "Pith review of Modeling the Effect of Sleeve Gastrectomy on Gastric Digestion in the Stomach: Insights from Multiphase Flow Modeling." pith.science (2026). https://pith.science/paper/6JEFKIVG

@misc{pith2026241118586,
  author       = {Pith},
  title        = {Pith review of: Modeling the Effect of Sleeve Gastrectomy on Gastric Digestion in the Stomach: Insights from Multiphase Flow Modeling},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/6JEFKIVG}},
  note         = {Machine review of arXiv:2411.18586}
}

read the original abstract

The geometry and motility of the stomach play a critical role in the digestion of ingested liquid meals. Sleeve gastrectomy, a common type of bariatric surgery used to reduce the size of the stomach, significantly alters the stomach's anatomy and motility, which impacts gastric emptying and digestion. In this study, we use an imaging data-based computational model, StomachSim, to investigate the consequences of sleeve gastrectomy. The pre-operative stomach anatomy was derived from imaging data and the post-sleeve gastrectomy shapes were generated for different resection volumes. We investigate the effect of sleeve sizes and motility patterns on gastric mixing and emptying. Simulations were conducted using an immersed-boundary flow solver, modeling a liquid meal to analyze changes in gastric content mixing and emptying rates. The results reveal that different degrees of volume reduction and impaired gastric motility have complex effects on stomach's mixing and emptying functions, which are important factors in gastric health of the patient. These findings provide insights into the biomechanical effects of sleeve gastrectomy on gastric digestion and emptying functions, highlighting the potential of computational models to inform surgical planning and post-operative management.

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Simulationoffallingdropletbythelat- tice Boltzmann method,

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Pith tools

Reviewed August 12, 2026 · model on record in the stance chip above.

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Laparo- scopic sleeve gastrectomy—volume and pressure assessment,

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Laparoscopic sleeve gastrectomy for morbid obesity,

Iannelli, A., Dainese, R., Piche, T., Facchiano, E., and Gugenheim, J., 2008, “Laparoscopic sleeve gastrectomy for morbid obesity,” World Journal of Gas- troenterology: WJG,14(6), p. 821

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The effect of sleeve gastrectomy on GLP-1 secretion and gastric emptying: a prospective study,

Sista,F.,Abruzzese,V.,Clementi,M.,Carandina,S.,Cecilia,M.,andAmicucci, G., 2017, “The effect of sleeve gastrectomy on GLP-1 secretion and gastric emptying: a prospective study,” Surgery for Obesity and Related Diseases, 13(1), pp. 7–14. Fig. 12 Schematic configuration of the two-dimensional falling drop simulation along with drop shapes at different time steps

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Changes in the anatomy and physiology of the distal esophagus and stomach after sleeve gastrectomy,

Csendes, A. and Braghetto, I., 2016, “Changes in the anatomy and physiology of the distal esophagus and stomach after sleeve gastrectomy,” J Obes Weight Loss Ther,6(1), p. 297

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C., Leang, Y

Wickremasinghe, A. C., Leang, Y. J., Johari, Y., Laurie, C., Nadebaum, D., Yue, H., Yap, K. S., Hebbard, G. S., Brown, W. A., and Burton, P. R., 2024, “Modified One Anastomosis Gastric Bypass Following Sleeve Gastrectomy for Severe Reflux and Delayed Gastric Emptying: A Prospective Trial with Clinical and Physiological Outcome Measures,” Obesity Surgery, ...

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