&Cartridge; physics 15, 146
Flow simulations suggest that differential flow velocity in the small intestine maximizes nutrient uptake while minimizing excess bacteria.
The bacteria that live in the human gut, also called the microbiome, help with metabolism, but they also use up nutrients and can grow out of control. Now, biophysicists have theoretically shown that the body can control bacterial growth by regulating the flow of fluid through the small intestine with two different patterns of muscle contraction . Their model accounts for the fluid dynamics of intestinal fluids and the effects of fluid flow on bacterial growth. The researchers hope that explaining the flow of fluid will contribute to a deeper understanding of the overall functioning of the gut.
The muscles of the small intestine have two different contraction patterns, but researchers don’t know why, as one is enough, says Karen Alim of the Max Planck Institute for Dynamics and Self-Organization in Germany. In a pattern called peristalsis, wave-like contractions in the gut muscles drive powerful flows of fluid, both forward and backward, along the length of the intestine, with the average flow pushing the fluid forward through the intestine. In the other pattern, called segmentation, contractions of the muscles encircling the gut wall result in a weaker flow with a more complex velocity structure that causes more mixing. Researchers have previously simulated fluid flows in the gut, but Alim and her colleagues wanted to study the consequences of the flow created by the two different muscle contraction patterns and look for effects on bacterial growth and nutrient uptake.
“Living systems are typically very complex,” says Alim. “Our goal was to gain a basic physical and mechanistic understanding of how the small intestine works.” In the simulations, Alim and colleagues approximated the small intestine as a uniform cylinder through which a nutrient-rich fluid flows. Using the equations of fluid dynamics, they simulated the effects of gut contractions in the two main patterns as both the bacteria and the gut wall compete for nutrients. They also explained the growth of the bacterial population as it moved through the gut and consumed nutrients. Although data on real intestinal flow are scarce, the researchers were able to determine basic parameters for their simulations from flow velocity measurements in the mouse intestine.
Peristalsis is the main driver of forward flow, and the team found that the strength of peristaltic contractions affects average flow velocity, with two important consequences. The slower the flow rate, the longer nutrients stay in the gut, allowing the body to absorb more nutrients. However, slower speeds also give the bacteria more time to multiply before being flushed out. In the simulations, the segmentation pattern tended to disrupt and evenly distribute the initially clumpy distribution of nutrients, directly aiding nutrient uptake.
The researchers believe their findings explain why the gut employs these two different types of muscle contraction. As they showed in further simulations, adopting slower speeds to improve nutrient extraction when the gut only uses peristalsis would also risk bacteria accumulating to dangerous levels. So when the bowel enters peristaltic mode, it makes sense to do so at a relatively high speed.
But alternating periods of peristalsis and segmentation reduce the effective rate of transport without sustaining the low rate for too long. The segmentation periods also increase nutrient uniformity and efficiency of absorption. Using the simulations, Alim and colleagues found that the gut can optimize the process by alternating the two patterns in the right way.
“The key parameter turns out to be the average flow rate,” says Alim, “which the small intestine can easily control by changing its contraction pattern. This allows the gut to optimize nutrient absorption while keeping bacterial growth in check.”
“This elegant study takes into account a factor that is almost always overlooked in research on gut dynamics, hydrodynamic flow,” says biophysicist Claude Loverdo of the Sorbonne University in Paris. “The authors convincingly show that the gut uses a clever strategy of switching between different flow regimes, using a slower-speed pattern right after eating to take up nutrients, followed by a higher-speed phase to flush out bacteria.”
– Mark Buchanan
Mark Buchanan is a freelance science writer who splits his time between Abergavenny, UK and Notre Dame de Courson, France.
- A. Codutti et al.“Altered fluxes balance nutrient uptake and bacterial growth along the gut,” physics Rev. Lett. 129138101 (2022).