Fluid dynamics simulations indicate that a variable flow velocity within the small intestine increases nutrient absorption while reducing excess bacteria.
The bacteria that live in the human gut, also called the microbiome, help with metabolism, but they also consume nutrients and can grow out of control. Biophysicists have now theoretically shown that the body can manage bacterial growth by regulating fluid flow through the small intestine using two different patterns of muscle contractions. . Their model explains fluid dynamics in the intestine and the effects of fluid flow on bacterial growth. The researchers hope that the interpretation of fluid flow will contribute to a deeper understanding of the functioning of the gut in general.
The muscles of the small intestine have two different patterns of contraction, says Karen Allem of the Max Planck Institute for Dynamics and Self-Regulation in Germany, but researchers don’t know why, as one appears to be sufficient. In one pattern, called peristalsis, wave-like contractions of the intestinal muscles along the length of the intestine lead to strong flows of fluid, back and forth, with the average flow pushing the fluid forward through the gut. In the other pattern, called segmentation, muscle contractions that run around the gut wall result in a weaker flow with a more complex velocity structure that leads to more mixing. The researchers previously simulated fluid flows in the gut, but Alem and her colleagues wanted to explore the consequences of the flow produced by the two different muscle contraction patterns, looking for effects on bacterial growth and nutrient absorption.
“Living systems are usually very complex,” Alim says. “Our goal was to gain a basic physical and mechanistic understanding of the functioning of the small intestine.” In simulations, Alim and colleagues approached the small intestine as a uniform cylinder through which a nutrient-laden fluid flows. Using fluid dynamics equations, they simulated the effects of gut contractions in two main patterns where bacteria and the gut wall compete for nutrients. It is also responsible for the growth of bacteria as they move through the gut and consume nutrients. Although data on true intestinal flows is scarce, the researchers were able to determine the key parameters of their simulation from measurements of flow velocity in the intestines of mice.
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 the nutrients remain in the intestine, allowing the body to absorb more nutrients. However, slower speeds give the bacteria more time to grow in number before being expelled. In the simulations, the fragmentation pattern tended to migrate and spread the initial lumpy distribution of nutrients uniformly, directly aiding nutrient uptake.
The researchers believe their findings explain why the gut uses these two distinct methods of muscle contraction. As they showed in more simulations, if the intestines used peristalsis alone, adopting slower speeds to improve nutrient extraction would also let bacteria build up to dangerous levels. So when the gut goes into peristalsis mode, it makes sense to do so at a relatively high speed.
But the alternating periods of peristalsis and segmentation reduce the effective transmission speed, without maintaining the low speed for too long. Fragmentation periods also enhance nutrient homogeneity and absorption efficiency. Using simulations, Alim and his colleagues found that the gut, by switching the two models in the right way, can improve the process.
“The key parameter turns out to be average flow velocity, which the small intestine can easily control by changing its contraction patterns. This allows the intestine to better absorb nutrients while keeping bacterial growth in place,” says Alim.
“This elegant study takes into account an often-overlooked factor in the research on gut dynamics, which is the hydrodynamic flow,” says biophysicist Claude Louverdo of the Sorbonne University in Paris. “The authors convincingly show that the gut exploits a clever strategy of switching between different flow regimes, using a slower-speed pattern immediately after eating to absorb nutrients, followed by a higher-velocity phase to flush out bacteria.”
– Mark Buchanan
Marc Buchanan is a freelance science writer who splits his time between Abergavenny, UK, and Notre Dame de Coursons, France.
- a. kodoti et al.Altering flows balances nutrient absorption and bacteria growth along the gut. Phys. Reverend Litt. 129138101 (2022).