Resistant starch therapy

This 2021 review subject was interactions among resistant starches and gut microbiota:

“Starch that reaches the large intestine without being fully digested is termed resistant starch (RS). Starch digestibility should be considered as a kinetic property (slower to faster) affected by host-specific factors, rather than as a binary trait (resistant or nonresistant).

RS is degraded by the colon’s complex ecosystem of microbes, triggering a cascading web of metabolic interactions. RS acts as a resource that is degraded and fermented by a hierarchy of specialized gut microbes:

  1. Primary degraders grow on RS in monoculture. They penetrate outer surfaces of intact RS granules, exposing pores and deeper concentric matrices while liberating oligosaccharides and generating metabolites like lactate and acetate.
  2. Secondary degraders grow on starch in monoculture, but degrade intact RS poorly or not at all. Instead, they may adhere to abrasions and pores on RS before participating in its degradation, and opportunistically utilize solubilized oligosaccharides produced by other RS degraders.
  3. Cross-feeders do not grow on starch in monoculture. They utilize by-products generated by upstream degraders, helping to maintain stoichiometric equilibrium and thermodynamically favorable (i.e. unconstrained) fermentation.

Together, the subsystem of microbes involved in RS degradation and fermentation participates in a complex network of cross-feeding interactions. In maintaining microbiome homeostasis, the RS nutrient web expands the scope of what could be considered a ‘beneficial’ gut microbe to a cluster of metabolically interconnected microbes.

1. Primary degraders such as acetate-producing Ruminococcus bromii are thought to be necessary for RS degradation in the human gut, where they unlock RS for other community members to degrade and ferment.

Ruminococcus genus

2. Secondary degraders possess extracellular amylases to degrade regular starch, but their contribution to initiating RS degradation is negligible compared to that of primary degraders. Instead, they may require primary degraders to erode smooth RS granule surfaces before adhering to RS and/or scavenging for ‘substrate spillover’ (i.e. excess oligosaccharides generated by primary degraders).

Eubacterium genus

Roseburia genus

3. Cross-feeders utilize starch by-products or metabolites generated by upstream RS degraders, such as acetate, lactate, formate, and succinate. Describing all known gut bacteria capable of utilizing these substrates exceeds the scope of this review, but one other example is noteworthy.

Faecalibacterium prausnitzii is a prominent butyrate-producing commensal, comprising 1.5% to 9.5% of fecal bacteria in European individuals. F. prausnitzii utilizes maltose and acetate to generate butyrate.

top 1-10 species

Microbiome sequencing data are compositional, meaning that gene amplicon read counts do not necessarily reflect bacterial absolute abundances. Instead, read counts are typically normalized to sum to 100%.

For this reason, relative abundances of smaller keystone communities (e.g. primary degraders) may increase, but appear to decrease simply because cross-feeders increase in relative abundance to a greater extent. These limitations illustrate the necessity of sufficiently powering RS interventions where microbiome composition is the primary endpoint, collecting critical baseline data and employing appropriate statistical techniques.”

https://www.tandfonline.com/doi/full/10.1080/19490976.2021.1926842 “Resistant starch, microbiome, and precision modulation”

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