Resistant Starch and the role of the microbiota

Most starches are easily digested in our small intestine but resistant starch resists digestion and reaches the colon intact. It is therefore a component of dietary fibre (see step 2.7). Once it reaches the colon it is fermented by gut microbes which produce short-chain fatty acids (SCFA). RS can be degraded (fermented) by two bacterial species, Ruminococcus bromii and Bifidobacterium adolescentis (and other related Bifidobacterium species), which are currently the only known human gut microorganisms with demonstrated RS degrading capabilities.

What affects starch digestibility? Starches with high levels of amylose tend to have higher levels of resistant starch. Other starch properties also influence digestibility including the size and type of starch granules which vary by plant origin and the structure and composition of starch polymers such as molecular size, amount of branching as well as glucose chain lengths. One way that scientists have tried to enhance resistant starch in staple food crops such as wheat and rice is by using crop genetics approaches (See a Case Study on bread in the next section for more on this).

Resistant starch supplementation in human dietary intervention trials have shown that there are changes in the gut microbiota but these are not consistent across all individuals and are likely depended on the type of resistant starch being consumed. Overall, there is some evidence that diets high in resistant starch can increase the ratio of Firmicutes to Bacteroidetes, which is associated with a healthier microbiome.