The impact of western diet on the mucus layer

Fiber ingestion helps ensure regular bowel movements. Moreover, since fiber is not digestible by human enzymes, it can also serve as a key nutrient for the gut microbiota as these microbes produce distinct enzymes that are able to ferment and degrade these fibers into important metabolites such as SCFAs.²⁸

FIGURE 4: Impact of Western diet versus diets rich in fiber and vitamins on local and systemic homeostasis and immunity.

Adapted from Siracusa F et al, 2019.²⁶

The intestinal mucus layer can also serve as an alternative energy source for certain gut microbes (80% of its mass being composed of sugars) when the diet is lacking in fiber.²⁹ This increase in mucus foraging by gut bacteria can prove detrimental as animal studies have shown that mice fed a diet with no fiber are more susceptible to intestinal infections and inflammation. This susceptibility was due to the resident microbiota eroding the mucus layer, such that it could no longer protect the underlying epithelium from invading pathogens.²⁹ Western diets shift the microbiota composition away from fiber degrading bacteria in favor of bacterial species that thrive on mucus (Fig 4).30 Thus, our Western diets may be leading to the loss of protective microbes and the expansion of microbes that weaken key defenses and barriers in the intestine, thereby helping trigger chronic intestinal inflammation.