The beneficial effects of dietary fibers on gastrointestinal inflammatory disorders has been widely studied. However, the consequences of dietary fiber intake on inflammation outside of the intestine, for instance, in the lung has been poorly documented.
Here, Trompette and colleagues showed that dietary fiber content changed the composition of the gut and lung microbiota in mice, in particular by altering the ratio of Firmicutes to Bacteroidetes. They observed that low-fiber diet (<0.3%) led to microbiota dominated by Firmicutes. By contrast, the proportion of Bacteroidaceae was found increased in mice fed a diet rich in fiber (4% supplemented with 30% cellulose or pectin). Interestingly, it was previously reported that Bacteroidetes phylum was potent at fermenting fiber into short-chain fatty acids (SCFAs), such as acetate and propionate. Then, the authors performed analysis to quantify the levels of SCFAs on the respective diets. In that way, the authors unravelled a mechanism by which dietary fiber content shaped the gut microbiota, increasing the levels SCFAs. In particular, they found that mice fed with a high-fiber diet had increased its circulating levels of SCFAs, and more interestingly, they were protected against allergic inflammation in the lung, whereas a low-fiber diet decreased levels of SCFAs and increased allergic airway disease. In particular, the researchers found that SCFAs had a role in driving bone marrow hematopoiesis, which in turn, was characterized by enhanced generation of macrophage and dendritic cell (DC) precursors, resulting in DCs with high phagocytic capacity but an impaired ability to promote T helper type 2 (TH2) cell effector function.
Taken together, these results led the authors to conclude that dietary fermentable fiber and SCFAs could define the immunological environment in the lung, which in turn, could influence the severity of allergic inflammation.