Abstract
α-Linolenic acid (ALA) is well-known for its anti-inflammatory activity. In contrast, the influence of an ALA-rich diet on intestinal microbiota composition and its impact on small intestine morphology are not fully understood. In the current study, we kept adult C57BL/6J mice for 4 weeks on an ALA-rich or control diet. Characterization of the microbial composition of the small intestine revealed that the ALA diet was associated with an enrichment in Prevotella and Parabacteroides. In contrast, taxa belonging to the Firmicutes phylum, including Lactobacillus, Clostridium cluster XIVa, Lachnospiraceae and Streptococcus, had significantly lower abundance compared to control diet. Metagenome prediction indicated an enrichment in functional pathways such as bacterial secretion system in the ALA group, whereas the two-component system and ALA metabolism pathways were downregulated. We also observed increased levels of ALA and its metabolites eicosapentanoic and docosahexanoic acid, but reduced levels of arachidonic acid in the intestinal tissue of ALA-fed mice. Furthermore, intestinal morphology in the ALA group was characterized by elongated villus structures with increased counts of epithelial cells and reduced epithelial proliferation rate. Interestingly, the ALA diet reduced relative goblet and Paneth cell counts. Of note, high-fat Western-type diet feeding resulted in a comparable adaptation of the small intestine. Collectively, our study demonstrates the impact of ALA on the gut microbiome and reveals the nutritional regulation of gut morphology.
Link to Full Text
Key Points
Information on the influence of ALA in normal gut homeostasis and its interplay with the commensal gut microbiota remains sparse. Nutritional studies on rats fed with perilla oil, a source rich in ALA, indicated a decrease in the Firmicutes to Bacteroidetes ratio and an increase in the abundance of Spirochaetes in the perilla oil group relative to normal lab chow. A study in mice showed that flaxseed/fish oil feeding rich in ω-3 PUFA promoted the growth of Bifidobacterium and improved metabolic outcome, indicated by reduced liver weight and hepatic triglyceride concentration compared to palm oil diet. One of the possible mechanisms by which ALA might beneficially impact host metabolism is through the production of conjugated fatty acids by intestinal bacteria. Conjugated isomers of ALA have gained attention due to their reported anti-inflammatory, anti-carcinogenic and anti-obesogenic properties. Short-term feeding of C57BL/6 mice with ALA and the ALA-derived metabolites of intestinal lactic acid bacteria affected intestinal immune homeostasis. ALA and its metabolite 13-hydroxy-9(Z),15(Z)-octadecadienoic acid promote the accumulation of anti-inflammatory M2 macrophages in the small intestinal lamina propria.
In this study, it was hypothesized that an ALA-rich diet leads to compositional changes in the commensal microbiota of the mouse small intestine. The potential impact of increased dietary amounts of ALA on gut morphology in the mid small intestine was assessed. The study demonstrates that ALA-rich high-fat diet leads to an altered composition of the commensal microbiota in the mid small intestine in healthy adult mice. The small intestine of the ALA diet-fed mice showed increased total ALA content along with changed villus morphology. Specifically, villus length was increased both in ALA-rich and HFD-fed mice, accompanied with an increased mucosal thickness and number of epithelial cells. The epithelial proliferation rate as well as the proportion of goblet and Paneth cells was significantly reduced in the ALA-rich diet and HFD groups.
With regards to the gut microbiome composition, an increase in Bacteroidetes and a corresponding reduction in the relative abundance of Firmicutes following ALA-rich diet was found. The difference in the Firmicutes/Bacteroidetes ratio between groups was not statistically significant, but this is most likely due to small the sample size. In this study, both Lactobacillus and Clostridium cluster XIVa associated OTUs showed decreased abundance in the ALA-rich diet group. The results could therefore imply that an ALA-rich diet might have the potential to shift the microbiota composition to the lean phenotype. In agreement with this assumption, the Firmicutes/Bacteroidetes ratio was significantly decreased in type II diabetic patients following 6 months of EPA and DHA-rich diet compared to baseline. Bacteroides-Prevotella abundance was increased.
ALA-enriched diet resulted in an expected increase in ALA levels in the small intestinal tissue of mice. A significant reduction in small intestinal AA levels in the ALA diet-fed group. The enrichment of ALA-derived metabolites in the small intestinal tissue was accompanied by the morphometric adaptation of the small intestine, as shown by increased mucosal thickness, increased villus length and an elevated number of epithelial cells per crypt-villus axis. These ALA-rich diet-induced elongated villus structures were associated with a vastly decreased epithelial cell proliferation rate. The study provides direct evidence for the functional role of an ALA-enriched diet in influencing the small intestinal architecture. The functional involvement of HFD-induced dysbiosis in pathways driving morphometric adaptation of the small intestinal architecture is subject to further investigation. This study on adult C57BL/6J mice revealed that an ALA-rich diet influences the microbiota composition and the predicted metagenome of the small intestine. The two-component system was downregulated, and the bacterial secretion system was upregulated in the ALA group, suggesting that an ALA-rich diet might affect bacterial adaptive responses. The percentage of goblet and Paneth cells per crypt-villus axis was dramatically reduced under ALA-rich diet and Western-type HFD feeding conditions.