Mice given the fermentable fibre inulin as a supplement to their HFD showed less weight gain and developed far less severe symptoms of metabolic syndrome, compared with those fed the HFD without the added prebiotic.
Inulin-fed mice showed substantially lower accumulation of body and liver fat, better glycaemic control and reduced cholesterol, discovered a research team from Georgia State University, Atlanta.
The type of fibre also appeared to be important. The researchers supplemented the HFD of another group of mice with cellulose, which is not readily fermentable by mice. In this case, only a small reduction in weight gain and minor improvement in blood sugar control occurred.
“We find that while fermentable (inulin), but not insoluble (cellulose), fibre markedly protected mice against HFD-induced metabolic syndrome,” wrote lead researcher Professor Andrew Gewirtz.
Inulin supplementation prevented colon mass reduction and the lower production of cells which make up the gut lining; effects which were notable in mice fed a pure HFD. Inulin also protected the supplemented mice from low-grade inflammation and bacterial penetration of the gut lining.
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Mice fed an HFD without the inulin supplement also suffered an almost 10-fold reduction in total faecal bacteria numbers compared with standard chow-fed mice. Again, inulin but not cellulose fully restored these numbers.
Additionally, an HFD diet severely reduced the diversity of the gut bacteria population, found the researchers. It also increased the ratio of Firmicutes to Bacteroidetes and the relative number of Proteobacteria.
Inulin was able to restore diversity and reverse these other relative changes in bacteria numbers to some extent. However, the effect was only partial. The microbiota composition of inulin-supplemented HFD mice therefore still had distinct differences from the chow-fed mice. Thus, the fermentable fibre was unable to restore the mouse gut bacteria fully to its original state.
This finding highlights a risk in using fibre supplements to manipulate bacterial populations without fully understanding the functionality of a particular microbiota, cautioned the researchers.
"Simply enriching processed food with purified fibres might offer some health benefits, but we're not ready to recommend it until we understand more of the very complex interplay between food, bacteria, and host," said Gewirtz.
Finally, the scientists found that interleukin-22 (a cytokine molecule that helps regulate inflammatory processes) was responsible for the protective effect induced by the inulin. This came as a surprise finding, as it was previously thought that short-chain fatty acids (SCFAs), produced by beneficial bacteria in the gut, were largely responsible for the benefits.
Consequently, the researchers concluded, “Further work is needed in this area to better define the role of SCFAs in protecting against metabolic syndrome.”
Source: Cell Host & Microbe
Published online, doi: 10.1016/j.chom.2017.11.003
“Fiber-Mediated Nourishment of Gut Microbiota Protects against Diet-Induced Obesity by Restoring IL-22-Mediated Colonic Health”
Authors: Jun Zou, Andrew T. Gewirtz et al.