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Gut Microbiome Offers Clues to Metabolically Healthy Brown Bear Weight Gain, Hibernation

Eurasian brown bear

NEW YORK (GenomeWeb) – A new Cell Reports study suggests the brown bear's transition from fat storage in summer to energy conservation over winter involves changes to gut microbial communities that coincide with metabolic shifts in these the periods.

The researchers analyzed gut microbial community membership and blood metabolite profiles in more than a dozen free-ranging Eurasian brown bears during hibernation in February and March and active phases after the bears roused in June, uncovering lower-than-usual microbial diversity in hibernation when energy conservation is imperative.

When the team used gut microbes from hibernating bears to colonize the guts of germ-free mice, it saw metabolic features reminiscent of those in blood samples from over-wintering bears. On the other hand, the brown bear's summer microbes produced plump, but glucose-tolerant mice.

"The restructuring of the microbiota into a more avid energy harvester during summer, which potentially contributes to the increased adiposity gain without impairing glucose metabolism, is quite striking," corresponding author Fredrik Bäckhed, a molecular and clinical medicine researcher affiliated with the University of Gothenburg and the University of Copenhagen, said in a statement.

When brown bears wake from their winter's hibernation each spring, they spend much of the following summer bulking up so they are burly enough to survive hibernation in a decreased metabolic state over roughly half the year. This not-inconsequential annual weight gain does not seem to cause the same sorts of metabolic problems obese humans often face, the team noted, prompting interest in the brown bear's secret to metabolic health.

"[T]he brown bear may constitute a model for healthy obesity and studying hibernation might be a promising approach to develop novel therapies for obesity," Bäckhed and co-authors wrote.

As part of the Scandinavian Brown Bear Research Project, the researchers used the Illumina MiSeq to do 16S rRNA sequencing on fecal DNA collected from 16 hibernating brown bears during February through March and from active brown bears a few months later.

Among the two-dozen bacterial phyla found in the brown bear stool samples, the team saw a preponderance of Actinobacteria, Proteobacteria, and Firmicutes in samples from active brown bears.

The brown bear gut microbiome contained fewer and less varied bacterial representatives during hibernation, with especially pronounced dips in Firmicutes and Actinobacteria bacteria, the researchers reported. On the other hand, levels of Bacteroidetes such as Bacteroides fragilis notched up in brown bears in this inactive state.

Overall, the investigators picked up significant shifts for microbes from 199 operational taxonomic units during the winter to summer transition. Almost 4,500 OTUs were identified across all of the brown bear gut microbial communities.

Along with microbial composition changes, the analysis highlighted broader season-related alterations in the bears' biology, namely a jump in blood levels of free cholesterol, triglycerides, and other lipids in the winter, when hibernating bears use lipolysis to tap stored fat for energy.

The presence of compounds produced by mitochondrial fatty acid oxidation also edged up in winter, the researchers found. In contrast, active summer bears had higher blood levels of metabolites associated with amino acid oxidation, glucose use and generation, lipid uptake, and red blood cell breakdown.

The team was able to reproduce some of the same metabolic features in germ-free mice transplanted with brown bear's summer or winter gut microbes and fed similar diets. For example, mice colonized with summer brown bear gut microbes ended the experiment heavier, with more robust fat stores, but no obvious signs of glucose intolerance.

In these mice, "summer microbiota promoted adiposity without impairing glucose tolerance," the study's authors wrote, "suggesting that seasonal variation in the microbiota may contribute to host energy metabolism in the hibernating brown bear."

Still, the work is still "very basic science," according to Bäckhed, meaning there's no quick fix for humans who want to gorge like summer bears without consequence. He and his colleagues also emphasized the need to verify their findings in captive brown bears with documented diet patterns.