How microbes help bears to hibernate

Gorging all summer and sleeping all winter – hibernating bears do it every year yet somehow manage to stay healthy. Now, Swedish scientists have discovered gut microbes are responsible for the extreme swings in metabolism, and it may lead to new treatments for obesity.

The authors collected faeces of 16 Swedish free-ranging Ursus arctos brown bears during both their active summer months and during hibernation, and analysed microbes living in them.

It turns out that the change in season came with a change in gut microbes. During hibernation, the bears had lower levels of the bacteria Firmicutes and Actinobacteria, and increased levels of Bacteroidetes. This trend is what you would see in the gut of a lean human, compared to an obese one.

The researchers then fed the microbes from summer and winter bears to germ-free mice.

After just two weeks, the mice fed with summer bear microbes gained more weight and fat than mice colonised with the winter bear gut microbes, even on the same diet.

“It’s interesting because it implies the [gut microbes] may actually be driving metabolism in way that could be harnessed,” says Margaret Morris, obesity pharmacologist at the University of New South Wales.

Despite the weight gain, the “summer” mice didn’t come down with the negative health effects usually associated with obesity, such as glucose intolerance. Understanding this “healthy” version of obesity, the authors say, could lead to new strategies for managing obesity in humans.

The scientists are next investigating the cause for the shift in gut microbes, which could be related to cold temperatures.

A study published in Cell last year showed mice exposed to cold temperatures experience shifts in gut microbe composition – enough to burn fat, improve glucose metabolism and reduce body weight.

“The microbiota may be a more important switch for energy metabolism and cold adaptation than previously appreciated,” said study author Fredrik Bäckhed.

“If we learn more about which bacteria and the functions that promote and/or protect against obesity in hibernating bears, we may identify new potential therapeutic targets.”

The study was published in Cell Reports.

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