Microbes may help bears stay healthy when fat for hibernation

Brown bears have all the luck. They can eat and eat and eat all summer, gain lots of weight and then lose it all by the next year. And they don’t have to worry about type 2 diabetes or other conditions that can plague humans who get too fat.

The big difference (one of them, anyway) between bears and people is that the bears hibernate in winter, and getting fat is the key to surviving six months of not eating. But hibernation alone doesn’t explain why the bears don’t suffer from the negative consequences of getting fat. So scientists looking for ways to help overweight people have looked to bears for clues to being fat and healthy.

A new study, published February 4 in Cell Reports, turns to the bears’ gut flora in that search. It’s not a stretch to think that these microbes could be involved, as they are known to help harvest energy from the diet. In addition, changes in the types of microbes found in the gut have been associated with obesity and type 2 diabetes.

To characterize the microbes living inside wild Eurasian brown bears, Fredrik Bäckhed of the University of Gothenburg in Sweden and colleagues started not with the bears themselves but with their poop. The team collected feces in summer and then during winter hibernation and determined the types of bacteria found within.

The summer feces had a rich diversity of bacteria dominated by the phyla Proteobacteria, Firmicutes and Actinobacteria. There were also Wolbachia, which can live symbiotically in insects that probably make up part of the bears’ summer meals.

In winter, when the bears weren’t eating anything, the diversity of bacteria in the feces dropped. There were plenty of Bacteroidetes, which can switch from surviving on sugars in the diet to sugars inside their host. And there were fewer Firmicutes, which need fiber from a host’s diet for food.

But are the changes in bacterial diversity just a reflection of the bears’ eating habits? Or are they somehow contributing to the bears’ health? To find out, the researchers transferred samples of the summer and winter gut microbes into mice that didn’t have gut bacteria of their own. The summer-microbe mice gained more fat and weight than the winter-microbe mice. The greater weight gain, though, did not appear to impair glucose tolerance, something that, in humans, is associated with insulin resistance and is considered a step on the path to diabetes.

The microbes, the team concludes, may indeed contribute to the bears’ healthy weight gain.

But it’s still only a maybe. The researchers admit that their study is an early step in understanding gut microbes’ role in weight gain before hibernation. Because they studied wild bears, the scientists had no control over what the animals ate and were limited in what kinds of data they could collect. A more detailed study with bears in captivity, one in which researchers could control diet and other factors, may yield different results. Such information could one day teach us how to be as healthy as a fat and happy brown bear.