Free-ranging brown bears (Ursus arctos) undergo cycles of active phase during the summer followed by prolonged hibernation periods during the winter. During the active phase brown bears overeat and double their fat depots in order to store energy for the hibernation and fasting period. Interestingly, it has been reported that although bears become obese during summer, they remain metabolically healthy.
The researchers studied how seasonal variation affect the gut microbiota and plasma metabolites in free-ranging brown bears and whether a seasonally altered gut microbiota can explain the healthy obesity phenotype during summer. Both microbiota composition and blood metabolites differed seasonally between hibernation and active phase in the brown bear. There were significantly altered bacterial phyla between summer and winter in bear microbiota. The winter microbiota comprised fewer bacterial taxa and was more homogeneous than the summer microbiota, which can be explained by seasonal diet variation. Specifically, hibernation microbiota had reduced levels of Firmicutes and Actinobacteria, and increased levels of Bacteroidetes, whereas summer microbiota was enriched in Proteobacteria, Firmicutes, and Actinobacteria. Besides, there were low abundant bacterial phyla only present in the summer microbiota. The highest microbiota heterogeneity during summer is a noticeable finding, as microbiota diversity is currently considered a new biomarker or indicator of health. On the other hand, several metabolites involved in lipid metabolism were also affected by hibernation. Indeed, serum levels of cholesterol esters, triglycerides, free cholesterol, and even chain acylcarnitines were significantly higher in winter, whereas lactate, several gluconeogenic amino acids, and total bile acid levels were decreased. These results suggest that during hibernation bears mobilize and oxide lipids as survival strategy, together with reduced glucose utilization and increased utilization of amino acids for gluconeogenesis.
To study whether seasonal microbiota variation affect host physiology, the researchers colonized germ-free mice with a summer or winter bear gut microbiota and followed them for 2 weeks. There was no seasonal difference in microbiota diversity, but mice colonized with a summer bear microbiota showed a significantly higher body-fat gain as compared with mice colonized with a winter bear microbiota. However, despite their increased fat mass, mice colonized with summer bear microbiota showed no statistical significantly differences in glucose metabolism than mice colonized with a winter bear microbiota. Therefore, some of the seasonal metabolic features of bear summer or winter microbiota were transferred to germ-free mice. Bears become insulin sensitive during their overeating/fattening phase of the seasonal cycle independent of fat accumulation, whereas in humans obesity and adiposity are closely linked to insulin resistance. Taken together these data, the researchers speculate that microbiota promote energy storage during summer and may be associated with the healthy obesity phenotype in brown bears.
In conclusion, bears’ shifting metabolic status between hibernation and active phase is linked to significant changes in their gut microbes. Further studies using bears in captivity in which dietary components can be controlled are required for linking dietary patterns with gut microbial enterotypes.
Sommer F, Stahlman M, Ilkayeva O, et al. The gut microbiota modulates energy metabolism in the hibernating brown bear Ursus arctos. Cell Rep. 2016; http://dx.doi.org/10.1016/j.celrep.2016.01.026.